In vitro induction and selection of fluoroquinolone-resistant mutants of Streptococcus pyogenes strains with multiple emm types.
OBJECTIVE
To perform a systematic analysis of point mutations in the quinolone resistance determining regions (QRDRs) of the DNA gyrase and topoisomerase genes of emm type 6 and other emm types of Streptococcus pyogenes strains after in vitro exposure to stepwise increasing concentrations of levofloxacin.


METHODS
Twelve parent strains of S. pyogenes, each with a different emm type, were chosen for stepwise exposure to increasing levels of levofloxacin followed by selection of resistant mutants. The QRDRs of gyrA, gyrB, parC and parE correlating to mutants with increased MICs were analysed for point mutations.


RESULTS
Multiple mutants with significantly increased MICs were generated from each strain. The amino acid substitutions identified were consistent regardless of emm type and were similar to the mechanisms of resistance reported in clinical isolates of S. pyogenes. The number of induction/selection cycles required for the emergence of key point mutations in gyrA and parC was variable among strains. For each parent-mutant set, when MIC increased, serine-81 of gyrA and serine-79 of parC were the primary targets for amino acid substitutions. No point mutations were found in the QRDRs of gyrB and parE in any of the resistant mutants sequenced.


CONCLUSIONS
Despite its intrinsic polymorphism in the QRDR of parC, emm type 6 is not more likely to develop high-level resistance to fluoroquinolones when compared with other emm types. All emm types seem equally inducible to high-level fluoroquinolone resistance.
Billal
Dewan S
DS
Department of Otolaryngology-Head and Neck Surgery, Wakayama Medical University, 811-1 Kimiidera, Wakayama, 641-8510, Japan.
Fedorko
Daniel P
DP
Yan
S Steve
SS
Hotomi
Muneki
M
Fujihara
Keiji
K
Nelson
Nancy
N
Yamanaka
Noboru
N
D000890:Anti-Infective Agents / Q000494:pharmacology*; D000942:Antigens, Bacterial / Q000145:classification*; D001425:Bacterial Outer Membrane Proteins / Q000145:classification*; D002352:Carrier Proteins / Q000145:classification*; D027081:DNA Gyrase / Q000235:genetics; D027101:DNA Topoisomerase IV / Q000235:genetics; D024881:Drug Resistance, Bacterial / Q000235:genetics; D024841:Fluoroquinolones / Q000494:pharmacology*; D008826:Microbial Sensitivity Tests; D017354:Point Mutation; D013297:Streptococcus pyogenes / Q000187:drug effects* / Q000235:genetics
D000890:Anti-Infective Agents; D000942:Antigens, Bacterial; D001425:Bacterial Outer Membrane Proteins; D002352:Carrier Proteins; D024841:Fluoroquinolones; C014689:streptococcal M protein; D027101:DNA Topoisomerase IV; D027081:DNA Gyrase