INCREASED EXPRESSION OF FIBRONECTIN-BINDING PROTEINS BY FLUOROQUINOLONE-RESISTANT STAPHYLOCOCCUS-AUREUS EXPOSED TO SUBINHIBITORY LEVELS OF CIPROFLOXACIN

Bacterial adhesion, which plays an important role in Staphylococcus au reus colonization and infection, may be altered by the presence of ant ibiotics or/and antibiotic resistance determinants. This study evaluat ed the effect of fluoroquinolone resistance determinants on S. aureus adhesion to solid-phase fibronectin, which is specifically mediated by two surface-located fibronectin-binding proteins. Five isogenic mutan ts, derived from strain NCTC 8325 and expressing various levels of qui nolone resistance, were tested in an in vitro bacterial adhesion assay with polymethylmethacrylate coverslips coated with increasing amounts of fibronectin. These strains contained single or combined mutations in the three major loci contributing to fluoroquinolone resistance, na mely, grlA, gyrA, and flqB, which code for altered topoisomerase IV, D NA gyrase, and increased norA-mediated efflux of fluoroquinolones, res pectively. Adhesion characteristics of the different quinolone-resista nt mutants grown in the absence of fluoroquinolone shelved only minor differences from those of parental strains. However, more important ch anges in adhesion were exhibited by mutants highly resistant to quinol ones following their exponential growth in the presence of one-quarter MIC of ciprofloxacin. Increased bacterial adhesion of the highly quin olone-resistant mutants, which contained combined mutations in grlA an d gyrA, was associated with and explained by the overexpression of the ir fibronectin-binding proteins as assessed by Western ligand affinity blotting. These findings contradict the notion that subinhibitory con centrations of antibiotics generally decrease the expression of virule nce factors by S. aureus. Perhaps the increased adhesion of S. aureus strains highly resistant to fluoroquinolones contributes in part to th at emergence in clinical settings.