key: cord-017131-rx1z4orm
authors: Patra, Amlan Kumar
title: An Overview of Antimicrobial Properties of Different Classes of Phytochemicals
date: 2012-02-18
journal: Dietary Phytochemicals and Microbes
DOI: 10.1007/978-94-007-3926-0_1
sha: 
doc_id: 17131
cord_uid: rx1z4orm

Plants produce a great diversity of phytochemicals, the beneficial properties of which have been used by humans for centuries since the advent of human civilization. With the discovery of effective and potent antimicrobial compounds, these synthetic antimicrobial compounds are widely used to prevent and cure microbial diseases. However, the development of antibiotic resistant strains of bacteria, reduced efficacy and safety of antimicrobials and the search of new antimicrobials against emerging incurable diseases by conventional antimicrobial agents have revived to explore phytochemicals as an alternative to synthetic antimicrobial compounds. Although numerous studies have been conducted in vitro and in vivo in the recent years on the efficacy of plant phytochemicals as antimicrobial agents, this chapter provides an overview of the antimicrobial properties of some major group of phytochemicals, namely, different phenolic compounds, alkaloids, saponins, iridoids and secoiridoids, polyacetylenes, glucosinolates, terpenoids, sulfinate, limonoids (tetranortepenoids) and anthranoids against pathogenic bacteria, fungi, viruses and commensal bacteria in the intestinal tracts of humans and animals. This chapter also discusses their antimicrobial mechanisms of action, the efficiency of different groups of phytochemicals against multiple-drug resistant bacteria, the effect of active dietary phytometabolites on the beneficial and pathogenic microbes of the gastrointestinal tracts and the outcomes of combination of phytofactors and drugs interactions.

Plants contain a wide array of phytochemicals , which have traditionally been utilized for centuries in folk medicines or ethnomedicines. The earliest information on the medicinal use of plants comes from China in 5000 BC (Greathead 2003 ) , from India (in Rigveda and Atharvaveda) in 2000 BC (Ramawat et al. 2008 ) , from Mesopotamia in 2600 BC (Newman et al. 2000 ) , and also from Egypt in about 1550 BC (Davidson and Naidu 2000 ) . The natural medicines were widely used until the fi rst half of the twentieth century, when a shift towards synthetic medicines that were more effective, patentable and highly profi table, occurred (Tyler 1999 ) . However, there have been increasing interests towards use of natural chemicals in medicinal purposes in recent years. These ethnomedicines are encouraging for both the public and national health care institutions as alternatives to synthetic drugs due to relatively lower incidences of adverse reactions compared to modern conventional pharmaceuticals along with their reduced cost (Nair et al. 2005 ) .

Recently, the growing occurrences of multi-drug resistant strains of bacteria and the appearance of strains with decreased susceptibility to antibiotics have led to a resurgence of research interests in the discovery of novel antimicrobial agents from natural sources for therapeutic and preventive purposes against microbial diseases, food preservatives and feed additives in the animal industry. The ethnopharmacologists, botanists, microbiologists and natural-product chemists are constantly in search of medicinal effi cacy of plants and their phytochemicals , since the reported data so far available on plants are comparatively meager compared to the vast number of plant population. Plants produce a great diversity of compounds. The structures of close to 50,000 compounds have already been elucidated and there are perhaps hundreds of thousands of such compounds in plants (Pichersky and Gang 2000 ) . Only a few of these are part of 'primary' metabolic pathways (those common to all organisms). The rest are secondary metabolites or phytochemicals whose biosynthesis is restricted to selected plant groups (Pichersky and Gang 2000 ) . Phytochemicals can be divided into many major classes depending upon the chemical structures, botanical origins, biosynthesis pathways or biological properties. The most phytochemical classifi cation scheme is based on chemical structures such as phenolics, alkaloids, saponins, terpenoids, limonoids, polyacetylenes and secoiridoids and so on. Numerous studies have been conducted in vitro and in vivo in the recent years on the effi cacy of plant phytochemicals as antimicrobial agents. This paper presents the antimicrobial properties of some major group of phytochemicals against pathogenic bacteria, fungi and virus, and benefi cial microbes of the gastrointestinal tracts and their mechanism of action.

Phenolic compounds are a group of phytochemicals , which have a phenol structure, i.e. an aromatic benzene ring bearing at least one hydroxyl substituent (Robbins 2003 ; Vermerris and Nicholson 2006 ) . Phenolic compounds are commonly found 1 An Overview of Antimicrobial Properties of Different Classes of Phytochemicals throughout the plant kingdom, where they protect the plants from microbial infections, ultraviolet radiation and chemical stressors. This large and diverse group of phytochemicals is classifi ed into many subclasses depending upon chemical structures and occurrence in plants. The commonly categorized subclasses of phenolic compounds are simple phenolics (resorcinol and phloroglucinol ), phenolic acids and aldehydes , coumarins , fl avonoids , chalcones , aurones , benzophenones , xanthones , stilbenes , benzoquinones , naphthaquinones , anthraquinones , betacyanins , lignans , and polyphenols (proanthocyanidin , galloyl, hexahydroxydiphenyl ester , hydroxy cinnamic acid , and phloroglucinol derivatives) (Vermerris and Nicholson 2006 ; Handique and Baruah 2002 ) . The detailed structures and chemistry of these phenolic compounds are presented elsewhere (Vermerris and Nicholson 2006 ) . Foods containing phenolics are becoming an important part of diets due to their potential anti-oxidative properties. Besides, these compounds have also potent anti-microbial properties.

The phenolic acid and aldehyde group of phenolic compounds is characterized by the presence of a carboxylic acid or aldehyde group substituted on a phenol (Table 1. 1 ; Vermerris and Nicholson 2006 ) . The naturally occurring phenolic acids generally have two characteristic constitutive carbon frameworks: the hydroxycinnamic and hydroxybenzoic structures (Robbins 2003 ) . Majority of cinnamic and benzoic acid derivatives in plants are linked through ester, ether or acetal bonds to structural components, polyphenols , organic acids (quinic , maleic , tartaric and shikimic acid ), glucose and terpenes (Robbins 2003 ) . Chlorogenic acid is an ester of quinic acid and caffeic acid . Some aldehyde analogues of phenols (e.g. vanillin ) are also grouped with phenolic acids (Robbins 2003 ) . The numbers and positions of the hydroxyl and other groups on the aromatic ring can produce a large number of compounds in this subclass (Robbins 2003 ; Vermerris and Nicholson 2006 ) . Phenolic acids are present in a wide range of plants including in many common foods such as tea, coffee and berries. Besides, phenolic acids and aldehydes could be formed by the intestinal microbial biotransformation of other phenolic compounds in the intestine, where they may infl uence intestinal microbiota.

A number of simple phenols and phenolic acids possess antibacterial, antiviral and antifungal activities against a wide range of microbes, but at different concentrations. Gallic acid and p -hydroxybenzoic acid reduced the viability of Camplylobacter jejuni at concentrations as low as 1 mg/L (Ganan et al. 2009 ) . Synaptic acid , vanillic acid , and caffeic acid were microbicidal at concentrations starting at 10 mg/L. Ferulic acid and cumaric acid were effective at a concentration of 100 mg/L (Ganan et al. 2009 ) . Ozçelik et al. ( 2011 ) recently tested some phenolic acids such as gallic acid, caffeic acid, chlorogenic acid , and quinic acid for their in vitro antiviral, antibacterial, and antifungal activities. All these phenolic acids were inhibitory to herpes simplex virus type 1 (HSV -1), whereas gallic acid, chlorogenic acid and quinic acid showed potent antiviral effect against parainfl uenza virus type 3 at the therapeutic range of 0.8-0.05 mg/L. In general, antibacterial activity of phenolic acids is stronger against Gram-positive bacteria than Gram-negative bacteria (Merkl et al. 2010 ; Cueva et al. 2010 ) . The outer membrane of Gram-negative bacteria provides them with a hydrophobic surface structure that is able to exclude certain hydrophilic molecules, making them inherently resistant to many antimicrobial agents including phenolic acids (Alakomi et al. 2007 ; Cueva et al. 2010 ) . Gram-positive bacteria are enclosed in a plasma membrane covered by a thick peptidoglycan wall and lack an outer membrane (Alakomi et al. 2007 ; Cueva et al. 2010 ) . Although, phenolic acids are effective against Gram-negative bacteria, their antimicrobial effect is strain dependent (e.g. different strains of Escherichia coli ; Cueva et al. 2010 ) .

