Extended-spectrum β-lactamase-producing Escherichia coli contributes to the survival of cefotaxime-susceptible E. coli under high concentrations of cefotaxime by acquisition of increased AmpC expression.

Extended-spectrum β-lactamase-producing Escherichia coli (ESBL-E) are becoming increasingly widespread in Vietnam. Antibiotics are detected in many Vietnamese foods; however, the effect of ESBL-E and antibiotic consumption on intestinal bacteria has not been studied sufficiently. Here, we investigated the effect of oral administration of ESBL-E (TB19) and cefotaxime on luminescence-emitting cefotaxime-sensitive E. coli (X14). Mice were given water containing TB19 and then received three injections of 1.0 × 108 CFU of X14 harboring a luciferase gene. The mice were administered 100 μg of cefotaxime and luminescent bacteria were monitored over 24 h, following which luminescent bacteria were isolated from mouse feces. Luminescence continued to be detected in mice administered TB19 24 h after cefotaxime ingestion. Fecal analysis revealed two types of luminescent colonies: cefoxitin-resistant E. coli (X14-R) and Pseudomonas aeruginosa. Pulse-field gel electrophoresis confirmed that X14-R was a clonal strain of X14, suggesting that X14 survived using ESBLs originating from TB19 and acquired cefoxitin resistance due to cefotaxime consumption. Moreover, in vitro analysis of X14 indicated that expression of the ampC gene was upregulated by cefotaxime. Overall, ESBL-E and cefotaxime promoted the expansion of cefoxitin-resistant E. coli in the absence of plasmid-mediated gene transfer.