key: cord-0943515-jxz75mdh
authors: Sanders, Mary Ellen
title: Probiotics: Considerations for Human Health
date: 2008-09-16
journal: Nutr Rev
DOI: 10.1301/nr.2003.marr.91-99
sha: 7386c08aea06ca0bbcf496ae6ff3f7fdfb254fb6
doc_id: 943515
cord_uid: jxz75mdh

Evidence for the role of probiotics in maintenance of health or prevention of disease is mounting and is supported in some cases by blinded, placebo‐controlled human trials. Today, in an era of antibiotic‐resistant pathogens and other looming microbial threats, the value of prevention of infection is recognized. Probiotics may play an important role in helping the body protect itself from infection, especially along the colonized mucosal surfaces of the gastrointestinal tract. Probiotic products are available in many different forms worldwide, including pills, powders, foods, and infant formula. In some cases, general health claims are made that cannot be substantiated for the specific strains and levels being used and consumers must therefore beware.

Probiotics, live microorganisms administered in adequate amounts that confer a health effect on the host, 1 are emerging as signi cant dietary ingredients in the eld of nutrition. An important function for probiotics is that they in uence the populations or activities of host micro ora residing in the alimentary canal, the vaginal tract, or the skin. Until recently, the role of gastrointestinal ora in human health was under-appreciated. The concept of colonization resistance (i.e., limiting action of the normal ora on colonization of the bowel by exogenous and endogenous potentially pathogenic microorganisms) has been recognized for decades, 2 but only recently has the signi cance of this concept resonated with the medical community. The following quote is revealing:

"The toll of human suffering that results from infection and, in particular, from infection with organisms that can be readily eliminated through the use of antimicrobial agents has engendered a clinical attitude that views those microorganisms as inimical agents of disease to be ruthlessly and utterly eradicated. Yet, the biological relationship between multicellular organisms and the microbial world is better viewed as one of symbiosis than of enmity. Therefore, it follows that preservation of the normal ora, rather than its deletion, can provide the greatest clinical bene t." 3 Although the bene ts of the normal ora are being acknowledged, the importance of probiotics extends further, because the application of exogenous microbes can also bene t the host. This concept is largely credited to Elie Metchnikoff, an early 20th century Russian scientist awarded the Nobel Prize for his innovative work in immunology. Today a growing industry has developed around the sale of probiotics in food, dietary supplements, and pharmaceutical formats for human and animal use. This article discusses the scienti c basis of probiotics and a perspective for their use.

Why consider the value of probiotics to human health? The gastrointestinal tract and its resident microbes have far-reaching implications for health. The normal microora limits the ability of potential pathogens to infect. Boosting the body's ability to resist infection prevents morbidity, decreases antibiotic use (and possibly the spread of antibiotic-resistant pathogens), and decreases the sequelae from some primary infections, which are rare, but nonetheless can be serious. A breakdown of tolerance to the intestinal ora is thought to be a key step in the development of in ammatory disorders of the intestine (e.g., Crohn's disease and ulcerative colitis) for which there is no cure and for which treatment can be as severe as surgical removal of the colon. Errant activities of some intestinal bacteria are thought to contribute to the generation of carcinogens, which may play a role in the genesis of colon cancer. Exposure of the immune cells of the intestinal tract to the right types of microbes in infancy may be important to the prevention of allergy development later in life. Taken together, these facts suggest that intervention at the level of micro ora may be important to health.

Dr. Sanders is with the Dairy and Food Culture Technologies, 7119 S. Glencoe Ct., Centennial, CO 80122-2526, USA.

Although the concept of probiotics was introduced in the early 20th century, the term was not coined until the 1960s. The de nition of the term has evolved through the years ( Table 1) . Characteristics of the de nition proposed by an Expert Consultation in an FAO/WHO report 1 follows.

Probiotics must be alive. Although it is recognized that dead cells may mediate physiologic bene ts, a different term should refer to these agents.

Probiotics are administered. A misuse of the term equates "probiotic" with native "bene cial" ora (presumably lactobacilli and bi dobacteria). Although native bene cial ora may be isolated, puri ed, characterized, evaluated, and ultimately used as probiotics, in their native state they are not probiotics.