Phenolic compounds are usually poorly absorbed in the small intestine, and thus most of the dietary phenolics accumulate in the colon (Clifford 2004 ; van Duynhoven et al. 2011 ) . Therefore, higher concentrations of phenolic acids may reach in the intestine than the concentrations in diets. Phenolics may selectively suppress or stimulate (Tzounis et al. 2008 ) . Chlorogenic, quinic and gallic acids stimulated growth of Lactobacillus collinoides relative to control cultures (no additive) up to concentrations of 1 g/L of tomato broth media. In contrast, growth of Lactobacillus brevis was little affected during early incubation, which has been suggested to be due to metabolism of these acids (Stead 1994 ) . From structure-activity relationship , phenols having different alkyl chain length with hydroxyl groups could be important for antimicrobial actions (Kubo et al. 1995 ) . p-Hydroxybenzoic acid, protocatechuric , gentisic acid , vanillic acid , ferulic acid , caffeic acid and their methyl, ethyl, propyl and butyl esters were investigated for antibacterial action. It has been reported that the antimicrobial effect of phenolic acids derivatives increased with the increasing length of the alkyl chain (Merkl et al. 2010 ) . The presence of hydroxyl groups on the phenol groups and oxidized status of phenol groups also determine the toxicity of microbes. The fl uidity of the cell membrane could be disturbed with increasing hydrophobic alkyl chains. The phenolic acids could enter the molecular structure of the membrane with the polar hydroxyl group oriented into the aqueous phase by hydrogen bonding and nonpolar carbon chain aligned into the lipid phase by dispersion forces (Kubo et al. 1995 ) . Thus, when the hydrophilic force exceeds hydrophobic one, the activity tends to disappear. Also, the number and position of substitutions in the benzene ring of the phenolic acids and the saturated side-chain length infl uenced the bacteriocidal effects of phenolic acids against the different microorganisms, but in different ways against Gram-positive and Gram-negative bacteria (Cueva et al. 2010 ) . For example, Cueva et al. ( 2010 ) showed that for benzoic and phenylacetic acids, E. coli was inhibited in the following order of potency: non-substituted > 4-hydroxy-3-methoxy-> 3-hydroxy-> 4-hydroxy-> 3,4-dihydroxy-substituted acid. For phenylpropionic acids, the order differed slightly: nonsubstituted > 4-hydroxy-> 3-hydroxy->3,4-dihydroxysubstituted acid. However, the potency of phenolic acids was in different order for Lactobacillus spp. For benzoic acids, the order of potency was: 4-hydroxy-> 3-hydroxy-> non-substituted > 4-hydroxy-3-methoxy-> 3,4-dihydroxy-substituted acids, except for Lactobacillus coryniformis CECT 5711 (4-hydroxy-> non-substituted > 3-hydroxy > 4-hydroxy-3methoxy-substituted acids). For phenylacetic acids, growth inhibition of lactobacilli was on the order of non-substituted > 3-hydroxy-> 4-hydroxy-> 3,4-dihydroxysubstituted acids. For phenylpropionic acids, growth inhibition was as follows: nonsubstituted > 4-hydroxy-> 3-hydroxy > 3,4-dihydroxy-substituted acids, except for Lactobacillus fermentum CECT 5716 (3-hydroxy > non-substituted > 4-hydroxyand 3,4-dihydroxy-substituted acids) and Lactobacillus plantarum LCH17 (nonsubstituted > 3-hydroxy-> 4-hydroxy-> 3,4-dihydroxy-substituted acids).

Coumarins are naturally found in many families of plants (Apiaceae, Asteraceae, Fabiaceae, Rosaceae, Rubiaceae, Rutaceae and Solanaceae) and microorganisms, and approximately 1,000 coumarins have been isolated from these sources (Weinmann 1997 ; Smyth et al. 2009 ) . Coumarins can be classifi ed into fi ve groups depending upon the structure, i.e. coumarins with substituents in benzene ring, coumarins with substituents in pyrone ring, furocoumarins, pyranocoumarins , and coumarin dimmers ( Fig. 1.1 ; Smyth et al. 2009 ) .

Coumarins exhibit a broad diversity for antimicrobial activity. O-acetylcolumbianetin, edultin, cniforin A, columbianadin and imperatorin isolated from the fruits of Cnidium monnieri (L.) Cuss exerted a little to no appreciable growth-inhibition of Gram-positive and Gram-negative bacteria (Ng et al. 1996 ) . An amino-coumarin -7-amino-4-methylcoumarin showed broad-spectrum antibacterial and antifungal activities (Liu et al. 2008 ) . Melliou et al. ( 2005 ) studied the antibacterial activity of pyranocoumarins using an agar disc diffusion method. Seselin, xanthyletin, 5-hydroxyseselin, and 7-hydroxyalloxanthyletin had no antibacterial effects. Coumarin derivatives such as 5-methoxyseselin and its brominated derivatives, alloxanthoxyletine, the acetylated derivatives, and dipetalolactone were active against all the tested bacteria. A seselin derivative, 3-bromo-4-benzoyloxyseselin showed moderate activity, while three coumarins containing acetoxy groups in pyrano ring were only active against the two Gram-positive bacteria. A new coumarin -cajanuslactone isolated from pigeon pea leaves showed anti-bacterial activity against Staphylococcus aureus (ATCC 6538), and the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) were 0.031 and 0.125 mg/mL, respectively . Some seselin derivatives, including derivatives of 5-methoxyseselin, were found to be potent against human immunodefi ciency virus (HIV ) (Xie et al. 1999 ) .

It has been suggested that the presence of oxygenated substituents in the ether or ester form usually enhances the antibacterial activity, while the presence of free hydroxyl group reduces the activity (Melliou et al. 2005 ) . This fact could be at least partially attributed to the reduced lipophilicity of the hydroxyl derivatives, which hinders the penetration through the bacterial cell wall (Melliou et al. 2005 

Flavonoids are one of the largest groups of secondary metabolites that are distributed in various plant species. They have signifi cant antioxidant properties, which are benefi cial for health. These polyphenolic compounds are constructed basically with an A and C ring of benzo-1-pyran-4-quinone and a B ring. The main classes of fl avonoids ( Fig. 1.2 ) (having a hydroxyl group at the 3-position), e.g. kaempferol , quercetin , galangin , datiscetin , morin , robinetin , isorhamnetin , tamarixetin , quercetagetin and myricetin ; (3) fl avanones (2-3 bond saturated), e.g. hesperetin , taxifolin, eriodictyol and naringenin ; (4) flavan-3-ol, e.g. catechin and epicatechin; (5) isoflavone , e.g. genistein , daidzein and coumestrol ; (6) anthocyanidins : cyanidin , delphinidin , pelargonidin and peonidin (Crozier et al. 2006 ) . The majority of fl avonoids commonly remain conjugated with sugars as glycosides .

Numerous fl avonoid derivatives showed antiviral activity against a wide range of viruses such as HSV , HIV , coxsackie B virus, coronavirus, cytomegalovirus, poliomyelitis virus, rhinovirus, rotavirus, poliovirus, sindbis virus, and rabies virus (De Bruyne et al. 1999 ; Evers et al. 2005 ; Nowakowska 2007 ) . Ozçelik et al. ( 2011 ) investigated the effects of quercitin, apigenin, genistein , naringin, silymarin and silibinin against HSV -1 and PI-3 virus. All fl avonoids inhibited HSV -1 activity, but only genistein inhibited parainfl uenza type-1 (PI-1) activity. Of the three fl avonoids (baicalin, rutin and naringin) examined by Ng et al. 1996 , baicalin was found to be the most potent in inhibiting the growth of S. aureus : 11 of the 16 strains tested were inhibited at 128 mg/L. However, no inhibitory activity of rutin and naringin against S. aureus was observed at 128 mg/L. At this concentration, naringin and baicalin inhibited two strains and rutin inhibited one strain of the eight P. aeruginosa strains tested. The fl avonoids compounds display different mode of antiviral action. For instance, baicalein probably block human cytomegalovirus infection at entry level while the primary mechanism of action for genistein may be to block immediateearly protein functioning off human cytomegalovirus (Evers et al. 2005 ) . Both these fl avoinoids did not inhibit the virus replication (Evers et al. 2005 ) .

Puupponen-Pimia et al. ( 2001 ) investigated 13 falovonoid compounds (apigenin, (+)-catechin , chlorogenic acid , cyanidin chloride, delphinidin chloride, isoquercitrin, kaempferol , cyanidin-3-glucoside (kuromanin), luteolin, myricetin , pelargonidin chloride, quercetin dehydrate and rutin trihydrate), and 4 phenolic acids (caffeic acid , 3-coumaric acid, ferulic acid , trans-cinnamic acid) on 7 Gram-positive lactic acid bacteria of intestines, Gram-negative E. coli CM 871 and Salmonella . Myrecetin strongly inhibited the growth of Lactobacillus as well as E. coli , but did not affect Salmonella . Luteolin was weakly inhibitory to Gram-positive lactic acid bacteria but not to Gram-negative bacteria. The anthocyanidins pelargonidin, delphinidin and cyanidin, as well as cyanidin-3-glucoside, only inhibited growth of E. coli and had no effect on other bacterial strains (Puupponen-Pimia et al. 2001 ) . However, phenolic acids did not inhibit lactic acid bacteria, but inhibited Gram-negative E. coli and Salmonella sp. Hatano et al. ( 2005 ) discussed that some prenylated fl avonoids such as licoricidin (an isofl avan) effectively suppressed the antibiotic resistance of methicillin-resistant S. aureus (MRSA ) compared to other fl avonoids. The addition of 4 m g/mL of licoricidin shifted the MIC of oxacillin from 128-256 to 8-16 m g/mL, and 8 m g/mL of licoricidin reduced it to less than 0.5 m g/mL. The requirement for dimethylallyl or equivalent substituents suggests the importance of affi nity for the bacterial cell membrane. 1 An Overview of Antimicrobial Properties of Different Classes of Phytochemicals Phenolic acids show greater antimicrobial potency than their corresponding fl avonoids precursors such as the monomers (+)-catechin and (−)-epicatechin (Ganan et al. 2009 ; Cueva et al. 2010 ) . Therefore, microbial transformations of dietary fl avonoid compounds in the intestine could lead to more potent microbialinhibitory compounds (phenolic acids ) and could reach greater concentrations in the intestine. This may selectively infl uence intestinal bacteria species, and therefore could affect the diversity and metabolic activity of the intestinal microbiota, including the transformation of phenolics in the gut (Cueva et al. 2010 ) .

