ABSTRACT
Abstract The gut of monogastric animals is colonised in the distal regions (ileum and colon) by a complex bacterial community in which anaerobic bacteria predominate numerically. Analysis of the composition of this community (generally referred to as the gut microflora) by the use of nucleic acid-based methods has revealed that many of the bacterial inhabitants have not yet been cultivated in the laboratory. Despite this handicap, information about the overall impact of the bacterial community on the host has been obtained by comparing the characteristics of germfree and conventional animals. In studying host-microflora relationships, however, it is essential to work with bacterial species that establish and persist within (colonise) the gut ecosystem rather than species that are merely transient. Differentiating between autochthonous and allochthonous species is therefore critical in investigations aimed at revealing the bacterial traits that are essential for life in the gut. Members of the genus Lactobacillus are ideal model bacteria with which to carry out such investigations because they predominate in the proximal regions of the gut of mice, poultry and pigs. It is estimated that hundreds of autochthonous bacterial species reside in the gut, yet the antigenic load associated with their cells does not stimulate a marked inflammatory response in the gut mucosa. In contrast, the presence of gut pathogens results in stimulation of the innate and adaptive immune systems and the eventual destruction of the pathogenic cells. Investigations to resolve the question as to how the mucosal immune system differentiates gut microflora from pathogenic species suggest that Toll-like receptors, oral tolerance mechanisms, and the production of secretory IgA molecules that coat the cells of members of the gut microflora, are involved.