Probiotics must deliver a measured health bene t, substantiated by studies conducted in the target host. Not speci ed in this de nition is that some bene ts may be physiologic markers presumed to be linked to a health bene t, such as lowering cholesterol. If used to substantiate ef cacy, such biomarkers should be validated.

Probiotics needn't be restricted to food applications or oral delivery. Probiotics used as pharmaceuticals or as topical agents are not excluded from this de nition.

A de nition of probiotics shouldn't limit the mechanism of action. A de nition stating that a probiotic must survive gastrointestinal tract transit or have an impact on normal micro ora is too restrictive, considering the wide range of proposed mechanisms that mediate probiotic function. The delivery of lactase by, for example, Streptococcus thermophilus, to the small intestine is recognized as probiotic activity, even though S thermophilus does not survive intestinal transit.

Not included in this de nition are stipulations for safety or for use of de ned strains. Safety is implied; the de nition states that the result of the probiotic is a health effect. In practice, a probiotic product should comprise one or more de ned strains. It is scienti cally untenable to validate probiotic function and monitor probiotic impact on a preparation of microbes of unknown composition. Probiotic activities have been deemed largely strain-speci c, so de nition to the strain level is important. Deposit of a probiotic strain into an internationally recognized culture collection is recommended.

It is important to have an agreed upon de nition by the scienti c community, even in the absence of regulatory de nitions. In the absence of a de nition, consumers cannot know what to expect from a product carrying this designation and there can be no common understanding among scientists about appropriate use of the term.

The impact of probiotics on diverse human physiologic endpoints has been tested. Recent reviews describe details of many human studies testing the ef cacy of probiotics. [11] [12] [13] [14] Table 2 lists the targets that have been evaluated. Although hundreds of publications on probiotics can be found in the scienti c literature, only a few strains acting on a few clinical targets enjoy the status of having "established effects." This judgment is made when considering the totality of the evidence (in vitro, animal, and human data) supported by a plausible mechanism of action. The key to comprehending that a wide range of physiologic parameters can be in uenced by probiotics is understanding the diverse effects of colonizing microbes.

As with research on any dietary component, some quali cations must be kept in mind when considering the body of published literature. First, generalizations about probiotics can be misleading. Different strains or combinations of strains cannot be expected to have the same 

Substances produced by microorganisms that promote the growth of other microorganisms 4 Organisms and substances that contribute to intestinal microbial balance 5 A live microbial feed supplement that bene cially affects the host animal by improving its intestinal microbial balance 6 A viable mono-or mixed-culture of microorganisms that, applied to animal or man, bene cially affects the host by improving the properties of the indigenous micro ora 7 Living microorganisms that, upon ingestion in certain numbers, exert health bene ts beyond inherent basic nutrition 8 A microbial dietary adjuvant that bene cially affects the host physiology by modulating mucosal and systemic immunity, as well as improving nutritional and microbial balance in the intestinal tract 9 A preparation of or a product containing viable, de ned microorganisms in suf cient numbers, that alter the micro ora (by implantation or colonization) in a compartment of the host and by that exert bene cial health effects in this host 10 Live microorganisms that, when administered in adequate amounts, confer a health bene t on the host 1 effect. Second, mechanisms are often not understood. In the absence of a credible hypothesis for the effect, studies documenting health effects must be considered preliminary. Third, effects should be evaluated with reference to the totality of the evidence. Private industry groups with a clear objective to establish functionality of their commercial products often fund studies. These studies should be con rmed in independent laboratories. Furthermore, negative results are rarely published. Therefore, the totality of the evidence is not made available for scienti c consideration. The willingness of editors of scienti c publications to publish negative results, especially of well-conducted human studies, should be encouraged.

