Use of Probiotics to Combat Gastrointestinal Infections

Probiotics have been shown to be useful in the treatment of a variety of gastrointestinal disorders, and the details are presented in Table 4. A number of these disorders have a significant inflammatory component in the small and/or large intestine and there is a growing body of research to suggest that probiotic bacteria may be useful particularly in many of these pediatric gastrointestinal conditions. Specific strains of Lactobacillus rhamnosus, Lb. reuteri, Lb. plantarum, Bifidobacterium lactis, and Saccharomyces cerevisiae (boulardii) have all been extensively studied. Probiotics can reduce the duration and severity of rotaviral enteritis, as well as decrease the risk of antibiotic-associated diarrhea in children and Clostridium difficile diarrhea in adults. Prevention of viral diarrhea in day-care centers as well as traveler's diarrhea has been demonstrated with some probiotics, although not all are equally effective (67). Small bowel bacterial overgrowth conditions may respond to probiotic use. How the probiotic bacteria counteract the inflammatory process by enhancing the degradation of external antigens, reducing the secretion of inflammatory mediators and maintaining the healthy gut microbiota by exclusion of pathogens is schematically shown in Figure 1.

Possible Mode of Action of Probiotics in Reducing the Duration of Diarrhea Several potential mechanisms have been proposed for how probiotics reduce the duration of rotavirus diarrhea, but none have been proven and each theory has its limitations. The first is competitive blockage of receptor sites (69) in which probiotics bind to receptors, thereby preventing adhesion and invasion of the virus. This concept might be plausible if there was evidence for specific receptor competition. In most cases, by the time a probiotic is ingested, the patient will already have had diarrhea for possibly 12 hours. By this time, the virus has infected mature enterocytes in the mid- and upper region of the small intestinal villi. The virus and/or its enterotoxin, NSP4, will then have disturbed fluid and electrolyte transport, thereby lowering fluid and glucose absorption. The toxin could have then potentially activated secretory reflexes, causing loss offluids from secretory epithelia, resulting in diarrhea (70). At best, subsequent competitive exclusion of viruses would only be effective for attachment of progeny, and it is not known whether such inhibition would reduce diarrhea. If probiotic organisms somehow competed with the toxin or peptides released from villous endocrine cells, it is feasible that the cascade that leads to diarrhea could be prevented.

The second potential mechanism may be that the immune response is enhanced by probiotics, leading to the observed clinical effect (45). This is supported by the protective effect which local immunoglobulin A (IgA) antibodies appear to confer against rotavirus (71). However, a problem with this theory is given that diarrhea appears to cease within 1 to 3 days in patients who would otherwise suffer for 4 to 6 days; the probiotics would need

Table 4 Examples of the Effects of Probiotics on Microbial Infections

Disorder

Subject

Probiotics

Effect

Reference

Infantile

Human

Lactobacillus

Reduced duration of

(43-47)

diarrhea

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