Molecular characterisation of the quinolone resistance-determining regions (QRDR) including gyrA, gyrB, parC and parE genes in Streptococcus pneumoniae.

ABSTRACT

Streptococcus pneumoniae is the leading cause of community-acquired pneumonia (CAP). Currently, empirical treatment with quinolones is being used due to the emergence of beta-lactam and macrolide resistance in S. pneumonaie. Although the prevalence of quinolone-resistant S. pneumoniae remains low, increasing numbers of resistant isolates are being seen. Genetic mechanisms leading to fluoroquinolone resistance in pneumococci are complex. This study aims to use molecular methods to characterise all isolates through sequence analysis of their QRDR regions. Thirty-two S. pneumoniae isolates were obtained from nasal swabs from adult and paediatric patients attending local general practices in Northern Ireland. Phenotypic minimum inhibitory concentration (MIC) was determined for Clinical and Laboratory Standards Institute (CLSI) broth microdilution against ciprofloxacin, levofloxacin and norfloxacin. Simultaneously, the QRDR regions of gyrA, gyrB, parC and parE were analysed by sequence typing for all pneumococci obtained. Only one isolate (3.1%) showed reduced susceptibility to ciprofloxacin and levofloxacin. Two amino acid positions were discordant in the S. pneumoniae R6 strain and eight (25%) and 23 (71.9%) isolates contained the mutations Ile460Val in gyrA and Lys137Asn in parC (deposited in GenBank, accession numbers GQ999587-GQ999589), respectively. No mutations were found in either the gyrB or parE loci. In conclusion, the study demonstrated increased fluoroquinolone resistance which could not be accounted for simply through QRDR mutations, and, reciprocally, that mutations in the QRDR region do not necessarily result in overt phenotypic resistance.