Background
The gut of animals, including humans, harbours complex communities of bacteria that are highly adapted to life in this site. As associates of animal hosts throughout evolution, these bacteria must have specialized molecular attributes that allow them to function, and hence persist in the gut environment. Nucleic acid-based methods of analysis have permitted the evaluation of these complex communities in terms of composition. Yet little is known of the functioning of bacterial cells in the gut (how they live there) and what differentiates these bacteria from those that inhabit terrestrial, aquatic, or plant-associated ecosystems (the basis of autochthony). Moreover, the ability of bacteria to persist in the gut is often strain specific but the molecular basis for this specificity is unknown. Lactobacilli are common inhabitants of gut ecosystems and can serve as model bacteria with which to investigate bacterial cell function in relation to life in the gut and autochthony.

L. reuteri 100-23 is a well-characterized rodent-specific strain that has been used in studies concerning the molecular basis of autochthony. Strain 100-23 forms a biofilm on the epithelial surface of the murine forestomach, and has been shown to participate in horizontal gene transfer in the gut. Furthermore, strain 100-23 is readily transformable at high levels by electroporation and can express heterologous genes, which are properties essential for post-genome sequence experimentation. Microbial ecological studies of strain 100-23 have been carried out using a unique mouse colony that we have maintained since the 1980s. The mice in this colony harbour a complex gut microbiota equivalent to their conventional counterparts except that lactobacilli, normally present in the murine gut, are absent. This Lactobacillus -free mouse colony is maintained in isolators by gnotobiotic technology and the animals provide a defined microbiological system with which to compare the colonization ability of lactobacilli as well as Lactobacillus -host interactions.

Taxonomic details
Lactobacillus reuteri (Kandler, O., Weiss, N. 1986. Regular nonsporing gram positive rods. In Sneath, D. H. A., Mair, N. C., Sharpe, M. E., Holt, J. H. eds. Bergey's manual of systematic bacteriology, volume 2, pp. 1208-1234. New York: Williams and Wilkins); Bacterium; Firmicutes .

Relevance to DOE mission
Lactobacillus reuteri strain 100-23 is a member of the complex bacterial community inhabiting the gastrointestinal tracts of mice. The study of this strain is therefore relevant to the "microbial consortia" mission.

Publications relating to L. reuteri strain 100-23
Heng, N. C. K., J. M. Bateup, D. M. Loach, X. Wu, H. F. Jenkinson, M. Morrison, and G. W. Tannock. 1999. Influence of different functional elements of plasmid pGT232 on maintenance of recombinant plasmids in Lactobacillus reuteri populations in vitro and in vivo. Appl. Environ. Microbiol . 65:5178-5185.

Heng, N. C. K., H. F. Jenkinson, and G. W. Tannock. 1997. Cloning and expression of an endo-1,3-1,4- b -glucanase gene from Bacillus macerans in Lactobacillus reuteri . Appl. Environ. Microbiol. 63:3336-3340.

McConnell, M.A., A. A. Mercer, and G. W. Tannock. 1991. Transfer of plasmid pAMß1 between members of the normal microflora inhabiting the murine digestive tract and modification of the plasmid in a Lactobacillus reuteri host.   Microb. Ecol.   Hlth. Dis. 4:343-35.

Tannock, G. W., C. Crichton, G. W. Welling, J. P. Koopman, and T. Midtvedt. 1988. Reconstitution of the gastrointestinal microflora of lactobacillus-free mice . Appl. Environ. Microbiol. 54:2971-2975.

Tannock, G. W., Ghazally, S., Walter, J., Loach, D., Brooks, H., Cook, G., Surette, M., Simmers, C., Bremer, P., Dal Bello, F., and Hertel, C. 2005. Ecological behavior of Lactobacillus reuteri 100-23 is affected by mutation of the luxS gene. Appl. Environ. Microbiol. 71:8419-8425.

Walter, J., Chagnaud, P., Tannock, G. W., Loach, D. M., Dal Bello, F., Jenkinson, H. F., Hammes, W. P., and Hertel, C. 2005. A high-molecular-mass surface protein (Lsp) and methionine sulfoxide reductase B (MsrB) contribute to the ecological performance of Lactobacillus reuteri in the gut of mice. Appl. Environ. Microbiol. 71:979-986 .

Walter, J., N. C. K. Heng, W. P. Hammes, D. M. Loach, G. W. Tannock, and C. Hertel. 2003. Identification of Lactobacillus reuteri genes specifically induced in the mouse gastrointestinal tract. Appl. Environ. Microbiol . 69:2044-2051.

Wesney, E., and G. W. Tannock. 1979. Association of rat, pig and fowl biotypes of lactobacilli with the stomach of gnotobiotic mice.   Microb. Ecol. 5:35-42.

Figure legend.
L. reuteri 100-23 cells adherent to squamous, keratinised epithelium in the mouse forestomach. Transmission electron micrograph of ruthenium red-stained preparation.