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

The aim of this study was to characterize quinolone resistance mechanisms i n strains of Streptococcus pneumoniae with increased MICs of ofloxacin. The se strains were also tested for their susceptibility to a battery of quinol one antimicrobial agents, including gemifloxacin. Of the S. pneumoniae isol ates used, 27 were susceptible to ofloxacin, 18 intermediate and 48 resista nt (ofloxacin MIC <4, 4 and >4 mg/L, respectively). In general, the ofloxac in-susceptible strains had no amino acid substitutions in GyrA, GyrB, ParC or ParE. Moderate increases in MIC were associated with substitutions in th e quinolone resistance-determining region (QRDR) of ParC, while the highest MICs were found for strains that also had substitutions in the QRDR of Gyr A. The most common substitutions were Ser79-->Phe in ParC and Ser81-->Phe i n GyrA. Other substitutions were identified within the QRDR of ParC and out side the QRDR of ParC and ParE; these did not appear to affect susceptibili ty. The effects of antimicrobial efflux pumps were studied by determining M ICs of a range of quinolones in the presence and absence of reserpine, an i nhibitor of Gram-positive efflux pumps. Our results indicated that high-lev el resistance, caused entirely by efflux, was seen in a minority of ofloxac in-resistant S. pneumoniae strains. Testing the susceptibility of quinolone -resistant strains to gemifloxacin, ciprofloxacin, norfloxacin, ofloxacin a nd trovafloxacin revealed that gemifloxacin was least affected by this larg e 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 a lso be effective against strains resistant to other quinolones.