Investigations into quinolone and fluoroquinolone resistance in Salmonella enterica

Infections with Salmonella enterica usually present as a self-limiting diarrhoeal disease. Occasionally, antimicrobial intervention is required for invasive infection or patients with underlying disease or at the extremities of age. In adults the therapeutic agents of choice are the fluoroquinolones, especially ciprofloxacin, which is also used in addition to third generation cephalosporins in children. Fluoroquinolone resistance is rare and is mostly detected in strains associated with foreign travel. Nevertheless, reports from around the world describing treatment failures with these drugs in nalidixic acid-resistant isolates have been increasing. To determine the levels of fluoroquinolone resistance in Scotland from 1990-2000, this study examined a collection of 180 isolates of Salmonella enterica isolated from human, veterinary and environmental sources. These were characterised using the genotypic methods of Plasmid Profile Analysis and Pulsed Field Gel Electrophoresis and levels of resistance were determined for quinolone and fluoroquinolone antimicrobials by the agar dilution method. By exposing susceptible isolates to subinhibitory concentrations of fluoroquinolones, resistant mutants were selected. Amplification and sequencing of the topoisomerase genes gyrA and parC was performed on a number of these mutants in an attempt to characterize the mutations. The prevalence of the recently described plasmid-borne resistance qnr genes was determined for a selection of 53 strains of Salmonella enterica from years 1997-2007 susceptible to nalidixic acid (40mg/L) but resistant to ciprofloxacin (0.125mg/L) in addition to 17 strains of Salmonella enterica from years 1997-2007 resistant to ciprofloxacin (0.125mg/L) and cefotaxime (1mg/L). The determined resistance levels indicated that with the exception of the isolates deemed fully susceptible by breakpoint method, all other isolates examined were resistant to the quinolone nalidixic acid. Resistance to fluoroquinolones was rare during this time as has been previously reported. With the exception of a single mutation in the parC gene, mutations, if present were confined to gyrA. Although significant increases in minimum inhibitory concentrations (MICs) were observed between wild-type isolates and selected mutants, only single point mutations were characterised. This, and the absence of mutations in some mutants with raised MICs compared to their wild-type, may indicate additional mechanisms of resistance such as increased efflux or porin changes not investigated in this study. Thirty-four from a total of 70 strains investigated for the presence of qnr genes were shown to harbour qnrA, qnrB, or qnrS. Twelve serotypes were represented, 7 of which have not previously been shown to harbour these genes. Positive strains were from human, environmental and veterinary sources; 58% of the strains of human origin were from patients with a history of foreign travel. Plasmid-mediated quinolone resistance has recently been identified in isolates of S. enterica in a number of countries at low prevalence. This study of Scottish isolates has identified a higher prevalence of qnr than expected and a wider dissemination of qnr resistance genes among different serotypes.