Association of norb overexpression and fluoroquinolone resistance in clinical isolates of Staphylococcus aureus from Korea

Abstract

Objectives: Although the prevalence of fluoroquinolone resistance among methicillin-resistant Staphylococcus aureus (MRSA) is known to be higher than in methicillin-susceptible S. aureus (MSSA), the reasons have never been identified. Methods:We randomly selected 115 isolates of S. aureus collected from 10 different hospitals in Korea between June 2009 and May 2011. To investigate the difference in fluoroquinolone resistance mechanisms between MRSA and MSSA, we evaluated gyrA and parC mutations and the relative expression of the multidrug efflux pump genes norA, norB and norC. Results: All 62 ciprofloxacin-resistant S. aureus had either gyrA or parC mutations. The S84L mutation of gyrA (59/62, 95.2%) and the S80F mutation of parC (61/62, 98.4%) were the most common. Fifty-eight (93.6%) strainshadboththeS84Lmutationof gyrAandtheS80Fmutation ofparC.Amongthe115isolates,norBoverexpression was the most common, occurring in 49 (42.6%) strains. There were only two (1.7%) strains with norA overexpression and none with norC overexpression. Strains overexpressing norB were more common among ciprofloxacin-resistant S. aureus (33/62, 53.2%) than ciprofloxacin-susceptible S. aureus (16/53, 30.2%) (P=0.013). When we analysed 62 ciprofloxacin-resistant S. aureus strains, those overexpressing norBweremore common in ciprofloxacin-resistant MRSA (28/46, 60.9%) than in ciprofloxacin-resistant MSSA (5/16, 31.3%) (P=0.041). Conclusions: IncreasedexpressionofnorB canbeafactor that contributesto ciprofloxacin resistance inMRSAstrains. © The Author 2013. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved.