Analysis of mutations within quinolone resistance determining regions of S. pneumoniae clinical isolates collected from TARGET surveillance during 2003

Abstract number: 1134_01_223

Morrissey I., Buckridge S., Farrell D.J.

Objectives:  

To compare fluoroquinolone (FQ) susceptibilities and determine the prevalence of QRDR mutations in recently circulating S. pneumoniae (SP) strains.

Methods:  

SP were collected from centres within Europe, USA, Mexico and South Africa. LEV, gatifloxacin (GATI) and moxifloxacin (MXF) MICs were determined by microbroth dilution. Approx. 20% of SP with LEV MIC of 1 mg/L and all SP with LEV MIC of 2 and above were chosen for analysis. QRDR regions for parC, parE, gyrA and gyrB were amplified by PCR and DNA sequences determined.

Results:  

No isolate of intermediate susceptibility to LEV (MIC = 4 mg/L) was found. Out of 3233 SP submitted 39 were LEV resistant (res). Of these, 33 were res to GATI and 10 res to MXF. QRDR amino acid changes are indicated in the Table. [Key: 1 ParC-S79F, 2 ParC-S79Y, 3 ParC-S79R, 4 ParC-D83N, 5 ParC-D83Y, 6 ParE-D435N, 7 GyrA-S81F, 8 GyrA-S81Y, 9 GyrA-E85A, 10 GyrA-E85G, 11 GyrA-E85K, ND not determined.] Other QRDR changes were also found but these were not related to FQ resistance (data not shown). Few isolates with LEV MIC of 1 mg/L possessed ‘^®silent’ mutations, but 42% of LEV MIC 2 mg/L did. These changes were mainly in the S79 residue of ParC (1 SP carried a S81F GyrA change). These ‘^®silent’ SP are only 1 step away from full LEV resistance. The mode GATI or MXF MIC for these SP was 0.5 or 0.25, respectively. Most LEV-res SP (MIC >= 8 mg/L) had 2 QRDR changes and those with additional changes had higher LEV MICs.

Conclusions:  

A large proportion of borderline LEV-susceptible isolates possessed silent parC mutations. These have the potential to select full FQ resistance in the future. The use of more potent FQs, such as MXF, that show less cross-resistance to LEV than GATI may reduce the likelihood of this occurring.