Fluoroquinolone Susceptibility Testing of Salmonella enterica: Detection of Acquired Resistance and Selection of Zone Diameter Breakpoints for Levofloxacin and Ofloxacin

Journal of clinical microbiology (Impact Factor: 3.99). 01/2014; 52(3). DOI: 10.1128/JCM.02679-13
Source: PubMed


Fluoroquinolones (e.g. ciprofloxacin) have become a mainstay for treating severe Salmonella infections in adults. Fluoroquinolone resistance in Salmonella is mostly due to mutations in the topoisomerase genes, but plasmid-mediated quinolone resistance (PMQR) mechanisms have also been described. In 2012, the Clinical and Laboratory Standards Institute (CLSI) revised the ciprofloxacin interpretive criteria (breakpoints) for disk diffusion and minimum inhibitory concentration (MIC) test methods for Salmonella. In 2013, CLSI published MIC breakpoints for Salmonella to levofloxacin and ofloxacin, but breakpoints for assigning disk diffusion results to susceptible (S), intermediate (I), or resistant (R) categories are still needed. In this study, MICs and inhibition zone diameters for nalidixic acid, ciprofloxacin, levofloxacin and ofloxacin were determined for 100 clinical isolates of non-Typhi Salmonella with or without resistance mechanisms. We confirmed that the new levofloxacin MIC breakpoints resulted in the highest category agreement (94%) when plotted against ciprofloxacin MICs and that the new ofloxacin MIC breakpoints resulted in 92% category agreement between ofloxacin and ciprofloxacin. By applying the new MIC breakpoints in MIC zone scattergrams for levofloxacin and ofloxacin, the following disk diffusion breakpoints generated the least number of errors: S≥28mm, I=19-27mm and R≤18mm for levofloxacin and S≥25mm, I=16-24mm and R≤15mm for ofloxacin. Neither the levofloxacin nor the ofloxacin disk yielded a good separation of isolates with and without resistance mechanisms. Further studies will be needed to develop a disk diffusion assay that efficiently will detect all isolates with acquired resistance to fluoroquinolones.

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