Enhanced in vivo fitness of fluoroquinolone-resistant Campylobacter jejuni in the absence of antibiotic selection pressure

Abstract

Campylobacter jejuni, a major foodborne human pathogen, has become increasingly resistant to fluoroquinolone (FQ) antimicrobials. By using clonally related isolates and genetically defined mutants, we determined the fitness of FQ-resistant Campylobacter in chicken (a natural host and a major reservoir for C. jejuni) in the absence of antibiotic selection pressure. When monoinoculated into the host, FQ-resistant and FQ-susceptible Campylobacter displayed similar levels of colonization and persistence in the absence of FQ antimicrobials. The prolonged colonization in chickens did not result in loss of the FQ resistance and the resistance-conferring point mutation (C257 --> T) in the gyrA gene. Strikingly, when coinoculated into chickens, the FQ-resistant Campylobacter isolates outcompeted the majority of the FQ-susceptible strains, indicating that the resistant Campylobacter was biologically fit in the chicken host. The fitness advantage was not due to compensatory mutations in the genes targeted by FQ and was linked directly to the single point mutation in gyrA, which confers on Campylobacter a high-level resistance to FQ antimicrobials. In certain genetic backgrounds, the same point mutation entailed a biological cost on Campylobacter, as evidenced by its inability to compete with the FQ-susceptible Campylobacter. These findings provide a previously undescribed demonstration of the profound effect of a resistance-conferring point mutation in gyrA on the fitness of a major foodborne pathogen in its natural host and suggest that the rapid emergence of FQ-resistant Campylobacter on a worldwide scale may be attributable partly to the enhanced fitness of the FQ-resistant isolates.

Fig. 1.

Colonization and persistence of FQ^R Campylobacter in the chicken host. (A) The number of chickens colonized with Campylobacter in each group (n = 18). Each data point represents the percentage of chickens colonized at each sampling time. The detection limit of the plating method is 100 cfu per g of fecal contents. (B) The resistance rates of Campylobacter isolated from each group. Ten to 15 isolates from each group at each sampling time were randomly selected and tested with ciprofloxacin Etest strips. MICs of ≥4.0 μg/ml are regarded as resistant.

Fig. 2.

Pairwise competition between FQ^R and FQ^S Campylobacter in the chicken host. Each data point represents the mean CI ± SE. Some SEs are smaller than the symbols. Seven to 10 birds were used in each group.