Multiple regulatory pathways associated with high-level ciprofloxacin and multidrug resistance in Salmonella enterica serovar enteritidis: involvement of RamA and other global regulators

Abstract

Mechanisms of antibiotic resistance were examined in nalidixic acid-resistant Salmonella enterica serovar Enteritidis field isolates displaying decreased susceptibility to ciprofloxacin and in in vitro-derived ciprofloxacin-resistant mutants (104-cip and 5408-cip). All field isolates harbored a single gyrA mutation (D87Y). Deletion of acrB and complementation with wild-type gyrA increased quinolone susceptibility. Selection for ciprofloxacin resistance was associated with the development of an additional gyrA (S83F) mutation in 104-cip, novel gyrB (E466D) and parE (V461G) mutations in 5408-cip, overexpression of acrB and decreased susceptibility to nonquinolone antibiotics in both mutants, and decreased OmpF production and altered lipopolysaccharide in 104-cip. Complementation of mutated gyrA and gyrB with wild-type alleles restored susceptibility to quinolones in 104-cip and significantly decreased the ciprofloxacin MIC in 5408-cip. Complementation of parE had no effect on quinolone MICs. Deletion of acrB restored susceptibility to ciprofloxacin and other antibiotics tested. Both soxS and marA were overexpressed in 104-cip, and ramA was overexpressed in 5408-cip. Inactivation of each of these global regulators lowered ciprofloxacin MICs, decreased expression of acrB, and restored susceptibility to other antibiotics. Mutations were found in soxR (R20H) and in soxS (E52K) in 104-cip and in ramR (G25A) in 5408-cip. In conclusion, both efflux activity and a single gyrA mutation contribute to nalidixic acid resistance and reduced ciprofloxacin sensitivity. Ciprofloxacin resistance and decreased susceptibility to multiple antibiotics can result from different genetic events leading to development of target gene mutations, increased efflux activity resulting from differential expression of global regulators associated with mutations in their regulatory genes, and possible altered membrane permeability.

FIG. 1.

Accumulation of ciprofloxacin in in vitro-selected ciprofloxacin-resistant serovar Enteritidis isolates (104-cip and 5408-cip) and their isogenic parent strains (104 and 5408) in the presence and absence of PAβN (80 μg/ml). Ciprofloxacin (10 μg/ml) was added to each bacterial suspension at time zero. PAβN was added at 6 min as indicated by the arrow. Each value represents the mean ± standard error of the mean of three or four separate experiments.

FIG. 2.

The detection of porins was carried out using the polyclonal antibodies directed against denatured OmpF porin (A) or the F4 polyclonal antibody directed against the L3 internal loop of E. coli porins (B). Lanes 1, 104; lanes 2, 104-cip; lanes 3, 5408; lanes 4, 5408-cip, lanes 5, NCTC 13349. Arrows indicate the migration of F and D porins, respectively.