Importance of the host specificity in the selection of probiotic bacteria

The gastrointestinal tract is a complex and dynamic ecosystem. Commensal microorganisms (C), which proliferate in the intestine from birth, are crucial for gut homeostasis while non commensal (NC) microorganisms are transient and enter the organism from the environment and foods. We studied comparatively the influence of oral administration of C and NC Lactobacillus fermentum and Lactobacilus acidophilus on the gut-associated lymphoid tissue (GALT) of conventional mice. To determine the importance of the selection of probiotic host-specificity bacteria with immunomodulating capacity, we examined the interaction with the gut by transmission electron microscopy and FITC-labelled bacteria. We compared the immuno-modulation capacities of C and NC strains by studying the number of IgA secreting cells and cytokine profile. No differences were found in the number of IgA+ cells; however, the pattern of cytokine response to C and NC bacteria was different. With regard to proinflammatory cytokine (IFN gamma and TNF alpha), we found that TNF alpha was mainly produced by NC bacteria, while C bacteria were able to elicit mainly IFN gamma. The regulatory cytokines (IL-10 and IL-4) were induced with different patterns for both C and NC strains. No differences in the pathway of internalization to the gut between C and NC were found. In summary, we determined that C and NC bacteria interact with the intestine in the same way; both C and NC bacteria were able to reinforce the surveillance of the gut mucosal immune system. The cytokine profile showed that C bacteria would be involved in the regulation of intestinal homeostasis rather than in the immune activation as the NC bacteria.