Efflux and target mutations as quinolone resistance mechanisms in clinical isolates of Streptococcus pneumoniae

The aim of this study was to characterize quinolone resistance mechanisms in strains of Streptococcus pneumoniae with increased MICs of ofloxacin. These strains were also tested for their susceptibility to a battery of quinolone antimicrobial agents, including gemifloxacin. Of the S. pneumoniae isolates used, 27 were susceptible to ofloxacin, 18 intermediate and 48 resistant (ofloxacin MIC <4, 4 and >4 mg/L, respectively). In general, the ofloxacin-susceptible strains had no amino acid substitutions in GyrA, GyrB, ParC or ParE. Moderate increases in MIC were associated with substitutions in the quinolone resistance-determining region (QRDR) of ParC, while the highest MICs were found for strains that also had substitutions in the QRDR of GyrA. The most common substitutions were Ser79-->Phe in ParC and Ser81-->Phe in GyrA. Other substitutions were identified within the QRDR of ParC and outside the QRDR of ParC and ParE; these did not appear to affect susceptibility. The effects of antimicrobial efflux pumps were studied by determining MICs of a range of quinolones in the presence and absence of reserpine, an inhibitor of Gram-positive efflux pumps. Our results indicated that high-level resistance, caused entirely by efflux, was seen in a minority of ofloxacin-resistant S. pneumoniae strains. Testing the susceptibility of quinolone-resistant strains to gemifloxacin, ciprofloxacin, norfloxacin, ofloxacin and trovafloxacin revealed that gemifloxacin was least affected by this large variety of resistance mechanisms and was the only quinolone with MICs of less than or equal to 0.5 mg/L for all strains in this study. These results suggest that gemifloxacin is highly potent against S. pneumoniae and may also be effective against strains resistant to other quinolones.