The concept of "proper strain selection" for identi cation of effective probiotics gures prominently in the literature. The best known commercial strains are accompanied by descriptions of the scienti c processes by which the strains were selected from among many other strains deemed less useful. Interestingly, however, the multitude of characteristics or tests (e.g., human origin, acid tolerance, bile resistance, adherence to human intestinal cell lines, bacteriocin production, and colonization, among others) used by most investigators to select these strains have not been validated as important criteria for probiotic functionality. For example, controlled studies with Note: At least one double-blind, placebo-controlled human trial supports many of these targets, although some trials may have been conducted in a small number of subjects. Some targets are only supported by in vitro or animal studies. Reviewed in Mercenier et al. 1 4 isogenic mutants with mutated capacity for the attribute in question should be conducted where possible. For many in vitro selection criteria, there is no scienti c substantiation beyond the assertion that "it stands to reason" that the attributes deemed essential for probiotic function are even important. This is not meant to discourage careful laboratory analysis of strains for probiotic use. It is intended to highlight the limitations of some selection criteria and point out the need to restrain from overextending the meaning of such evaluations. Certain in vitro analyses are critical to responsible probiotic characterization, such as thorough taxonomic evaluation (using DNA-based and phenotypic techniques), strain identity patterns (using a combination of phenotypic, morphologic, and DNA-based techniques), safety assessment, and metabolic, enzymatic, and physiologic capabilities. Until those characteristics are proven to be important to in vivo function by controlled studies, however, their importance should not be assumed.

Probiotics are available in products such as food, dietary supplements, and drugs. In the United States, there are no approved probiotic drugs for human use, although one product, Preempt, which was developed by the United States Department of Agriculture, is available as an animal drug (the microbial preparation is used on newly hatched chicks to help prevent colonization by pathogens). Probiotics sold as pharmaceuticals are available in other countries. In the United States, food products containing probiotic bacteria are almost exclusively dairy products. Approximately 80% of the yogurt manufactured in the United States contains Lactobacillus acidophilus added as a probiotic. Some products also contain Bi dobacterium strains. In addition to yogurt, probiotic bacteria are added to milk, some cottage cheeses, and a few niche products such as rice milk. A fermented milk beverage, Actimel (Dannon, Tarrytown, NY) is a distinctive product in the United States available nationwide at a natural foods grocery chain. It contains 10 10 Lactobacillus casei per 100-mL serving. Actimel is labeled as a dietary supplement and is unique for a fermented dairy beverage in the United States in that it lists the probiotic level (10 10 /serving) on the label. Outside the United States, probiotic-containing food products include cheese, toddler formula, and juices. Products such as breakfast cereal and nutrition bars containing probiotics are not currently available, but they present attractive possibilities for future probiotic products. Recently, the United States Food and Drug Administration indicated that it had no questions regarding the effort of Nestle (Lausanne, Switzerland) to af rm the generally recognized as safe (GRAS) status of Strepto-coccus thermophilus and Bi dobacterium lactis for use in formula for infants greater than 4 months of age; this opened the door for the sale of probiotic-containing formula in the United States.

The manufacturer often determines strategies for formulation of U.S. dairy products with probiotic bacteria. The U.S. standard of identity for yogurt requires that yogurt be made with Lactobacillus bulgaricus and Streptococcus thermophilus as fermenting starter cultures, but no levels are speci ed. Probiotic bacteria such as Lactobacillus acidophilus or Bi dobacterium spp. are allowable in yogurt and milk, but no minimum levels are required. Some commercially used strains are listed in Table 3 . Culture manufacturers recommend approximately 10 6 probiotic bacteria per gram of yogurt and unfermented acidophilus milk. Even if this formulation recommendation is followed, there is no guarantee that the product contains this level at the time of consumption. Viable counts may fall below these levels if the bacteria used are not biologically stable in the particular food formulation.

Recognizing the importance of assuring consumers of the presence of live cultures with possible bene cial effects, the National Yogurt Association (McLean, VA) established a "Live Active Culture" seal that helps consumers identify yogurt containing live, active cultures; this seal can be used by any yogurt manufacturer on labels of yogurts meeting minimum standards. Use of the seal requires refrigerated yogurt to contain 10 8 viable lactic acid bacteria per gram at the time of manufacture. The seal can also be used on frozen yogurts containing 10 7 viable lactic acid bacteria per gram at the time of manufacture. However, these standards do not pertain to each species or strain listed on the label. Therefore, counts of the starter culture S thermophilus may meet the standard, but counts of L acidophilus or Bi dobacterium may not. The consumer therefore suffers from a lack of clear labeling standards for probiotic-containing products.