Epigallocatechin gallate exerted strong antibacterial growth against Gram-positive bacteria than against Gram-negative bacteria (Yoda et al. 2004 ; Engels et al. 2009 ) . It has been stated that Gram-positive bacteria absorb more epigallocatechin gallate into their peptidoglycan cell wall and aggregate its presence, while Gram-negative bacteria do not aggregate and absorb less epigallocatechin gallate (Ikigai et al. 1993 ; Engels et al. 2009 ) because of the repulsive negative charge of lipopolysaccharides on the surfaces of Gram-negative bacteria. The binding of epigallocatechin gallate to peptidoglycan disrupts its function in osmotic protection, cell division, and cell wall biosynthesis (Yoda et al. 2004 ) . Detailed information of antimicrobial activities of fl avonoids has been discussed elsewhere in this book (Chap. 2 ).

Some phenolic acids (ellagic and gallic acids) or fl avonoids (fl avan-3-ol, fl avan-3-4-diol or fl avan-4-ol) in plants are esterifi ed or polymerized into dimeric, oligomeric or polymeric compounds. Most abundantly present polyphenolic compounds in plants are tannins , which are usually of two types: hydrolysable tannins (HT) and condensed tannins (CT ). The HT are complex molecules with a polyol as a central core such as glucose, glucitol, quinic acids, quercitol and shikimic acid that is partially or totally esterifi ed with a phenolic group, i.e. gallic acid (3,4,5-trihydroxy benzoic acid ; gallotannins) or gallic acid dimmer hexahydroxydiphenic acid (ellagitannins) (Haslam 1989 ) . The CT (proanthocyanidins) are mainly polymers of the fl avan-3-ols (epi)catechin and (epi)gallocatechin units, which are linked by C4-C8 and C4-C6 interfl avonoid linkages (Ferreira et al. 1999 ; Hagerman and Butler 1989 ) .

The polyphenols also exert a wide range of antibacterial and antifungal activities. Ellagitannin extracts inhibited a range of pathogenic organisms including Vibrio cholerae , Shigella dysenteriae and Campylobacter spp . (Silva et al. 1997 ; Puupponen-Pimia et al. 2002 ) . Puupponen-Pimia et al. ( 2005 ) reported that berry extracts exhibit selective inhibitory properties against intestinal bacteria such as Staphylococcus , Salmonella , Listeria and Lactobacillus strains, and the selective inhibitory actions varied with berry extracts. In general, pathogenic Staphylococcus and Salmonella were sensitive to various berry extracts and ellagitannins fractions, while pathogenic Listeria and benefi cial Lactobacillus were not inhibited. Rauha et al. ( 2000 ) studied antimicrobial effects of some berry extracts against food spoilage and poisoning bacteria. The widest antibacterial activity was present in berries belonging to the genus Rubus (cloudberry and raspberry) that are rich in ellagitannins. Ellagic acid has been reported to exhibit a dose-dependent inhibitory effect (IC50 = 1 mM) on Helicobacter pylori isolated from peptic ulcer patients (Chung 1998 ) . Tannins isolated from Dichrostachys cinerea roots exerted antimicrobial effects against S. aureus , E. coli , Shiegella spp. and P. aeruginosa with MIC of the tannins ranging between 4.0 and 5.5 mg/mL, while the MBC ranging between 4.5 and 6.0 mg/mL (Banso and Adeyemo 2007 ) . Gallotannins extracted from the mango seed kernel inhibited the growth of Gram-positive food spoilage bacteria and decreased the growth of Gram-negative E. coli , but did not affect lactic acid bacteria (Engels et al. 2009 ) . The antibacterial properties of cranberry juice with inhibition of E. coli adherence to mucosal surfaces by cranberry juice is reported to be associated with the presence of proanthocyanidins (Howell et al. 1998 ) .

Many polyphenols have antiviral activities against different types of viruses (De Bruyne et al. 1999 ; Cheng et al. 2002 ) . It has been suggested that prodelphinidin B-2 3 ¢ -O -gallate (a proanthocyanidin gallate isolated from green tea leaf) showed anti-HSV -2 properties with the mechanism of inhibiting the attachment and penetration between cells and viruses possibly through the instability of viral glycoproteins (Cheng et al. 2002 ) . The structure and functional groups of the polyphenol compounds may determine the effectiveness of the antiviral activities (De Bruyne et al. 1999 ) .

The content of small-molecular phenolic compounds have greater infl uence on the antibacterial activity of extracts than tannins (Nazaruk et al. 2008 ) . Thus, polyphenols could be cleaved by bacterial enzymes to form a number of phenolic acids in the intestine, where they may infl uence the microbial populations (Bock and Ternes 2010 ) . Engels et al. ( 2009 ) recently studied the effects of gallotannins with different galloyl units from mango seed kernel on various Gram-positive and Gram-negative bacteria. Gallotannins showed antibacterial activities with MICs ranging from 0.1 g/L for S. aureus to 3.3 g/L for Pediococcus acidilactici . They also observed that degree of galloylation did not affect the growth of bacteria. It has been suggested that the antibacterial activities of gallotannins are due to their strong affi nity for iron and the inactivation of membrane-bound proteins (Engels et al. 2009 ) . It has also been shown that gallotannins changed the morphology of Bacillus subtilis , which has been hypothesized due to inhibition of cell division by binding of gallotannins to the cell wall or inhibition of enzymes involved in cell separation (Engels et al. 2009 ) .

Naphthoquinones are widely distributed in plants, fungi, and some animals. Lapachol, plumbagone, juglone and lawsone are naturally occurring naphthoquinones 1 An Overview of Antimicrobial Properties of Different Classes of Phytochemicals of plant origin that have antimicrobial effects against various pathogenic bacteria and fungi. Adeniyi et al. ( 2000 ) reported that two dimeric naphthoquinones, diospyrin and isodiospyrin , isolated from the root of Diospyros piscatoria (Gurke), a common ingredient in several folk medicines, exhibited a broad spectrum of antibacterial activity against S. pyogenes and S. pneumoniae (MICs of diospyrin ranged from 1.56 to 50 m g/mL) Salmonella choleraesuis serotype typhi ( S. typhi ) and Mycobacterium chelonae (MICs of diospyrin were between 25 and 100 m g/mL). Isodiospyrin was more active than its racemic isomer diospyrin (MICs against Gram-positive bacteria ranged from 0.78 to 50 m g/mL, while those against Pseudomonas aeruginosa and S. typhi ranged from 50 to 100 m g/mL). Another naphthoquinones, lapachol and b -lapachone, found in species of Tabebuia, had relevant effects against Candida albicans, Candida tropicalis, and Cryptococcus neoformans, and were more active than the reference standard, ketoconazole. Lapachone showed strong antimicrobial activity than lapachol against the fungi (Guiraud et al. 1994 ) . Methanol extract from the dried inner bark of Tabebuia impetiginosa exhibited potent antibacterial activity against H. pylori which contained lapachol and anthraquinones (Park et al. 2006 ) .

Alkaloids have been defi ned as N-heterocyclic basic metabolites, although the defi nition does not clearly separate from other N-containing compounds. Alkaloids have been classifi ed in many ways depending upon biogenic precursors or carbon skeleton characteristics. They have a great structural diversity compared with other classes of phytochemicals . Alkaloids are generally known according to their carbon skeleton structures. Pyridine (e.g. piperine), piperidine , quinoline , indole , pyrrolidine , quinazoline , isoquinoline , glyoxaline , lupinane , tropan , phenanthridine , imidazoline , alkaloidal amines and terpenoid types of alkaloids are commonly found in plants (Hegnauer 1988 ) .

Alkaloid fractions isolated from Strychnos potatorum L.f. (Loganiaceae) seeds, which were of indole type, were tested for their antimicrobial properties against some pathogenic Gram-positive, Gram-negative and acid-fast bacteria and fungi. These fractions had shown considerable antimicrobial activity against both bacteria and fungi at the tested concentrations (100 and 200 m g/mL). Further, the growth of Proteus vulgaris , S. aureus , Salmonella typhimurium, Vibrio cholerae , Mycobacterium tuberculosis, Aspergillus niger and C. albicans were signifi cantly inhibited (Mallikharjuna and Seetharam 2009 ) . Similarly, two benzophenanthridine alkaloids , dihydrochelerythrine and dihydrosanguinarinealkaloid constituents of Bocconia arborea showed considerable antimicrobial activity against Gram-positive and Gram-negative bacteria and C. albicans ( Navarro and Delgado 1999 ) .

