Contribution of topoisomerase IV and DNA gyrase mutations in Streptococcuspneumoniae to resistance to novel fluoroquinolones

In this study, we assessed the activity of ciprofloxacin, levofloxacin, spa rfloxacin, and trovafloxacin against clinical isolates of Streptacoccus pne umoniae that were resistant to the less-recently developed fluoroquinolones by using defined amino acid substitutions in DNA gyrase and topoisomerase IV. The molecular basis for resistance was assessed by using mutants select ed with trovafloxacin, ciprofloxacin, and levofloxacin in vitro, This demon strated that the primary target of trovafloxacin in S. pneumoniae is the Pa rC subunit of DNA topoisomerase IV, similar to most other fluoroquinolones. However, first-step mutants bearing the Ser79-->Phe/Tyr substitution in to poisomerase IV subunit ParC were susceptible to trovafloxacin with a minimu m inhibitory concentration of 0.25 mu g/ml, and mutations in the structural genes for both topoisomerase IV subunit ParC (parC) and the DNA gyrase sub unit (gyrA) were required to achieve levels of resistance above the breakpo int, The data also suggest that enhanced activity of trovafloxacin against pneumococci is due to a combination of factors that may include reduced eff lux of this agent and an enhanced activity against both DNA gyrase and topo isomerase IV.