Dietary supplements are another source of probiotics. These products, usually available in pill or capsule form, are produced by approximately 80 different companies in the United States. The product diversity is much greater than what is found in dairy products and this market segment is growing faster than the probioticcontaining dairy segment in the United States. Whereas dairy products containing probiotics generally contain Lactobacillus or Bi dobacterium species, dietary supplement (or biotherapeutic) products on the worldwide market also contain Enterococcus species, Bacillus species, Escherichia coli, and/or Saccharomyces. Unfortunately, the lack of standards (whether imposed by government or industry) has resulted in products that are not labeled properly. 35 Some of this mislabeling is reasonably harm-less. However, persistent use of the term "Lactobacillus sporogenes," when in fact the organism is a Bacillus species, perhaps coagulans, is not a benign error. 36 Some products contain Enterococcus but do not list this on the label. Enterococcus species have been associated with transferable antibiotic resistance genes and are a leading source of nosocomial infections in the United States. 37 Franz et al. 38 commented that in the past, Enterococcus species were considered ". . . harmless commensals with low pathogenic potential," but today they ". . . may be considered opportunistic pathogens." Although infection resulting from Enterococcus consumption in foods or probiotic supplements has not been reported, consumers should be properly informed about product contents.

Labeling of U.S. food or dietary supplement products must conform to U. S. food regulations. In short, both foods and dietary supplements are allowed to make statements relating the food or supplement to the normal functioning of the human body (structure/function claims), but statements regarding their role in the prevention, treatment, cure, diagnosis, or mitigation of disease are expressly forbidden because these claims are limited to use on drugs. Manufacturers of dietary supplements must notify the U.S. Food and Drug Administration of structure/function statements they intend to make on products, but premarket approval of such statements is not required. Examples of some structure/func-tion statements used on U.S. probiotic dietary supplement products are shown in Table 4 .

The perception of probiotic products in Europe (especially northern Europe) and Asia differs from that in the United States. Currently, there is no harmonized legislation in Europe for the use of probiotics, but in general, little can be legally claimed. In Japan, there is legal recognition of functional foods (Foods for Specied Health Use, or FOSHU) and several probiotic products and strains have been granted FOSHU status by the Japanese Ministry of Health. Recently, the FAO and WHO jointly convened an expert consultation on foods containing probiotics. The report has been posted. 1 Guidelines for the safe use and labeling of probiotics developed as a follow-up to the expert consultation are also available. 42

One hundred percent safety can never be guaranteed. However, many species of lactobacilli are integral to the production of fermented foods and have been consumed safely as part of these foods for millennia. In addition, both bi dobacteria and some species of lactobacilli are normal, nonpathogenic inhabitants of the human intestinal tract. Intestinal lactobacilli (some species not normally associated with fermented foods) and bi dobacte- ria have been used in dietary supplements for decades, with an overwhelming record of safe consumption. The conclusion of a recent review of safety of lactobacilli and bi dobacteria is that there is essentially no risk with the oral consumption of lactobacilli or bi dobacteria by healthy people. 9 In patients with certain underlying diseases or conditions, however, some caution should be exerted. In fact, native lactobacilli and bi dobacteria have been documented as causative agents of infections in patients with compromised health. [43] [44] [45] Some metabolic end products may be a concern. Speci cally, Dlactic acidosis can result in people with short bowel syndrome if the probiotic produces D-lactic acid from sugar metabolism. 46 This situation can be avoided by choosing probiotics that exclusively produce the L-isomer of lactic acid. There are only two documented cases of association between probiotic Lactobacillus consumption and infection, and this association was correlative, not causal. Two different probiotic preparations containing L rhamnosus were involved with adverse effects: a liver abscess in a 74-year-old diabetic woman with hypertension 47 and endocarditis in a 67-year-old undergoing tooth extraction with a history of mitral valve prolapse. 48 These reports suggest that lactobacilli can behave opportunistically, albeit extremely rarely. In a discussion of safety of probiotics, however, one must consider the broad range of microbes beyond the lactobacilli and bi dobacteria that are used as probiotics. Strains of Enterococcus, Bacillus, and Escherichia should be evaluated comprehensively for safety. Even the yeast, Saccharomyces, can be problematic under certain circumstances. Hennequin et al. 49 documented 13 cases of fungemia owing to vascular catheter contamination in hospitalized patients consuming Saccharomyces as a probiotic; this was likely due to cross contami-nation of catheters when the probiotic was administered. Enterococcus species are a leading cause of nosocomial infections worldwide and are an important reservoir of transferable antibiotic resistance genes. A group of experts advised that Enterococcus not be used as a probiotic in foods. 1 Bacillus infections linked to probiotic consumption include three reports 50-52 detailing seven cases of B subtilis bacteremia, septicemia, and cholangitis, all in patients with underlying disease.

