What is it?
A widely used definition of probiotics is “Live microbes that, when administered in adequate amounts, confer a health benefit on the host.” The basic idea is that all animals have a great number and variety of microorganisms living in and on their bodies, and that many of these microbes are commensuals, organisms that benefit from living on the host and that in return convey some benefit to the animal. Such benefits include inhibiting disease-causing organisms from colonizing the host, producing nutrients for the host, and possibly participating in the normal development and regulation of the immune system. Animals raised in laboratories without any colonizing microbes, and animals with abnormal numbers and types of microbes, often experience disease, so it is believed that an appropriate microbial ecology is important for normal health and function.
Though many organisms have been used, most probiotic products contain bacteria, often from the Bifidobacteria or Lactobacillus groups, or Saccharomyces (brewer’s yeast). While some of the bacteria suggested as probiotics are natural parts of the gut ecology, many strains are not, and brewer’s yeast of course does not normally live in or on animals.
To be potentially useful, a probiotic organism must not normally be capable of causing disease (though many are opportunistic pathogens, able to cause disease under certain circumstances). It must be able to survive the acidic environment of the stomach and at least temporarily survive in the intestines, where most normal bacterial flora is found. Typically, probiotic organisms do not colonize individual animals well and must be taken continuously to be present in any number. Finally, a probiotic preparation must contain enough live organisms in an appropriate delivery vehicle to enable colonization of the intestines.
While the scientific principles behind probiotics are sound, there are a number of problems with the practical use of such preparations. Every individual animal has a complex ecology of microbes colonizing it, and the specific type and number depend on species, environment, community, and individual genetics. Even for humans, as many as 80% of the organisms we harbor have yet to be identified or studied, and much less is know about the microbial ecology of companion animals. So trying to manipulate this ecology and the health of the individual by adding a few bacteria, often of a strain not naturally found in the individual, may not make sense. Also, numerous tests have shown that commercial probiotic products often do not contain the organism they claim, they are sometimes contaminated with undesirable organisms, and they may have too few microbes or non-living microbes in them. Without any formal government regulation or monitoring it is difficult to know whether individual products are safe or effective.
Does It Work?
Much laboratory research has been done on many different potential probiotics, and there is good evidence to support the principle that such organisms can influence health and disease. Clinical studies in humans, however, have often produced mixed or disappointing results.
Based on clinical trials in humans, the balance of the evidence supports the effectiveness of some probiotic preparations for prevention and treatment of diarrhea caused by antibiotics, infection, or radiation treatment and for treatment of lactose intolerance. There is also good evidence for their use in irritable bowel syndrome and some other inflammatory gastrointestinal tract diseases. The evidence is inconclusive for most uses of probiotics, including in preventing or treating traveller’s diarrhea, urinary tract infections, fatty liver syndrome, and Helicobacter infections responsible for stomach ulcers. Research evidence does not support the use of probiotics in Crohn’s disease, ulcerative colitis, asthma, allergic eczema, or rheumatoid arthritis.
Further research may find other potential benefits from probiotics, but because the composition and function of the normal microbial ecology is so poorly understood, it is not yet clear which conditions may benefit from the use of which organisms. At this time clinical studies are largely based on haphazard selection of products and conditions rather than on strong, plausible scientific hypotheses.
While there has been extensive research on the effect of probitics in livestock, there are very few published studies on their use in companion animals. Studies in horses have shown poor colonization of human-derived organisms. Several trials have failed to show any benefit of probiotics on the shedding of disease-causing organisms such as Salmonella. One small study suggested some benefit from brewer’s yeast in treatment of diarrhea, although the duration o hospitalization and the overall outcome was not affected by the treatment.
In dogs, human-derived organisms are also ineffective at colonizing the intestines. Dog-derived microbes do seem to reach and survive in the intestines, but no clinical for these has yet been demonstrated. And in cats, there are no published research studies on the use of probiotics for prevention or treatment of disease.
