P1002 Paper Poster Session V Assaying and explaining multidrug resistance in Gram-negative bacteria Evaluation of various quinolone antimicrobial disks to screen for fluoroquinolone resistance in Salmonella spp

I. Akyar1,2, A. Karatas2,3, O. Karatuna2,4 1Acibadem University- School of Medicine, Dept of Microbiology, Istanbul, Turkey 2Acibadem Labmed Clinical Laboratories, Istanbul, Turkey 3Acibadem University- School of Medicine-Dept of Microbiology, Istanbul, Turkey

Objectives: It’s important to differentiate Salmonella isolates with low-level fluoroquinolone (FQ) resistance (i.e. MIC >0.06 mg/L) from wild type isolates because there is clinical evidence that these isolates respond poorly to FQ treatment in systemic infections. EUCAST recently recommended the use of 5 μg disk to screen for all currently defined FQ resistance mechanisms in Salmonella spp. The objective of this study was to investigate the correlation between ciprofloxacin MIC values and zone diameters of various quinolone antimicrobial disks to evaluate the performance of pefloxacin disk and possible utility of the other disks. Methods: Consecutive Salmonella isolates (n=198) from clinical samples were tested for ciprofloxacin MIC with broth microdilution method at the range 0.002 – 2 mg/L. Disk diffusion test was performed on Mueller Hinton agar plates (bioMérieux, France) with the following disks (Bioanalyse, Turkey): (100 µg), ciprofloxacin 1 (µg), ciprofloxacin (5 µg), (5 µg), (10 µg), (10 µg), (10 µg), garenoxacin (5 µg), (1 µg), (5 µg), (1 µg), (30 µg), (2 µg), (2 µg), (10 µg), pefloxacin (5 µg), (5 µg), (10 µg). Escherichia coli ATCC 25922 was used for quality control and EUCAST’s tentative quality control criteria were followed for pefloxacin 5 µg disk. Results: Ciprofloxacin susceptible (MIC <=0.06 mg/L) isolates constituted %84.3 (n=167), low-level resistant (MIC 0.12-0.5 mg/L) isolates constituted 11.6% (n=23), and high-level resistant (MIC >=1 mg/L) isolates constituted 4.0% (n=8) of the study isolates. Among the tested antimicrobial disks the best correlation between ciprofloxacin MIC and range of zone diameters was observed for pefloxacin 5 µg (100% success in discriminating wild type isolates with MICs <= 0.06 mg/L from isolates with acquired resistance with MIC of >0.06 mg/L), followed by nalidixic acid 30 µg (failure in discriminating 3.5% of the isolates) (Table 1). Conclusion: The high ciprofloxacin susceptibility rate (84.3%) among the study isolates caused a disproportionate distribution of wild type and non-wild type isolates included in the study, yet EUCAST’s recommended pefloxacin 5 µg disk performed excellently for our isolate collection. The traditional antimicrobials nalidixic acid 30 µg and ciprofloxacin 5 µg used for the same purpose in the past, exhibited poor performance and caused overlapping of wild type and non-wild type isolates (3.5% and 7.6%, respectively). We tested various additional quinolone antimicrobials of which some are indicated for veterinary use only, however none of them performed satisfactorily.