Mechanisms of disease: the hygiene hypothesis revisited.

In industrialized countries the incidence of diseases caused by immune dysregulation has risen. Epidemiologic studies initially suggested this was connected to a reduction in the incidence of infectious diseases; however, an association with defects in immunoregulation is now being recognized. Effector T(H)1 and T(H)2 cells are controlled by specialized subsets of regulatory T cells. Some pathogens can induce regulatory cells to evade immune elimination, but regulatory pathways are homeostatic and mainly triggered by harmless microorganisms. Helminths, saprophytic mycobacteria, bifidobacteria and lactobacilli, which induce immunoregulatory mechanisms in the host, ameliorate aberrant immune responses in the setting of allergy and inflammatory bowel disease. These organisms cause little, if any, harm, and have been part of human microecology for millennia; however, they are now less frequent or even absent in the human environment of westernized societies. Deficient exposure to these 'old friends' might explain the increase in immunodysregulatory disorders. The use of probiotics, prebiotics, helminths or microbe-derived immunoregulatory vaccines might, therefore, become a valuable approach to disease prevention.

Modulation of the microbial ecology of the human colon by probiotics, prebiotics and synbiotics to enhance human health: an overview of enabling science and potential applications.

The application of probiotics and prebiotics to the manipulation of the microbial ecology of the human colon has recently seen many scientific advances. The sequencing of probiotic genomes is providing a wealth of new information on the biology of these microorganisms. In addition, we are learning more about the interactions of probiotics with human cells and with pathogenic bacteria. An alternative means of modulating the colonic microbial community is by the use of prebiotic oligosaccharides. Increasing knowledge of the metabolism of prebiotics by probiotics is allowing us to consider specifically targeting such dietary intervention tools at specific population groups and specific disease states.

Probiotics to enhance anti-infective defences in the gastrointestinal tract.

Several clinical studies have demonstrated the therapeutic and/or prophylactic efficacy of specific probiotics against acute viral gastroenteritis and antibiotic-associated diarrhoea (including Clostridium difficile infection). Emerging evidence also suggests beneficial effects against Helicobacter pylori infection. The evidence of efficacy against traveller's diarrhoea remains, however, inconclusive. The precise mechanisms by which probiotics potentiate host gastrointestinal defences and mediate protection are not fully known. There is evidence to suggest, however, that probiotics might contribute to host defence by reinforcing non-immunological defences and stimulating both specific and non-specific host immune responses. Little is known about the relative importance of the probiotic-stimulated mechanisms in host protection. This review summarises the evidence for the anti-infective effects of probiotics and discusses the effect of orally delivered probiotics on non-immunological and immunological defence mechanisms in the host, especially in the gastrointestinal tract.

Immune protection mediated by the probiotic Lactobacillus rhamnosus HN001 (DR20) against Escherichia coli O157:H7 infection in mice.

This study investigated the protective effects of feeding the immunoenhancing probiotic Lactobacillus rhamnosus HN001 against Escherichia coli O157:H7 infection in murine (BALB/c and C57BL/6 mice) challenge infection models. Mice were fed milk-based diets supplemented with L. rhamnosus HN001 (3 x 10(8) cfu g(-1)) for 7 days prior to and following oral challenge with E. coli O157:H7. Morbidity and feed intake were measured for 1 week following challenge; pathogen translocation to spleen, liver and blood, and humoral and cellular immunological responses (specific antibody and phagocytosis) were measured in a sub-sample of ostensibly healthy animals 1 week post-challenge. Results showed that, after challenge, L. rhamnosus HN001-fed mice exhibited lower cumulative morbidity and bacterial translocation rates, compared to non-probiotic-fed control mice. Significantly higher intestinal anti-E. coli IgA responses and blood leucocyte phagocytic activity were recorded among probiotic-fed mice compared to controls. These results demonstrate that feeding the probiotic L. rhamnosus HN001 to mice can reduce the severity of E. coli O157:H7 infection, and suggest that this reduction may be associated with enhanced humoral and cellular immune responses.

Dietary intake of Lactobacillus rhamnosus HNOO1 enhances production of both Th1 and Th2 cytokines in antigen-primed mice.

Probiotic lactobacilli have been proposed as a potential oral bacteriotherapeutic means of modulating immune phenotype expression in vivo, via their ability to promote cytokine production. This study investigated the ability of a known interferon (IFN)gamma-promoting probiotic (Lactobacillus rhamnosus HNOOI) to modulate cytokine production in mice expressing an on-going Th2-type immune response. BALB/c mice were primed to ovalbumin in alum adjuvant to invoke antigen-specific Th2 cytokine-secreting cell populations. Mice that were fed Lb. rhamnosus HN001 during antigen sensitization produced higher levels of lymphocyte-derived IFNgamma, but also interleukin (IL)-4 and IL-5, in comparison to control animals. Although HN001 was additionally shown to induce pro-IFNgamma monokine (IL-12, IL-18) secretion in macrophages in vitro, its ability to invoke mixed lymphocyte cytokine production during an on-going Th2-type immune response in vivo suggests that this probiotic is a general immunostimulatory agent, in contrast to the pro-Th1/anti-Th2 immunoregulation reported for some strains of IFNgamma-promoting lactobacilli.