The marketing literature accompanying veterinary commercial products often claims safety and efficacy based on company studies or other unpublished research. While these findings are interesting, it has been clearly shown that industry-sponsored research is more likely than independant research to find results in favor of the product being tested. In addition, such marketing materials deliberately avoid any research that does not favor the product, so while useful such materials cannot be relied upon exclusively to decide if a product is safe or effective.
Is It Safe?
There are very few reports of disease or harm caused by probiotics. Individuals with ineffective immune systems (such as the elderly, pregnant women, people with HIV or receiving chemotherapy) are at increased risk of disease from all microbes, and there have been cases of severe illness caused by bacterial or yeast infections from probiotic products given to the patient or to others in their hospital unit. Very sick or immunocompromised individuals should not use these products. Interestingly, however, newborns and infants have often been subjects in studies of probiotics, and the majority show no adverse effects.
Some probiotic organisms are closely related to disease-causing microbes. Enterococcus faecium is a widely available veterinary probiotic related to other members of the Enteroccocus group, which commonly cause serious infections. Such organisms have the potential to cause illness or to share genes for antibiotic resistance with other, disease-causing members of the same group.
Some risks from probiotics are related to their intended functions, to interact with the microbial flora and affect the immune system. In horses, one study of a probiotic given to prevent diarrhea found the product actually caused diarrheal illness is neonatal foals. A study in dogs showed the probitoic Enterococcus faecium potentially improved the attachment to the intestines of the disease-causing organism Campylobacter. And other studies have shown some inflammatory conditions, such as allergic rhinitis, can increase in people given probiotics for another condition. So while the balance of the evidence is that these products are quite safe, because they do have real effects on the body they can potentially have unintended or undesirable effects.
Ø A normal microbial flora is beneficial, and perturbations in the normal flora are associated with disease, so the principle that manipulating the microbial ecology can affect health is reasonable.
Ø The normal microbial flora is complex and poorly understood, so how to appropriately manipulate it to achieve health benefits is not yet clear.
Ø Clinical studies in humans are mixed, showing benefits from some probiotic products for some conditions, no benefit in other cases, and inconclusive results for many products and conditions.
Ø There is little reliable research in companion animals regarding the safety or efficacy of probiotic products.
Ø The risks of probiotics are probably very low. Individuals with compromised immune systems are at greatest risk and should not be exposed to probiotics. There is some limited potential for these products to cause disease even in healthy individuals.
References and More Information
The Cochrane Collaboration, The Cochrane Reviews, a searchable database of systematic reviews of the human medical literature at http://www.cochrane.org/reviews/
Crislip, M. Probiotics. Science-Based Medicine Blog. http://www.sciencebasedmedicine.org/?p=344#more-344
FAO/WHO. Guidelines for the evaluation of probiotics in food. http://www.who.int/foodsafety/fs_management/en/probiotic_guidelines.pdf
Rice, L.B., et al. Transferable, Plasmid-Mediated VanB-Type Glycopeptide Resistance in Enterococcus faecium. Antimicorbial Agents and Chemotherapy 1998 April; 42(4): 963–964
Rinkinen, M., et al. Interaction between probiotic lactic acid bacteria and canine enteric pathogens: a risk factor for intestinal Enterococcus faecium colonization? Vet Microbiol 92: 111-119
Versalovic, J., Wilson, M., Eds. Therapeutic Microbiology: Probiotics and Related Strategies, ASM Press, Washington, DC, 2008
Weese, J.S. Microbiologic evaluation of commercial probiotics. JAVMA 2002; 220(6): 794-797
Weese, J.S., Rousseau, J. Evaluation of Lacotbacilluc pentosus WE7 for prevention of diarrhea in neonatal foals. JAVMA 2005; 226(12): 2031-2034
Wynn, S.G. Probiotics in veterinary practice. JAVMA 2009; 234(5): 606-613
© Brennen McKenzie, 2008