Sensitivity of DNA and RNA viruses to alkaloids may differ. Ozçelik et al. ( 2011 ) investigated various alkaloids namely yohimbine and vincamine (indole -type), scopolamine and atropine (tropane-type), colchicine (tropolone-type), allantoin (imidazolidine-type), trigonelline (pyridine-type) as well as octopamine, synephrine, and capsaicin (exocyclic amine-type) for their antiviral activities against DNA virus herpes simplex (HSV -1) type 1 and RNA virus parainfl uenza type-3 (PI-3). All the alkaloids were effective against HSV -1 at 0.05-1.6 mg/L, but atropin and octopamine showed potent antiviral activities against PI-3 at 0.05-0.8 mg/L (Ozçelik et al. 2011 ) . Antibacterial alkaloids from Chelidonium majus Linn, i.e. benzo [c] phenanthridine -type alkaloids, 8-hydroxydihydrosanguinarine, 8-hydroxydihydrochelerythrine were potently active against MRSA strains with MICs/MBCs ranged from 0.49 to 15.63 and 1.95 to 62.50 m g/mL, respectively (Zuo et al. 2008 ) .

There are two rich sources of organosulphur compounds from plants; (1) Alliaceae family containing alliin -alliinase system and (2) Cruciferae ( Brassicacae ) family e.g. Brassica juncea , Wasabia japonica (wasabi), Armoracia rusticana (horseradish) and Brassica oleracea (caulifl ower) containing glucosinolate-myrosinase (Mithen 2006 ) . A number of sulphur-containing compounds can be derived from these plants through the action of myrosinase and alliinase enzymes.

The primary sulphur-containing constituents in Alliums spp. (e.g. A. sativum (garlic), A. cepa (onion), A. porrum (leek)) and Brassica spp. (e.g. cabbage, kale, caulifl ower and turnip) are S -alk(en)yl-L-cysteine sulphoxides and g -glutamyl-S -alk(en)yl-Lcysteine sulphoxides (Block et al. 1992 ; Ross and Milner 2007 ;  Fig. 1.3 ) . The content of S-alk(en)yl-L-cysteine sulphoxides in garlic may range from 0.53% to 1.3% of fresh weight with S -allyl-L-cysteine sulphoxide (alliin ) being the largest contributor. By the action of alliinase enzyme present inside the cells, these compounds are converted into thiosulfi nate (a functional group consisting of the linkage R-S(=O)-S-R'), which are then spontaneously and enzymatically converted into a large array of volatile compounds, e.g. diallyl disulphide, diallyl trisulphide, allyl methyl disulphide and dipropyl and disulphide (Mithen 2006 ) .

Antimicrobial activities of garlic and onion against a wide range of Grampositive and Gram-negative bacteria, virus and fungi are known for many years (Ankri and Mirelman 1999 ) . The antifungal activities of garlic oils appear to be more than the antibacterial activity (Avato et al. 2000 ) . Extracts of garlic exhibit the most potent antibacterial activity, followed by onion, and Brassica including cabbage (Kyung and Lee 2001 ) . The principal antimicrobial compounds of Allium and Brassica are allicin ( S -allyl-L-propene thiosulfi nate ) and methyl methanethiosulfinate, respectively (Kyung and Lee 2001 ) . These compounds are derived from S -allyl and S -methyl derivatives of L-cysteine sulfoxide , respectively. Avato et al. ( 2000 ) tested different mixtures of garlic distilled oils containing diallyl disulfi de (DDS) and diallyl trisulfi de (DTS), ranging from 1% to 51% and 88% to 38%, respectively, against yeasts ( C. albicans, C. tropicalis and B. capitatus ), Gram-positive bacteria ( S. aureus and B. subtilis ) and Gram-negative bacteria ( P. aeruginosa and E. coli ). Incubation of garlic extracts made up of 1% DDS and 88% DTS did not show growth inhibition against all the tested microorganisms, whereas garlic oils with higher quantities of DDS showed signifi cant inhibitory activity, increasing with the increase of DDS amount, thus implicating the DDS as the active antimicrobial agent (Avato et al. 2000 ) . It has been reported that allicin (MIC, 6 m g/mL; MBC, 6 m g/mL) was more potent than DDS (MIC range, 100-200 m g/mL; MBC range, 100-200 m g/mL), its corresponding sulfi de, but of a strength similar to that of diallyl tetrasulfide (MIC range, 3-6 m g/mL; MBC range, 3-6 m g/mL) against H. pylori (O'Gara et al. 2000 ) . Kyung and Fleming ( 1997 ) investigated the different S-compounds found in cabbages on the growth of 15 bacteria and 4 fungi. S -Methyl-L-cysteine sulfoxide, sinigrin , and dimethyl sulfi de at 500 ppm did not inhibit the growth of any of the bacteria and yeasts. Dimethyl disulfi de at 500 ppm retarded the growth of some bacteria, but was not bactericidal to any of the test microorganisms. Dimethyl trisulfi de , methyl methanethiosulfi nate and methyl methanethiosulfonate had MICs of 200 ppm, between 50 and 200 ppm, and between 20 and 100 ppm, respectively for bacteria, and 20 ppm, between 6 and 10 ppm and between 50 and 500 ppm for yeasts, respectively (Kyung and Fleming 1997 ) .

There are numerous reports showing the effectiveness of garlic or allicin as antimicrobial agents in comparison to antibiotics (Fujisawa et al. 2009 ; Cai et al. 2007 ) . Also, allicin with antibiotics may synergistically augment the antimicrobial actions (Cai et al. 2007 ; An et al. 2009 ) . Besides, thiosulfi nates and their derivatives show promising activity against multidrug resistant bacteria including MRSA (Ankri and Mirelman 1999 ; Fujisawa et al. 2009 ) . The main mode of action of thiosulfi nate derivatives have been proposed to be due to its chemical reaction with the thiol groups of various enzymes (Ankri and Mirelman 1999 ) and thus antimicrobial properties of allicin may be abolished by cysteine, coenzyme A and glutathione (Fujisawa et al. 2009 ) . Antimicrobial activity of the diallyl sulfi des has been reported to increase with the number of sulfur atoms (O'Gara et al. 2000 ) .

Glucosinolates are the sulphur-containing metabolites found in large number of edible plants. Over 120 glucosinolates are present in 16 families of dicotyledonous angiosperms, most of which are clustered within the Brassicaceae and Capparaceae (Fahey et al. 2001 ) . Allyl (sinigrin ) and 3-butenyl (gluconapin) glucosinolate are found in brown mustard, p -hydroxybenzyl glucosinolate in white mustard, allyl and other glucosinolate in horseradish and wasabi, methylthiopropyl in cabbage and 2-hydroxy 3-butenyl glucosinolate in rapeseed ( Fig. 1.4 ; Fahey et al. 2001 ; Mithen 2006 ) .

The antibacterial and antifungal properties of glucosinolates are known for a long time (Fahey et al. 2001 ) . Intact glucosinolates do not show antimicrobial action, but the hydrolysis products of glucosinolates are active against various microorganisms (Manici et al. 1997 ; Tierens et al. 2001 ) . Aires et al. ( 2009a ) observed that the in vitro growth inhibition and the sensitivities of the individual bacteria are infl uenced by the structure of glucosinolates and their hydrolysis products. The most effective glucosinolate hydrolysis products were the isothiocyanates ; sulforaphane and benzyl isothiocyanate were the strongest inhibitory against the growth of human pathogenic bacteria. Regarding action of glucosinolates products on the type of bacteria, 4-methyl sulfi nyl butylisothiocyanate exhibited antibacterial activity against a larger range of bacteria. Indole-3-carbinol had some inhibitory effects against the Gram-positive bacteria, but had no effect against the Gram-negative bacteria. Indole-3-acetonitrile had some inhibitory activity against the Gram-negative bacteria. Glucosinolates, nitriles and amines were ineffective at the doses up to 3 m mol (Aires et al. 2009b ) . Saavedra et al. ( 2010 ) evaluated the in vitro antibacterial actions of different classes of common dietary phytochemicals , i.e. simple phenolics -tyrosol, gallic acid, caffeic acid , ferulic acid , and chlorogenic acid ; chalcone -phloridzin; fl avan-3-ol -(−) epicatechin; secoiridoid -oleuropein glucoside; glucosinolate hydrolysis products -allyl isothiocyanate, benzyl isothiocyanate and 2-phenylethyl isothiocyanate) against four pathogenic microbes. All of the isothiocyanates had signifi cant antimicrobial activities, while the phenolics were much less effi cient. No antimicrobial activity was observed with phloridzin. Allyl isothiocyanate from cabbage had an MIC between 50 and 500 ppm for bacteria and between 1 and 4 ppm for yeasts (Kyung and Fleming 1997 ) .

Iridoids is a group of cyclic monoterpenoids having iridane skeleton (cis-2-oxabicycle-(4.3.0)-nonane), which mostly remain as glycosides ( Fig. 1.5 ; Perez et al. 2005 ) . Secoiridoids derive from iridoids by the elimination of the link 7-8 to yield the basic structure (Perez et al. 2005 ) . This group of phytochemicals is found in a number of folk medicinal plants and many of them possess signifi cant biological and pharmacological activities (Dinda et al. 2009 ) .

