Probiotics are live microorganisms that are believed to exert a beneficial effect when administered in an adequate amount. The daily dose is usually 109 or 1010, which is low considering that there are up to 1014 bacteria in the colon. Although uncertain, it is believed that the beneficial activities of such low numbers are due to colonization by probiotics of the same gut compartment as pathogens. The exact mechanism of action of probiotics is unknown; however, in vitro and in vivo studies have shown that these organisms can alter gut immune function, directly affect the growth of other bacteria, or affect products of pathogenic microbial growth. Metabolites, cell wall components, and DNA of probiotics have been shown to interact with the gut immune system. This can result in antiinflammatory actions, changes in cytokine production, amplification of beta–defensins, changes in how antigens are presented to immune cells, or even promotion of survival and growth of intestinal epithelial cells. Probiotics can directly affect pathogenic bacteria by producing antimicrobial substances such as bacteriocins, produce deconjugated bile acids (which are more antimicrobial than bile acids), bind iron (which is an essential element for all bacteria except lactobacilli), prevent adhesion of pathogens, prevent invasion of pathogenic bacteria, or prevent bacteria from producing toxins. In vitro studies suggest there may be some anticancer effects, but this has not yet been confirmed in vivo. Probiotics are an area of intense research as an alternative for antibiotics and antiinflammatory drugs.

Commentary: The use of probiotics in veterinary medicine is increasing, yet there is still only limited information on their efficacy. A quick PubMed search identified several recently published studies on their use in treatment of gastrointestinal disease. Those studies were small and the results suggest probiotics may be beneficial; however, it cannot be emphasized enough that large randomized controlled studies are needed. This is an exciting area of research and probiotics should be investigated with the same scrutiny as any other drug.

Mechanisms of probiotic actions: A review. Oelschlaeger TA. INT J MED MICROBIOL 300:57-62, 2010.