Escherichia coli F-18 makes a streptomycin-treated mouse large intestine colonization factor when grown in nutrient broth containing glucose

Streptomycin-induced inflammation enhances Escherichia coli gut colonization through nitrate respiration

Treatment with streptomycin enhances the growth of human commensal Escherichia coli isolates in the mouse intestine, suggesting that the resident microbial community (microbiota) can inhibit the growth of invading microbes, a phenomenon known as “colonization resistance.” However, the precise mechanisms by which streptomycin treatment lowers colonization resistance remain obscure. Here we show that streptomycin treatment rendered mice more susceptible to the development of chemically induced colitis, raising the possibility that the antibiotic might lower colonization resistance by changing mucosal immune responses rather than by preventing microbe-microbe interactions. Investigation of the underlying mechanism revealed a mild inflammatory infiltrate in the cecal mucosa of streptomycin-treated mice, which was accompanied by elevated expression of Nos2, the gene that encodes inducible nitric oxide synthase. In turn, this inflammatory response enhanced the luminal growth of E. coli by nitrate respiration in a Nos2-dependent fashion. These data identify low-level intestinal inflammation as one of the factors responsible for the loss of resistance to E. coli colonization after streptomycin treatment.

Our intestine is host to a complex microbial community that confers benefits by educating the immune system and providing niche protection. Perturbation of intestinal communities by streptomycin treatment lowers “colonization resistance” through unknown mechanisms. Here we show that streptomycin increases the inflammatory tone of the intestinal mucosa, thereby making the bowel more susceptible to dextran sulfate sodium treatment and boosting the Nos2-dependent growth of commensal Escherichia coli by nitrate respiration. These data point to the generation of alternative electron acceptors as a by-product of the inflammatory host response as an important factor responsible for lowering resistance to colonization by facultative anaerobic bacteria such as E. coli.

Neither motility nor chemotaxis plays a role in the ability of Escherichia coli F-18 to colonize the streptomycin-treated mouse large intestine

Escherichia coli F-18, isolated from the feces of a healthy human in 1977, is an excellent colonizer of the streptomycin-treated mouse large intestine and displays normal motility and chemotaxis ability. A chemotaxis-defective derivative of E. coli F-18, E, coli F-18 CheA-, and a nonflagellated derivative, E. coli F-18 Fla-, were constructed. These strains were found to colonize the streptomycin-treated mouse large intestine as well as E. coli F-18 when mice were fed both E. coli F-18 and either the CheA- or Fla- derivative at high levels (10(10) CFU of each strain per mouse) or low levels (10(4) CFU of each strain per mouse). Furthermore, E. coli F-18 lost motility and chemotaxis ability when grown in colonic or cecal mucus in vitro despite retaining the ability to synthesize flagella. Thus, it appears that neither motility nor chemotaxis plays a role in the ability of E. coli F-18 to colonize because this strain becomes functionally nonmotile upon growth in the streptomycin-treated mouse large intestine.