A number of iridoids and secoiridoids (nepetalactones from Serbian Nepeta species , Nestorović Three iridoids, phloyoside1, phlomiol, and pulchelloside 1, isolated from the rhizomes of the Iranian fl ora Eremostachys laciniata ( Lamiaceae ) had low to moderate levels of antibacterial activity (MIC = 0.05-0.50 mg/mL) against fi ve bacterial strains, Bacillus cereus , Citrobactor freundii , Proteus mirabilis , P. aeruginosa , S. aureus (Modaressi et al. 2009 ) . Out of these three compounds, pulchelloside 1 showed highest antibacterial activity against B. cereus , penicillin-resistant E. coli , P. mirabilis and S. aureus with an MIC value of 0.05 mg/mL. Nestorović et al. ( 2010 ) investigated the nepetalactones content in the methanol extracts of the shoot cultures of three endemic Serbian Nepeta species: Nepeta rtanjensis , N. sibirica and N. nervosa , and evaluated the antimicrobial activity of these extracts against eight bacterial strains E. coli , P. aeruginosa , S. typhimurium , Listeria monocytogenes , Enterobacter cloacae (human isolate), B. cereus (clinical isolate), Micrococcus fl avus and S. aureus , and eight fungal species: Aspargillus fl avus , Aspargillus fumigatus , Aspargillus niger , Fusarium sporotrichoides , Fulvia fulvum, Penicillium funiculosum, P. ochrochloron and Trichoderma viride . Trans, cisnepetalactone was present in shoots of N. rtanjensis , while cis,trans -nepetalactone stereoisomer was present in N. sibirica . No nepetalactone was observed in shoots of N. nervosa . All these extracts had signifi cant antibacterial and antifungal activities against all the tested species. N. rtanjensis extract showed the strongest antibacterial activity with MIC of 50 m g/mL. N. nervosa and N. sibirica extracts showed antibacterial activities with MIC of 50-100 and 100 m g/mL, respectively. Similarly, N. rtanjensis, N. nervosa and N. sibirica extracts showed MIC of 25-5, 50-100 and 25-100 m g/mL, respectively. The presence of trans-nepetalactone in N. rtanjensis extract was probably responsible for strongest activity against bacteria and fungi, while cis-nepetalactone in N. sibirica extract showed higher antibacterial and antifungal activity than that of N. nervosa extract. (Geng et al. 2009a (Geng et al. , b, 2011 . Iridoid aglycone moieties, but not its glycosides , exhibit the antiviral activities. Zhang et al. ( 2009 ) studied an anti-hepatitis C virus pseudoparticles (HCVpp) entry essay on both aqueous and methanol extracts of the fl owering tops of Lamium album . Iridoid glucoside lamalbid isolated from the methanol extract was inactive against HCVpp, whereas its aglycone, and epimers named lamiridosins A and B present as major constituents in the aqueous extract signifi cantly inhibited in vitro HCV entry (IC50 value of 2.31 m M). These were nontoxic to the Hep G2 2.2 cells at a concentration of 50 m g/mL. They also demonstrated that the parent iridoid glycosides did not show anti-HCV entry activity, but the aglycones of shanzhiside methyl ester, loganin, loganic acid, verbenalin, eurostoside and picroside II exhibited signifi cant anti-HCV entry and anti-infectivity activities.

Chemically, saponins are a group of high molecular-weight glycosides , in which saccharide chain units (1-8 residues) are linked to a triterpene (triterpene saponins ) or steroidal (steroid saponins) aglycone moiety, i.e. sapogenin (Fig. 1.6 ) . They occur in a wide variety of plants with triterpene saponins (in soybean, alfalfa, quillaja, and guar), and are more widely distributed in nature than steroidal (in yucca, tomato, and oats) saponins ( Hostettmann and Marston 1995 ) . The steroidal saponins may possess furostanol or spirostanol (e.g. smilagenin and sarsapogenin) moiety. The saccharide chains are commonly attached at the C3 position (monodesmosidic), but some sapogenins contain two saccharide chains (bidesmosidic) attached at the C3 and C17 (via C28) position (Vincken et al. 2007 ) . A large number of saponins could be possible depending upon the modifi cations of the ring structure of aglycone moieties and number of sugars added to it, and in turn producing different biological properties.

Many plant extracts containing saponins from various plants and purifi ed saponins show antimicrobial activities at different concentrations (Sen et al. 1998 ; Avato et al. 2006 ) . However, the types of saponins exhibit different spectra of antimicrobial effects. Oleanolic acid isolated from the root bark of Newbouldia laevis have broad-spectrum antimicrobial activity against 6 Gram-positive, 12 Gramnegative bacterial species and three Candida species (Kuete et al. 2007 ) . b -sitosterol-3-O -b -d-glucopyranoside isolated from this plant also showed antibacterial effects on three Gram-positive, six Gram-negative bacterial species and three Candida species. A saponin fraction from the stem of Y. schidigera exhibited potent growthinhibitory activity with MIC ranging from 31.3 to 125 m g/mL against certain food-deteriorating yeasts ( C. albicans ), fi lm-forming yeasts ( Debaryomyces hansenii, Pichia nakazawae, Zygosaccharomyces rouxii ), dermatophytic yeasts ( Candida famata, Hansenula anomala, Pichia carsonii ), and against brewer's yeast ( Saccharomyces cerevisiae ) (Miyakoshi et al. 2000 ) . Different saponins, i.e. tigogenin from Tribulus terrestris , dioscin from the rhizomes of Smilacina atropurpurea , minutosides from bulb of Allium leucanthum were very active against different fungal strains such as C. albicans , C. glabrata and Cryptococcus neoformans (Zhang et al. 2006a, b ; Barile et al. 2007 ) . Saponins appear to have stronger activities against fungi, and act by disrupting the membrane integrity of fungal cells.

Different extraction procedures and storage may affect the antimicrobial action of saponins probably due to chemical transformation of saponins (Guclu-Ustundag and Mazza 2007 ) . Commercially produced quillaja ( Quillaja saponaria ) and yucca ( Yucca schidigera ) saponins showed different antibacterial activities against E. coli , suggesting that saponins from various commercial sources differ in their biological activities (Sen et al. 1998 ) . In this study, commercial saponin-rich quillaja and yucca extracts exhibited antibacterial activity against S. aureus and E. coli at different concentrations. The antimicrobial activity of saponins may also be modifi ed by the pH of media. The tea saponins exhibited greater antimicrobial activities against Gram-positive S. aureus (MIC <0.006 vs. >0.2), Gram-negative E. coli (MIC 0.003 vs. >0.2) and C. albicans (MIC <0.006 vs. >0.2) at low pH 4 than high pH 8.5 (Li et al. 2009 ) .

Some saponins , in general, exhibit stronger antimicrobial activity against Gram-positive bacteria than against Gram-negative bacteria (Avato et al. 2006 ) . Saponins fraction from soapnut pericarps ( Sapindus mukurossi , Tanaka et al. ( 1996 ) and guar ( Cyamopsis tetragonoloba , Hassan et al. 2010a, b ) showed greater antibacterial activity against Gram-positive bacteria than against Gram-negative bacteria. Conversely, saponins isolated from orchid tree ( Bauhinia variegata L.) bark exhibited greater antibacterial activity for Gram-negative bacteria than Gram-positive bacteria at concentrations ranging from 2.5 to 10 mg/mL (Morrissey and Osbourn 1999 ) .

The relationships between saponin structures and antimicrobial activity are strongly noted. The structure of sapogenin moiety, chain length and composition of sugars infl uences the antimicrobial activities. The Y. schidigera saponin fraction possessing a trisaccharide chain without any oxygen functionalities at C-2 and/or C-12 of the aglycone exhibited potent anti-yeast activity, while saponins with 2b-OH or 12-keto groups showed very weak or no activity. Low activity was observed for saponins with a disaccharide chain and no activity was observed for the aglycones obtained after acid hydrolysis (Miyakoshi et al. 2000 ) . Yang et al. ( 2006a ) noted that no activity was observed in the hecogenin saponins when its sugar moiety was less than four monosaccharide units. Pentaglycoside was more active than tetraglycoside and shows extended antifungal spectrum against A. fumigatus . In the diosgenin saponin series, saponins with only triglycosides are active against C. albicans and C. glabrata , while the diosgenin saponins with monoglycoside and diglycoside did not show any activity. Again, within the group of tigogenin saponins, their antifungal capacity was slightly infl uenced by the composition of the sugar moiety. The replacement of a glucosyl unit with a xylosyl unit showed enhanced activity against A. fumigatus . Avato et al. ( 2006 ) suggested that the sugar moiety is not important for the antimicrobial effi cacy from their study since antibacterial activity increased from the saponin extracts to the sapogenin samples.

