Mutation in grlA and norA genes in Fluoroquinolone resistant strains of S. aureus

Background and Aim. Fluoroquinolones are one of the most commonly used broad spectrum classes of antibiotics with increasingly drug resistance. The present study was aimed to investigate mutation in grlA and norA genes in fluoroquinolone resistant strains of S. aureus.
In this cross-sectional study, 85 strains of S. aureus was isolated from different clinical samples by biochemical and molecular tests. Susceptibility to fluoroquinolones in the isolates was tested by the disc diffusion method and broth macrodilution method was used to investigate ciprofloxacin minimum inhibitory concentration. Mutations in the quinolone resistance-determining regions (QRDR) in chromosomal grlA gene and norA gen mutations was investigated in quinolone-resistant S. aureus isolates, by PCR amplification of grlA and norA genes and sequencing.
Results. Out of 85 tested bacteria, 17 (20%) isolates recognized as fluoroquinolones resistant. Minimum inhibitory concentration of ciprofluxacin for resistant isolates ranged between 32 to 512 µg/ml. Two single point mutations S80F and P144S and one double mutation S80F P144S were determined in grlA gene of Fluoroquinolone resistant strains of S. aureus but none of resistant strains showed mutation in norA gene.
Conclusion. The obtained results demonstrated that grlA gene mutation is one of the most important mechanisms of resistance to ciprofloxacin in clinical isolates of S. aureus in Rasht and most of resistant isolates acquired common mutation in codon 80.