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Different Escherichia coli strains generally have the same metabolic capacity for growth on sugars in vitro, but they appear to use different sugars in the streptomycin-treated mouse intestine (Fabich et al., Infect. Immun. 76:1143-1152, 2008). Here, mice were precolonized with any of three human commensal strains ( E. coli MG1655, E. coli HS, or E. coli Nissle 1917) and 10 days later were fed 10 5 CFU of the same strains. While each precolonized strain nearly eliminated its isogenic strain, confirming that colonization resistance can be modeled in mice, each allowed growth of the other commensal strains to higher numbers, consistent with different commensal E. coli strains using different nutrients in the intestine. Mice were also precolonized with any of five commensal E. coli strains for 10 days and then were fed 10 5 CFU of E. coli EDL933, an O157:H7 pathogen. E. coli Nissle 1917 and E. coli EFC1 limited growth of E. coli EDL933 in the intestine (10 3 to 10 4 CFU/gram of feces), whereas E. coli MG1655, E. coli HS, and E. coli EFC2 allowed growth to higher numbers (10 6 to 10 7 CFU/gram of feces). Importantly, when E. coli EDL933 was fed to mice previously co-colonized with three E. coli strains (MG1655, HS, and Nissle 1917), it was eliminated from the intestine (E. coli strains can provide a barrier to infection and suggest that it may be possible to construct E. coli probiotic strains that prevent growth of pathogenic E. coli strains in the intestine.

Previously we showed that the tricarboxylic acid (TCA) cycle operates as a full cycle during Salmonella enterica serovar Typhimurium SR-11 peroral infection of BALB/c mice (M. Tchawa Yimga et al., Infect. Immun. 74:1130-1140, 2006). The evidence was that a Δ sucCD mutant (succinyl coenzyme A [succinyl-CoA] synthetase), which prevents the conversion of succinyl-CoA to succinate, and a Δ sdhCDA mutant (succinate dehydrogenase), which blocks the conversion of succinate to fumarate, were both attenuated, whereas an SR-11 Δ aspA mutant (aspartase) and an SR-11 Δ frdABCD mutant (fumarate reductase), deficient in the ability to run the reductive branch of the TCA cycle, were fully virulent. In the present study, evidence is presented that a serovar Typhimurium SR-11 Δ frdABCD Δ sdhCDA double mutant is avirulent in BALB/c mice and protective against subsequent infection with the virulent serovar Typhimurium SR-11 wild-type strain via the peroral route and is highly attenuated via the intraperitoneal route. These results suggest that fumarate reductase, which normally runs in the reductive pathway in the opposite direction of succinate dehydrogenase, can replace it during infection by running in the same direction as succinate dehydrogenase in order to run a full TCA cycle in an SR-11 Δ sdhCDA mutant. The data also suggest that the conversion of succinate to fumarate plays a key role in serovar Typhimurium virulence. Moreover, the data raise the possibility that S. enterica Δ frdABCD Δ sdhCDA double mutants and Δ frdABCD Δ sdhCDA double mutants of other intracellular bacterial pathogens with complete TCA cycles may prove to be effective live vaccine strains for animals and humans.

Previously, we reported that the mouse intestine selected mutants of Escherichia coli MG1655 that have improved colonizing ability (M. P. Leatham et al., Infect. Immun. 73: 8039-8049, 2005). These mutants grew 10 to 20% faster than their parent in mouse cecal mucus in vitro and 15 to 30% faster on several sugars found in the mouse intestine. The mutants were nonmotile and had deletions of various lengths beginning immediately downstream of an IS 1 element located within the regulatory region of the flhDC operon, which encodes the master regulator of flagellum biosynthesis, FlhD 4 C 2 . Here we show that during intestinal colonization by wild-type E. coli strain MG1655, 45 to 50% of the cells became nonmotile by day 3 after feeding of the strain to mice and between 80 and 90% of the cells were nonmotile by day 15 after feeding. Ten nonmotile mutants isolated from mice were sequenced, and all were found to have flhDC deletions of various lengths. Despite this strong selection, 10 to 20% of the E. coli MG1655 cells remained motile over a 15-day period, suggesting that there is an as-yet-undefined intestinal niche in which motility is an advantage. The deletions appear to be selected in the intestine for two reasons. First, genes unrelated to motility that are normally either directly or indirectly repressed by FlhD 4 C 2 but can contribute to maximum colonizing ability are released from repression. Second, energy normally used to synthesize flagella and turn the flagellar motor is redirected to growth.