Terpenoid compounds derive from a basic structure of C5 isoprene units. They are classifi ed according to the number of isoprene unit involved for their synthesis, i.e. monoterpenoid (C10), sesquiterpenoids (C15), diterpenoids (C20), sesterterpenoids (C25) and triterpenoids (C30). They can be acyclic (myrcene and geraniol), monocyclic (cymene and carvacrol), bicyclic (pinene) and tricyclic with different groups (alcohol, phenol, and aldehyde). The most commonly occurring essential oils (EO) are included in two chemical groups (Fig. 1.7 ) : terpenoids (monoterpenoids and sesquiterpenoids) and phenylpropanoids, which are synthesized through mevalonate and shikimic acid metabolic pathways, respectively (Gershenzon and Croteau 1991 ; Calsamiglia et al. 2007 ) . Among these two classes, terpenoids are the more diversifi ed group of plant bioactives abundantly found in many herbs and spices (Gershenzon and Croteau 1991 ) . Within terpenoids, the most important components of EO of the majority of plants belong to the monoterpenoids and sesquiterpenoids (Gershenzon and Croteau 1991 ; Calsamiglia et al. 2007 ) . Phenylpropanoids have a side chain of three carbons bound to an aromatic ring of C6 (Calsamiglia et al. 2007 ) . Phenylpropanoids are less abundant compounds of EO compared with terpenoid family, but some plants contain in signifi cant proportions. The EO are a group of secondary plant metabolites obtained from volatile fractions of plants by steam distillation process (Gershenzon and Croteau 1991 ) . The EO are used traditionally by humans, for many centuries, which provide characteristic fl avor and aroma specifi c to many plants, and are used as antimicrobial agents and preservatives. The EO have diverse chemical composition, nature and biological properties. The EO can be obtained from fl owers, petals, leaves, stems, fruits, roots and barks and the concentrations of EO in these parts depends upon the stage of growth, environmental conditions (Hart et al. 2008 ) .

A number of EO are known for their strong anti-microbial activities against many pathogenic and non-pathogenic bacteria and fungi. Curcumin and its derivatives, the phenylpropanoids, are the principal compounds in rhizome of Curcuma longa (turmeric), which exhibit antibacterial properties against different bacteria and fungi. Essential oil fractions of turmeric inhibited the growth of pathogenic Gram-positive ( S. aureus and Staphylococcus epidermidis ) and Gram-negative ( E. coli , P. aeruginosa and S. typhimurium ) bacteria (Singh et al. 2002 ) . The EO fraction was more effective against Gram-positive compared to Gram-negative strains, and was comparable to standard antibiotics gentamycin, ampicillin, doxycycline and erythromycin in these strains (Singh et al. 2002 ) . A recent study by De et al. ( 2009 ) demonstrated that curcumin inhibited the growth of different clinical isolates of H. pylori with MICs ranging from 5 to 50 m g/mL. The gingerols, another phynylpropanoids from Zingiber offi cinalis (zinger), possess antifungal and antibacterial properties (Park et al. 2008 ) . Ginger extract containing gingerol inhibited the growth of H. pylori with MICs ranging from 0.8 to 12.5 m g/mL (Mahady et al. 2003 ) .

Constituents of EO differ in their antimicrobial activity against bacteria and fungi. Investigating the antimicrobial properties (18 bacterial species and 12 fungi) of fi ve EO constituents (cineole, citral, geraniol, linalool and menthol), Pattnaik et al. ( 1997 ) showed that linalool had the most antibacterial activity and inhibited 17 bacteria, followed by cineole, geraniol (each of which inhibited 16 bacteria), menthol and citral aromatic compounds, which inhibited 15 and 14 bacteria, respectively. However, the antifungal activities of these EO constituents did not follow the pattern of antibacterial activities. Citral and geraniol oils were the most effective against fungi (inhibiting all 12 fungi), followed by linalool (inhibiting 10 fungi), cineole and menthol (each of which inhibited 7 fungi) compounds (Pattnaik et al. 1997 ) .

It has been suggested that the pH of EO in culture media may modify antimicrobial properties. For example, anise oil had higher antifungal activity at pH 4.8 than at 6.8, while the oil of Cedrus deudorawas was most active at pH 9 (Janssen et al. 1987 ) . The structure and stereochemistry of the essential oils have profound infl uences on the antimicrobial activities. Alkenyl substituents incorporated into nonphenolic ring structures of essential oils such limonene showed increased antibacterial activities compared with alkyl substituents such as p -cymene with alkylation showing more inhibitory effect on Gram-negative bacteria (Dorman and Deans 2000 ) . From stereochemistry of EO, it has been reported that a -isomers such as a -pinene are less active relative to b -isomers such as geraniol and nerol; cis -isomers are inactive contrary to trans -isomers; compounds with methyl-isopropyl cyclohexane rings are the most active; or unsaturation of the cyclohexane ring further increases the antibacterial activity, e.g. terpinolene, terpineol and terpineolene (Hinou et al. 1989 ; Dorman and Deans 2000 ) . However, Griffi n et al. ( 1999 ) reported that the specifi city and level of antimicrobial activity of terpenoids were not always characterized by the functional groups, but were associated with hydrogen-bonding parameters, and for Gram-negative organisms a combination of hydrogen-bonding parameters and molecular size parameters. The antimicrobial properties of EO from different sources have been discussed in details elsewhere (Chap. 5 ).

Chemically, limonoids are unique secondary metabolites, characterized by a tetranortriterpenoid skeleton with a furan ring ( Fig. 1.8 ) . They are commonly isolated from Citrous and Maliaceae plants (Hallur et al. 2002 ; Rahman et al. 2009 ; Vikram et al. 2010 ) . Besides their health promoting effects, various limonoids have been shown to possess antibacterial, antifungal and antiviral effects (Govindachari et al. 2000 ; Battinelli et al. 2003 ; Atawodi and Atawodi 2009 ) .

Various limonoid compounds such as mahmoodin , azadirone , epoxyazadiradione, nimbin , gedunin, azadiradione , deacetylnimbin and 17-hydroxyazadiradione, isolated from various parts of Azadirachta indica (Meliaceae family) have been reported to have antimicrobial activities (Siddiqui et al. 1992 ; Govindachari et al. 2000 ; Atawodi and Atawodi 2009 ) . Rahman et al. ( 2009 ) tested two limonoids isolated from the seeds of Swietenia mahagoni ( Meliaceae family), swietenolide and 2-hydroxy-3-O-tigloylswietenolide against various multiple-drug-resistant bacterial strains including Gram-positive ( S. aureus , S. pneumoniae and Haemophilus influenzae ) and Gram-negative ( E. coli , Klebsiella pneumoniae, Salmonella typhi, and Salmonella paratyphi ) strains. The most potent activity of swietenolide was observed against H. infl uenzae , S. typhi , and S. paratyphi , whereas 2-hydroxy-3-O-tigloylswietenolide was most active against S. pneumoniae, S. typhi, and S. paratyphi . The lowest activity was observed against K. pneumoniae for both compounds. The limonoids compounds may exhibit antibacterial properties against pathogenic bacteria by disrupting the quorum sensing system and biofi lm production. Vikram et al. ( 2010 ) demonstrated limonin, nomilin, obacunone, deacetyl nomilin and limonin 17-O-b -D-glucopyranoside purifi ed from seeds of grapefruits to possess the anti-quorum sensing activity and inhibitory effect on biofi lm formation of pathogenic E. coli O157:H7 with obacunone exhibiting strong antagonistic activity.

Limonoids also have signifi cant antiviral activity. Limonin and nomilin showed inhibitory effects on HIV -1 replication in peripheral blood mononuclear cells and monocytes/macrophages, which was not cytotoxic at the active concentrations (Battinelli et al. 2003 ) . The antiviral activity was not much infl uenced by structural differences by limonin and nomilin in this study (Battinelli et al. 2003 Parida et al. ( 2002 ) demonstrated in an in vivo study that azadirachtin obtained from A. indica inhibited dengue virus type-2 replication as confi rmed by the absence of dengue-related clinical symptoms in sucking mice and absence of virus specifi c 511 bp amplicon.

More than 700 polyacetylene compounds have been characterized from plants, which are mainly prominent in the Asteraceae, Apiaceae and Campanulaceae including many medicinal plants from various parts of the world (Hudson 1989 ) . Food plants of the Apiaceae plant family such as carrots, celery, parsley, fennel and parsnip contain a group of bioactive aliphatic C17-polyacetylenes including falcarinol, falcarindiol, panaxydiol, and polyacetylene 8-O-methylfalcarindiol (Zidorn et al. 2005 ; Christensen and Brandt 2006 ) . Avato et al. ( 1997 ) investigated the different polyacetylene compounds from the aerial organs of Bellis perennis L. Of the major constituents, methyl deca-4,6-diynoate and deca-4,6-diynoic acid, and their structural analogues, i.e. deca-4,6-diyne, dimethyl octa-3,5-diyne-1,8-dioate and deca-4,6-diyne-1,10-dioic acid, deca-4,6-diynoic acid and deca-4,6-diyne-1,10-dioic acid showed antimicrobial activity against Gram-positive and Gram-negative bacteria, respectively.