In general, considerations for safety should include knowledge of history of safe use under the recommended route of administration (some non-oral applications have been proposed, e.g., intravaginal), the health status of the consumer, the frequency of association of species with infection, the likelihood of production of potentially deleterious metabolic end-products (including D-lactic acid), association with transferable antibiotic resistance, sensitivity to therapeutic antibiotics (for use if an infection does occur), and relation to species that produce hemolysins, mammalian toxins, or other virulence factors. The documentation of safety is an important reason why probiotics should be de ned strains, properly classi ed with regard to taxonomy, and properly named according to current conventions of bacterial nomenclature.

Probiotics hold much promise. However, the data supporting their use is emerging and has limitations. Some key areas for research include:

De nition of physiologically relevant and validated biomarkers useful for assessing the impact of probiotics on human health Follow-up of preliminary studies with con rmatory Validation of in vitro assays; tests using isogenic strains that differ only in one parameter (i.e., isolate a non-adhering mutant or non-bacteriocin-producing strain and determine if its physiologic effect is any different from parent strain) would be useful in this regard Epidemiologic study of probiotic effects, possible in populations with a signi cant penetration of probiotic use Some novel applications and research with probiotics have emerged recently. Patton et al. 53 genetically engineered a non-pathogenic strain of E coli to express galactosyl transferase derived from a Neisseria strain. This enzyme attached galactosyl residues that mimicked the receptor for shiga toxin onto the lipopolysaccharide core on the surface of the host E coli. The engineered E coli was capable of titrating the shiga toxin from a liquid medium. Mice treated with a lethal dose of shiga toxinproducing E coli survived the infection.

Another area in development takes advantage of the gut mucosal surface, a key site for development of immune response. Delivery of antigens to the gut mucosal surface via engineered lactococci and lactobacilli has been investigated. These live microbes would essentially serve as oral vaccines, with targets such as rotavirus, 54 foot-and-mouth disease virus, 55 and bovine coronavirus. 56 In another approach using live microbes, Steidler et al. 57 engineered a strain of Lactococcus lactis to secrete the anti-in ammatory cytokine interleukin-10. Intragastric administration of this bacterium caused a 50% reduction in colitis in a mouse model of chemically induced colitis, suggesting a new therapeutic approach to the treatment of in ammatory bowel diseases. Although these strategies require the use of recombinant genetic techniques, the successes of the approaches suggest great potential for the novel use of live microbes to improve health.

The ability to analyze bacterial communities has undergone tremendous advancement with the application of polymerase chain reaction technologies and appropriate DNA probes. One technique, terminal restriction fragment polymorphism (TRFP), is especially valuable for determining the impact probiotics have on intestinal microecology. 58 Instead of relying on culture techniques for determining perhaps six to ten large groups of bacteria in the intestine, this technique allows semiquantitative resolution of dozens of native bacterial species comprising 99.99% of the bacteria in a community. If applied to fecal or intestinal samples, the changes in bacterial communities detected by TRFP can be indicative of the impact of probiotics.

Lastly, as with virtually all branches of biology, genomic sequencing and subsequent functional genomic efforts will have a great impact. Knowledge of the genome of probiotic bacteria will greatly enhance efforts to elucidate mechanisms and improve or design strains for maximum physiologic effectiveness. Currently, the genomes of several lactobacilli are fully sequenced (although not all sequences are in the public domain) and additional species of both lactobacilli and bi dobacteria are in progress.

