Mutations in parC and gyrA genes among levofloxacin resistant clinical isolates of Streptococcus pneumoniae

Abstract number: 1134_03_192

Alonso R., Mateo E., Calvo F., Cisterna R.

Objectives:  

The incidence of levofloxacin resistance in clinical isolates of S. pneumoniae is relatively low. LVX resistance requires at least 2 mutations in the quinolone resistance determining region (QRDR) of topoisomerase IV and DNA gyrase. The purpose of this study was to characterize among recent clinical isolates of S. pneumoniae the mutations that conferred resistance to levofloxacin.

Methods:  

Twenty-four levofloxacin-resistant pneumococci (MICs > 4 mg/L) isolated from respiratory samples during 2004 were analysed. Minimal inhibitory concentrations (CMIs) to levofloxacin, gatifloxacin, moxifloxacin, and gemifloxacin were determined by agar dilution method using Mueller–Hinton agar with 5% horse blood. The QRDR regions of parC and gyrA were amplified by PCR and their DNA sequence determined.

Results:  

Levofloxacin resistance was associated with combinations of at least two amino acid substitutions, most commonly involving ParC Ser79Phe (23/25), and GyrA Ser81Tyr (20/25). Of the 24 strains, three strains showed single parC mutation (S79F) and a double gyrA mutation (S81Y + E85G). One strain showed a K137N substitution in ParC and no mutations were found in the QRDR of gyrA. This strain may be harbour mutations in gyrB gene. None of the 24 strains was susceptible to gatifloxacin; twelve and twenty isolates were susceptible to moxifloxacin (CMI < 4 mg/L) and gemifloxacin (CMI < 1 mg/L), respectively.

Conclusions:  

The results suggest that resistance to levofloxacin require at least two substitutions of amino acids within the QRDR region of gyrase and topoisomerase IV, and that there is considerable cross-resistance among fluoroquinolones associated with these changes.