Analysis of grlA mutations in clinical isolates of Staphylococcus aureus with different levels of quinolone resistance

Induction of fibronectin-binding proteins and increased adhesion of quinolone-resistant Staphylococcus aureus by subinhibitory levels of ciprofloxacin

We recently reported that strain EN1252a, a fluoroquinolone-resistant derivative of Staphylococcus aureus NCTC8325 with mutations in grlA and gyrA, expressed increased levels of fibronectin-binding proteins (FnBPs) and showed a significantly higher attachment to fibronectin-coated polymer surfaces after growth in the presence of subinhibitory concentrations of ciprofloxacin. The present study evaluated the occurrence and frequency of fluoroquinolone-induced FnBP-mediated adhesion in clinical isolates of fluoroquinolone-resistant methicillin-resistant S. aureus (MRSA) and methicillin-susceptible S. aureus (MSSA). Eight of ten MRSA isolates and four of six MSSA isolates with grlA and gyrA mutations exhibited significant increases in attachment to fibronectin-coated surfaces after growth in the presence of one-quarter the MIC of ciprofloxacin. Fluoroquinolone-induced FnBP-mediated adhesion of one clinical MRSA strain and the double mutant strain EN1252a also occurred on coverslips removed from the subcutaneous space of guinea pigs. For strain EN1252a, the regulation of fnb transcription by sub-MICs of ciprofloxacin was studied on reporter plasmids carrying fnb-luxAB fusions. One-quarter of the MIC of ciprofloxacin significantly increased fnbB, but not fnbA, promoter activity of the fluoroquinolone-resistant mutant but not its fluoroquinolone-susceptible parent ISP794. This response was abolished by pretreatment with rifampin, indicating an effect at the level of transcription. Activation of the fnbB promoter was not due to an indirect effect of ciprofloxacin on growth rate and still occurred in an agr mutant of strain EN1252a. These data suggest that sub-MIC levels of ciprofloxacin activate the fnbB promoter of some laboratory and clinical isolates, thus contributing to increased production of FnBP(s) and leading to higher levels of bacterial attachment to fibronectin-coated or subcutaneously implanted coverslips.

In vitro activities of 13 fluoroquinolones against Staphylococcus aureus isolates with characterized mutations in gyrA, gyrB, grlA, and norA and against wild-type isolates

The in vitro activities of 13 fluoroquinolones (FQs) were tested against 90 Staphylococcus aureus clinical isolates: 30 wild type for gyrA, gyrB, grlA and norA and 60 with mutations in these genes. Clinafloxacin (CI-960), sparfloxacin, and grepafloxacin were the most active FQs against wild-type isolates (MICs at which 90% of isolates were inhibited, 0.06 to 0.1 microgram/ml). Mutations in grlA did not affect the MICs of newer FQs. grlA-gyrA double mutations led to higher MICs for all the FQs tested. Efflux mechanisms affected the newer FQs to a much lesser extent than the less recently developed FQs.