Analysis of plasmid-mediated quinolone and oxyimino-cephalosporin resistance mechanisms in Uruguayan Salmonella enterica isolates from 2011-2013

This study characterised the mechanisms of fluoroquinolone and oxyimino-cephalosporin resistance in human Salmonella enterica isolates in Uruguay. Salmonella enterica isolates were collected from 2011-2013 and were selected based on non-susceptibility to ciprofloxacin and/or oxyimino-cephalosporins. The disk diffusion assay was performed for various antibiotics, and the ciprofloxacin minimum inhibitory concentration (MIC) was determined following CLSI guidelines. Genetic relatedness was determined following PulseNet protocols. Extended-spectrum β-lactamases, ampC alleles and plasmid-mediated quinolone resistance were characterised by PCR and sequencing. Plasmid analyses were carried out by conjugation or transformation assays, and plasmid-encoded genes were identified by PCR. Mutations in the quinolone resistance-determining region of gyrases were sought by PCR and sequencing. Among 579 isolates, 105 (18.4%) ciprofloxacin-non-susceptible (CIP-NS) isolates, 9 (1.6%) oxyimino-cephalosporin-resistant isolates and 2 (0.3%) isolates resistant to both antibiotic families were detected. Thirteen isolates carried qnrB alleles (twelve qnrB19 and one qnrB2), four carried blaCTX-M-8, two blaCTX-M-14, two blaSHV-2 and three blaCMY-2-like genes. No correlation was found between mutations in gyrases and ciprofloxacin MICs. Several co-circulating clones of S. enterica ssp. enterica serovar Typhimurium were detected; conversely, S. enterica ssp. enterica serovar Enteritidis corresponded mainly to a single circulating clone. Nine (75%) of twelve of CIP-NS extraintestinal isolates shared the same pulsotype with intestinal isolates. During the study period, the frequency of CIP-NS isolates increased, albeit with ciprofloxacin MICs of 0.125-0.5 mg/L. Detection of the same quinolone-resistant clones recovered both from intestinal and extraintestinal samples highlights the significance of epidemiological surveillance of antibiotic susceptibility for every human Salmonella isolate.