Effect of suprainhibitory concentrations of quinolones on hydrophobicity and motility of Serratia marcescens

The effect of suprainhibitory concentrations of quinolones (ciprofloxacin, enoxacin and norfloxacin) on the growth, hydrophobicity and motility of a nosocomial pathogen Serratia marcescens was studied. A postantibiotic effect (PAE) was induced by 2x of 4x MIC concentrations for 0.5 h. By using the 2x MIC concentrations all three quinolones induced equally long PAE approximately 1 h. The longest PAE of 5.4 h at 4x MIC concentration was induced by enoxacin. The results obtained showed that suprainhibitory concentrations of quinolones significantly stimulated the adhesion of S. marcescens to xylene, with the exception of enoxacin, which inhibited the adhesion to 61.2% at 4x MIC concentration. These results correlated with those in the salt aggregation test. The adhesion of strains to nitrocellulose filters did not influence the aftereffect of suprainhibitory concentrations of quinolones. Exposure of bacterial cells to suprainhibitory concentrations of ciprofloxacin and norfloxacin caused a reduction in motility, while this effect was more distinct at 4x MIC concentration. The results suggest that any consideration of postantibiotic effects should include the residual antibiotic effects on virulence factors, in addition to the defined suppression of bacterial regrowth.