Much remains to be learned about the role of probiotics in human health. This is clearly an emerging area of science and one requiring con rmation of ef cacy and mechanisms of action in controlled studies. However, the cumulative information that does exist has begun to establish a credible hypothesis about the role of probiotics in enhancing human health. The ongoing multidisciplinary approach to exploration of this eld will be invaluable to this effort.

Evaluation of health and nutritional properties of probiotics in food including powder milk with live lactic acid bacteria

Colonization resistance

Gastrointestinal ora and its alterations in critical illness

Probiotics: growth promoting factors produced by microorganisms

Probiotics, the other half of the antibiotic story

Probiotics in man and animals

Probiotics: a general view

Demonstration of safety of probiotics-a review

Probiotics, prebiotics, and synbiotics-approaching a de nition

Probiotics in health and disease in the pediatric patient

Probiotic spectra of lactic acid bacteria (LAB)

Probiotics as biotherapeutic agents: present knowledge and future prospects

Role of probiotics in food hypersensitivity

Probiotics in primary prevention of atopic disease: a randomized placebo-controlled trial

Increased bacteria urease activity in faeces in juvenile chronic arthritis: evidence of altered intestinal micro ora?

Probiotics against allergy: data, doubts, and perspectives

Effect of long-term consumption of a probiotic bacterium, Lactobacillus rhamnosus GG, in milk on dental caries and caries risk in children

Protective role of probiotics and prebiotics in colon cancer

Lactic acid bacteria and cancer: mechanistic perspective

Effects of probiotic bacteria on diarrhea, lipid metabolism, and carcinogenesis: a review of papers published between 1988 and 1998

Probiotics and intestinal disease: a clinical perspective

Lactobacillus feeding reduces endotoxemia and severity of experimental alcoholic liver (disease)

Use of fermented foods to combat stunting and failure to thrive

Favourable effect of an acidi ed milk (LC-1) on Helicobacter pylori gastritis in man

A placebocontrolled study of the effect of sour milk on blood pressure in hypertensive subjects

Probiotics: effects on immunity

A review of the role of the gut micro ora in irritable bowel syndrome and the effects of probiotics

Probiotics and in ammatory bowel disease: from fads and fancy to facts and future

Reduction of axaluria after an oral course of lactic acid bacteria at high concentration

Probiotics-compensation for lactase insuf ciency

Effect of oral administration of freeze-dried Lactobacillus acidophilus on small bowel bacterial overgrowth in patients with end stage kidney disease: reducing uremic toxins and improving nutrition

Probiotics for urogenital health

Species-speci c identi cation of commercial probiotic strains

Lactobacillus sporogenes" is not a Lactobacillus probiotic

Incidence and virulence factors and antibiotic resistance among enterococci isolated from food

Enterococci at the crossroads of food safety?

Guidelines for the evaluation of probiotics in food

Safety of lactic acid bacteria and their occurrence in human clinical infections

Lactic acid bacteria and human clinical infection

Lactobacilli and bacteremia in southern Finland

D-lactic acidosis in short bowel syndrome

Liver abscess due to a Lactobacillus rhamnosus strain indistinguishable from L. rhamnosus strain GG

Lactobacillus endocarditis caused by a probiotic organism

Possible role of catheters in Saccharomyces boulardii fungemia

The trouble in tracing opportunistic pathogens: cholangitis due to Bacillus in a French hospital caused by a strain related to an Italian probiotic? Microbiol Ecol Health Dis

Recurrent septicemia in an immunocompromised patient due to probiotic strains of Bacillus subtilis

Nosocomial bacteremia caused by Bacillus species

A new biological agent for treatment of shiga toxigenic Escherichia coli infections and dysentery in humans

Bovine rotavirus nonstructural protein 4 produced by Lactococcus lactis is antigenic and immunogenic

The potential of Lactobacillus as a carrier for oral immunization: development and preliminary characterization of vector systems for targeted delivery of antigens

Heterologous protein secretion in Lactococcus lactis: a novel antigen delivery system

Treatment of muring colitis by Lactococcus lactis secreting interleukin-10

Terminal restriction fragment patterns: a tool for comparing microbial communities and assessing community dynamics