Polyacetylene carboxylic acids, 13( E ),17-octadecadiene-9,11-diynoic acid (13,14-dihydrooropheic acid, and the known 17-octadecene-9,11,13-triynoic acid (oropheic acid, isolated from the stem bark of Mitrephora celebica demonstrated signifi cant activity against MRSA and Mycobacterium smegmatis (Zgoda et al. 2001 ) . Similarly, pentayne diol, a polyacetylene which was isolated from Bidens pilosa (a traditional medicinal herbs) showed highly potent and extensive inhibitory activities against several Gram-positive and Gram-negative pathogenic bacterial species, including MRSA , and vancomycin-resistant Enterococcus faecalis and C. albicans (Tobinaga et al. 2009 ) . In a recent fi nding, a polyacetylene compound from Carlina acaulis , i.e. carlina oxide exhibited strong antibacterial activity against two MRSA strains, Streptococcus pyogenes, P. aeruginosa, C. albicans , and C. glabrata with less toxicity to human HeLa cells (Herrmann et al. 2011 ) .

Anthranoid compounds are widely distributed in various plants particularly in Aloe , Cassia, Rheum , Cassia and Frangula , which are traditionally used in ethnomedicine for laxative and cathartic action (Paneitz and Westendorf 1999 ) . Naturally occurring anthranoids can be chemically described as dihydroxyanthraquinones, -dianthrones and -anthrones, often present in plants as glycones (Table 1. 2 ; Paneitz and Westendorf 1999 ) . Different anthranoids such as aloe-emodin, rhein, emodin, physcion and chrysophanol occur in Rheum species.

Anthranoids have shown antimicrobial properties in different studies. The anthranoid compounds from the rhizome of Rheum emodi exhibited antibacterial and antifungal activities (Babu et al. 2003 ) . The antimicrobial effects of the three anthraquinones on S . aureus found to be in the order of rhein > emodin > 1,8-dihydroxyanthraquinone (Wu et al. 2006 ) . Similarly, Wang et al. ( 2010 ) demonstrated that the sequence of antimicrobial activity against Bifi dobacterium adolescentis of the fi ve hydroxyanthraquinones was rhein > emodin > aloe-emodin > chrysophanol > physician. They also suggested the infl uence of substituent groups on phenyl ring in hydroxyanthraquinones against B. adolescentis activity might be related with the polarity and the sequence was carboxyl > hydroxyl > hydroxylmethyl > methyl and methoxyl. Prenylated anthranoids from leaves of Harungana madagascariensis have shown to inhibit Bacillus megaterum (Kouam et al. 2007 ) . Additionally, the effect of emodin with antibiotics (ampicillin and oxacillin) was found to be synergistic or partially synergistic against MRSA, where emodin reduced the MICs of the antibiotics (Lee et al. 2010 ) . However, some of the anthranoids have potent mutagenic effect (Paneitz and Westendorf 1999 ) , which is required to consider when evaluating the antimicrobial properties of these compounds.

There is considerable evidence that a number of phytochemicals have potential to become useful antimicrobial agents that could be employed as preventative or treatment therapies against microbial and viral diseases. Although, there are some encouraging effects in vivo to inhibit pathogenic microbes without affecting benefi cial bacteria in the gastrointestinal tracts, more studies would be required for the safety and effi cacy of these phytochemicals to establish whether they could offer therapeutic benefi ts over conventional therapies. Besides, the combination of some antimicrobial drugs and phytochemicals may act as better antimicrobial agents than antimicrobial drugs alone. For example, the application of dual combinations demonstrated synergy between streptomycin and gallic acid, ferulic acid , chlorogenic acid , allylisothiocyanate and 2-phenyle thylisothiocyanate against the Gram-negative bacteria. Moreover, they can act synergistically with less effi cient antibiotics to control bacterial growth (Saavedra et al. 2010 ) . 3,4-dihydroxyphenylacetic acid and 3-hydroxyphenylacetic acid increased the susceptibility of S . enterica subsp. enterica serovar Typhimurium strains for novobiocin. In addition, organic acids present in berries, such as malic acid, sorbic acid, and benzoic acid , were shown to be effi cient permeabilizers of Salmonella as shown by an increase in the 1-N-phenylnaphthylamine uptake assay and by lipopolrsaccharide release (Alakomi et al. 2007 ) . Cinnamon essential oil and its major component (trans-cinnamaldehyde) enhanced the antibacterial activity of clindamycin against a toxicogenic strain of Clostridium diffi cile ) . In addition, the enhancement activity of different essential oils ( Mentha longifolia L. and Mentha spicata L.) and different monoterpenes (piperitone, carvone and menthone) on the antibacterial activity of nitrofurantoin has been reported (Rafi i and Shahverdi 2007 ; Shahverdi et al. 2004 ) . The antibacterial activity of cefi xime, cephotaxime, vancomycin and tetracycline was also increased by curcumin (Moghaddam et al. 2009 ) . Allicin has a synergistic effect with amphotericin B against C. albicans via enhancing the phospholipid peroxidation reaction in vitro and in vivo , which suggests that allicin could reduce the amphotericin B dose to lessen side effects ) . Due to the growing use of phytochemicals and other dietary phytochemical-rich supplements, it is required to understand whether problems might arise from using these preparations in combination with conventional drugs. There is lack of comprehensive studies that can establish the consequences of phytochemicals-drug interactions. However, all these evidence also suggest that intake of phytochemicals rich foods could be considered in future research while antimicrobial agents are applied to the body.

Plant genomes contain 20,000-60,000 genes, and about 15-25% of these genes encode enzymes for secondary metabolism (Bevan et al. 1998 ; Somerville and Somerville 1999 ) . The genome of a plant species encodes only a small fraction of all the enzymes that are required to synthesize the entire set of secondary metabolites found throughout the plant kingdom (Pichersky and Gang 2000 ) . Identifi cation of particular genes for target phytochemicals and the genetic engineering techniques could allow expressing the biosynthetic pathways of some phytochemical synthesis in organisms such as E. coli , B. subtilis or S. cerevisiae . For example, Miyahisa et al. ( 2006 ) reported that introduction of four genes for a phenylalanine ammonia-lyase, cinnamate/coumarate:CoA ligase, chalcone synthase, and chalcone isomerase, in addition to the acetyl-CoA carboxylase, in E. coli cells resulted in effi cient production of (2S)-naringenin from tyrosine and (2S)-pinocembrin from phenylalanine. Finally, the possibility of using phytochemicals as antimicrobial compounds would be a paradigm shift towards the potential health benefi ts and safety overcoming the problem of microbial resistance to drugs.

Antibacterial activity of diospyrin, isodiospyrin and bisisodiospyrin from the root of Diospyros piscatoria (Gurke) (Ebenaceae)

Initial in vitro evaluations of the antibacterial activities of glucosinolate enzymatic hydrolysis products against plant pathogenic bacteria

The antimicrobial effects of glucosinolates and their respective enzymatic hydrolysis products on bacteria isolated from the human intestinal tract

Weakening of salmonella with selected microbial metabolites of berryderived phenolic compounds and organic acids

Allicin enhances the oxidative damage effect of amphotericin B against Candida albican s

Antimicrobial properties of allicin from garlic

Azadirachta indica (neem): a plant of multiple biological and pharmacological activities

Antimicrobial activity of polyacetylenes from Bellis perennis and their synthetic derivatives

Allylsulfi de constituents of garlic volatile oil as antimicrobial agents

Antimicrobial activity of saponins from Medicago sp.: structure-activity relationship

Antimicrobial constituents from the rhizomes of Rheum emodi

Evaluation of antibacterial properties of tannins isolated from Dichrostachys cinerea

Saponins from Allium minutifl orum with antifungal activity

Effect of limonin and nomilin on HIV-1 replication on infected human mononuclear cells

Analysis of 1.9 Mb of contiguous sequence from chromosome 4 of Arabidopsis thaliana

Allium chemistry: HPLC analysis of thiosulfi nates from onion, garlic, wild garlic (Ramsons), leek, scallion, shallot, elephant (great-headed) garlic, chive, and Chinese chive

The phenolic acids from bacterial degradation of the mangiferin aglycone are quantifi ed in the feces of pigs after oral ingestion of an extract of Cyclopia genistoides (honeybush tea)

Antibacterial activity of allicin alone and in combination with beta-lactams against Staphylococcus spp. and Pseudomonas aeruginosa

Invited review: essential oils as modifi ers of rumen microbial fermentation

Iridoids from the aerial parts of Verbena littoralis ( Verbenaceae )

Antiviral properties of prodelphinidin B-2 3 ¢ -O-gallate from green tea leaf

Bioactive polyacetylenes in food plants of the Apiaceae family: occurrence, bioactivity and analysis

Inhibitory actions of ellagic acid on growth and aryl amine N-acetyltransferase activity in strains of Helicobacter pylori from peptic ulcer patients

Diet-derived phenols in plasma and tissues and their implication for health

Plant secondary metabolites and the human diet

Antimicrobial activity of phenolic acids against commensal, probiotic and pathogenic bacteria

Non-glycosidic iridoids from Cymbaria mongolica

Phyto-phenols. In: Naidu AS (ed) Natural food antimicrobial systems

Antimicrobial activity of curcumin against Helicobacter pylori isolates from India and during infections in mice

Biological evaluation of proanthocyanidin dimers and related polyphenols

Naturally occurring iridoids, secoiridoids and their bioactivity. An updated review, Part 3

