Association of norB overexpression and fluoroquinolone resistance in clinical isolates of Staphylococcus aureus from Korea.
OBJECTIVE
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%) strains had both the S84L mutation of gyrA and the S80F mutation of parC. Among the 115 isolates, norB overexpression 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 norB were more common in ciprofloxacin-resistant MRSA (28/46, 60.9%) than in ciprofloxacin-resistant MSSA (5/16, 31.3%) (P = 0.041).


CONCLUSIONS
Increased expression of norB can be a factor that contributes to ciprofloxacin resistance in MRSA strains.
Division of Infectious Diseases, Massachusetts General Hospital and Harvard Medical School, Boston, MA, USA.
Yee Gyung
YG
Kwak
Que Chi
QC
Truong-Bolduc
Hong
H
Bin Kim
Kyoung-Ho
KH
Song
Eu Suk
ES
Kim
David C
DC
Hooper
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000096:biosynthesis / Q000235:genetics; D002939:Ciprofloxacin / Q000494:pharmacology; D024881:Drug Resistance, Bacterial; D024841:Fluoroquinolones / Q000494:pharmacology*; D015870:Gene Expression; D006761:Hospitals; D006801:Humans; D007723:Korea; D026901:Membrane Transport Proteins / Q000096:biosynthesis* / Q000235:genetics; D013203:Staphylococcal Infections / Q000382:microbiology*; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000235:genetics / Q000302:isolation & purification
D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D024841:Fluoroquinolones; D026901:Membrane Transport Proteins; D002939:Ciprofloxacin
S. aureus; ciprofloxacin; efflux pumps