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Members of the genus Lactobacillus are common inhabitants of the gut, yet little is known about the traits that contribute to their ecological performance in gastrointestinal ecosystems. Lactobacillus reuteri 100-23 persists in the gut of the reconstituted Lactobacillus-free mouse after a single oral inoculation. Recently, three genes of this strain that were specifically induced (in vivo induced) in the murine gut were identified (38). We report here the detection of a gene of L. reuteri 100-23 that encodes a high-molecular-mass surface protein (Lsp) that shows homology to proteins involved in the adherence of other bacteria to epithelial cells and in biofilm formation. The three in vivo-induced genes and lsp of L. reuteri 100-23 were inactivated by insertional mutagenesis in order to study their biological importance in the murine gastrointestinal tract. Competition experiments showed that mutation of lsp and a gene encoding methionine sulfoxide reductase (MsrB) reduced ecological performance. Mutation of lsp impaired the adherence of the bacteria to the epithelium of the mouse forestomach and altered colonization dynamics. Homologues of lsp and msrB are present in the genomes of several strains of Lactobacillus and may play an important role in the maintenance of these bacteria in gut ecosystems.The guts of mammals are colonized by a complex collection of microorganisms (the gut microbiota) that influence biochemical, physiological, immunological, and nonspecific disease resistance characteristics of the host (10). The bacterial composition of this ecosystem has been shown to be remarkably stable over time (43). Members of the gut microbiota have presumably coevolved with their host species and must possess traits that enable them to establish and maintain themselves in a lotic (flowing) and competitive environment. On the other hand, there are microbes present in the gastrointestinal tract that have a transient existence (allochthonous components), and this situation raises questions about what factors distinguish resident members of the microbiota from allochthonous members (27).Lactobacillus reuteri strain 100-23 is a true (autochthonous) resident of the murine gut because it adheres to the nonsecretory epithelium of the forestomach and persists at constant population levels in particular regions of the gut throughout the life of the murine host (13, 39).We maintain a colony of mice whose gut microbiota is devoid of lactobacilli but is otherwise equivalent to that of conventional mice (32). These reconstituted Lactobacillus-free (RLF) mice permit the study of Lactobacillus traits against a standardized bacteriological background. L. reuteri 100-23 and the RLF mouse gut therefore provide an appropriate model system with which to study the molecular basis of Lactobacillus autochthony.While L. reuteri 100-23 inhabits the murine gut, genes encoding methionine sulfoxide reductase (msrB), xylose isomerase (xylA), and a protein with low similarity to methionine synthase II (met) are specifically induced (38). Add...

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