Antimicrobial agents from plants: antibacterial activity of plant volatile oils

Antimicrobial activity of gallotannins isolated from mango ( Mangifera indica L.) kernels

Human cytomegalovirusinhibitory fl avonoids: studies on antiviral activity and mechanism of action

The chemical diversity and distribution of glucosinolates and isothiocyanates among plants

Condensed tannins

Antibacterial potential of garlic-derived allicin and its cancellation by sulfhydryl compounds

Antimicrobial activity of phenolic compounds of wine against Campylobacter jejuni

Swerilactones A and B, anti-HBV new lactones from a tradtional Chinese herb: Swertia mileensis as a treatment for viral hepatitis

Swerilactones C and D, anti-HBV new lactones from a traditional Chinese herb: Swertia mileensis

Anti-hepatitis B virus active lactones from the traditional Chinese herb: Swertia mileensis

Antifungal activity of some tetranortriterpenoids

Cantleyoside dimethyl acetal, a new antimicrobial iridoid glycoside from the aerial parts of Pterocephalus perennis

Plants and plant extracts for improving animal productivity

The role of structure and molecular properties of terpenoids in determining their antimicrobial activity

Saponins: properties, applications and processing

Comparison of antibacterial and antifungal activities of lapachol and B-lapachol

Choosing appropriate methods and standards for assaying tannins

Three new tetranortriterpenoids from neem seed oil

Polyphenolic compounds: an overview

Plant extracts to manipulate rumen fermentation

Haemolytic and antimicrobial activities of saponin-rich extracts from guar meal

Hemolytic and antimicrobial activities differ among saponin-rich extracts from guar, quillaja, yucca, and soybean

Effects of tannins and related polyphenols on methicillin-resistant Staphylococcus aureus

Biochemistry, distribution and taxonomic relevance of higher plant alkaloids

Carlina oxide -a natural polyacetylene from Carlina acaulis ( Asteraceae ) with potent antitrypanosomal and antimicrobial properties

Antimicrobial activity screening of 32 common constituents of essential oils

Saponins

Inhibition of the adherence of P-fi mbriated Escherichia coli to uroepithelial surfaces by proanthycyanidin extracts from cranberries

Plant photosensitizers with antiviral properties

Bactericidal catechins damage the lipid layer

Antimicrobial activities of essential oils. A 1976-1986 literature review on possible applications

A novel antibacterial iridoid and triterpene from Caiophora coronata

Cajanuslactone, a new coumarin with anti-bacterial activity from pigeon pea

Antimicrobial prenylated anthracene derivatives from the leaves of Harungana madagascariensis

Structural functions of antimicrobial long-chain alcohols and phenols

Antimicrobial activity of the methanolic extract and of the chemical constituents isolated from Newbouldia laevis

Antimicrobial activity of sulfur compounds derived from cabbage

Antimicrobial activities of sulfur compounds derived from S-alk (en) yl-L-cysteine sulfoxides in Allium and Brassica

Synergistic effect of emodin in combination with ampicillin or oxacillin against methicillin-resistant Staphylococcus aureus

Effects of pH on antioxidant and antimicrobial properties of tea saponins

Antimicrobial activity of an endophytic Xylaria sp.YX-28 and identifi cation of its antimicrobial compound 7-amino-4-methylcoumarin

Ginger ( Zingiber offi cinale Roscoe) and the gingerols inhibit the growth of Cag A+ strains of Helicobacter pylori

In vitro antimicrobial screening of alkaloid fractions from Strychnos potatorum

In vitro fungitoxic activity of some glucosinolates and their enzyme-derived products toward plant pathogenic fungi

Natural and synthetic 2,2-dimethylpyranocoumarins with antibacterial activity

Antimicrobial and antioxidant properties of phenolic acids alkyl esters

Plant secondary metabolites, occurrence, structure and role in the human diet

Combinatorial biosynthesis of fl avones and fl avonols in Escherichia coli

Antiyeast steroidal saponins from Yucca schidigera (Mohave yucca), a new antifood-deteriorating agent

Antibacterial iridoid glucosides from Eremostachys laciniata

The combination effect of curcumin with different antibiotics against Staphylococcus aureus

Fungal resistance to plant antibiotics as a mechanism of pathogenesis

Antibacterial activity of some selected Indian medicinal fl ora

Two antimicrobial alkaloids from Bocconia arborea

Polyphenolic compounds and in vitro antimicrobial and antioxidant activity of aqueous extracts from leaves of some Cirsium species

Nepetalactone content in shoot cultures of three endemic Nepeta species and the evaluation of their antimicrobial activity

The infl uence of natural products upon drug discovery

Examination of coumarins, fl avonoids and polysaccharopeptide for antibacterial activity

A review of anti-infective and anti-infl ammatory chalcones

Activities of garlic oil, garlic powder, and their diallyl constituents against Helicobacter pylori

Cytotoxicity, antiviral and antimicrobial activities of alkaloids, fl avonoids, and phenolic acids

Anthranoid contents of rhubarb ( Rheum undulatum L.) and other Rheum species and their toxicological relevance

Inhibitory potential of neem (Azadirachta indica Juss) leaves on dengue virus type-2 replication

Antibacterial activity of Tabebuia impetiginosa Martius ex DC (Taheebo) against Helicobacter pylori

Antibacterial activity of [10]-gingerol and [12]-gingerol isolated from ginger rhizome against periodontal bacteria

Antibacterial and antifungal activity of aromatic constituents of essential oils

Iridoids and secoiridoids from Oleaceae

Genetics and biochemistry of secondary metabolites in plants: an evolutionary perspective

Antimicrobial properties of phenolic compounds from berries

Development of functional ingredients for gut health

Berry phenolics selectively inhibit the growth of intestinal pathogens

Comparison of essential oils from three plants for enhancement of antimicrobial activity of nitrofurantoin against enterobacteria

Antibacterial activity of two limonoids from Swietenia mahagoni against multiple-drug-resistant (MDR) bacterial strains

The chemical diversity of bioactive molecules and therapeutic potential of medicinal plants. In: Ramawat KG (ed) Herbal drugs: ethnomedicine to modern medicine

Antimicrobial effects of Finnish plant extracts containing fl avonoids and other phenolic compounds

Phenolic acids in foods: an overview of analytical methodology

Antimicrobial activity of phenolics and glucosinolate hydrolysis products and their synergy with streptomycin against pathogenic bacteria

Effect of Quillaja saponaria saponins and Yucca schidigera plant extract on growth of Escherichia coli

Piperitone from Mentha longifolia var. chorodictya Rech F. reduces the nitrofurantoin resistance of strains of enterobacteriaceae

Trans-cinnamaldehyde from Cinnamomum zeylanicum bark essential oil reduces the clindamycin resistance of Clostridium diffi cile in vitro

Constituents of Azadirachta indica : isolation and structure elucidation of a new antibacterial tetranortriterpenoid, mahmoodin, and a new protolimonoid, naheedin

Antimicrobial activity of Terminalia macroptera root

Cytotoxicity and antibacterial studies of iridoids and phenolic compounds isolated from the latex of Himatanthus sucuuba

Antibacterial activity of Curcuma longa rhizome extract on pathogenic bacteria

A study of the antimicrobial activity of selected naturally occurring and synthetic coumarins

Plant functional genomics

The effect of chlorogenic, gallic and quinic acids on the growth of spoilage strains of Lactobacillus collinoides and Lactobacillus brevis

Study of the role of antimicrobial glucosinolate-derived isothiocyanates in resistance of Arabidopsis to microbial pathogens

Isolation and identifi cation of a potent antimalarial and antibacterial polyacetylene from Bidens pilosa

Phytomedicines: back to the future

Flavanol monomer-induced changes to the human fecal microfl ora

Metabolic fate of polyphenols in the human superorganism

Saponins, classifi cation and occurrence in the plant kingdom

Grapefruit bioactive limonoids modulate E. coli O157:H7 TTSS and biofi lm

Microcalorimetric assay on the antimicrobial property of fi ve hydroxyanthraquinone derivatives in rhubarb ( Rheum palmatum L.) to Bifi dobacterium adolescentis

History of the development and application of coumarin and coumarin-related compounds

Antimicrobial properties and toxicity of anthraquinones by microcalorimetric bioassay

Anti-AIDS agents. 37. Synthesis and structureactivity relationships of (3 ¢ R,4 ¢ R)-(+)-cis-khellactone derivatives as novel potent anti-HIV agents

Antifungal activity of C-27 steroidal saponins

Five new iridoids from Patrinia rupestris

Different susceptibilities of Staphylococcus and Gram-negative rods to epigallocatechin gallate

Polyacetylene carboxylic acids from Mitrephora celebica

Antifungal activities and action mechanisms of compounds from Tribulus terrestris L

Atropurosides A-G, new steroidal saponins from Smilacina atropurpurea

Lamiridosins, hepatitis C virus entry inhibitors from Lamium album

Polyacetylenes from the Apiaceae vegetables carrot, celery, fennel, parsley, and parsnip and their cytotoxic activities

Antibacterial alkaloids from Chelidonium majus Linn (Papaveraceae) against clinical isolates of methicillin-resistant Staphylococcus aureus