Abrams, Gerald D. (The University of Michigan Medical School, Ann Arbor), and Jane E. Bishop. Effect of the normal microbial flora on the resistance of the small intestine to infection. J. Bacteriol. 92:1604-1608. 1966.-Mucosal structure in the small intestine is known to be influenced by the normal microbial flora. This suggests that mucosal resistance to invasion by enteric pathogens might also be affected by the flora. To assess this possibility, germ-free and conventional mice were challenged with Salmonella typhimurium, and both the growth of organisms within the intestinal lumen and the translocation to mesenteric lymph nodes were studied quantitatively. There were significantly more organisms 24 hr after intragastric challenge in the mesenteric nodes of germ-free animals than in those of conventional ones. However, since intraluminal growth in the intestine was also greater in germ-free animals, no conclusion could be drawn about mucosal resistance per se. Results were similar when the challenge was intraduodenal. However, when intestinal emptying was prevented by ileal ligation before challenge, both intraluminal growth and translocation of S. typhimurium were equal in the two groups of mice. It is concluded from these data, as well as from preliminary dye studies of intestinal motility, that the normal flora does not influence mucosal resistance directly, but may alter enteric infection by affecting intestinal emptying.