A study on the resistance of Staphylococcus aureus and the mechanisms of its resistance to fluoroquinolone

Objective: To investigate the resistance of Staphylococcus aureus(SA) and the mechanisms of its resistance to fluoroquinolones (FQ).

Methods: The susceptibility of SA (200 strains) to 12 antibiotics was detected by disc diffusion, The minimal inhibitory concentrations (MICs) of 52 strains to three FQ were determined by agar dilution method. 52 strains resistant to ciprofloxacin (MIC> or =4 mg/L) were studied for the presence of point mutations in the gyrA gene and grlA gene by polymerase chain reaction-restriction fragment length polymorphisms (PCR-RFLP) method and for the expression of norA gene by reserpine reverse test respectively.

Results: 34% of the strains were resistant to oxacillin(methicillin resistant Staphylococcus aureus, MRSA) and other antimicrobials as well, but no vancomycin resistant strain was found. The resistance rate of MRSA to ciprofloxacin was 79.4% and cross-resistance existed. It was found that 42 strains (80.8%) had a mutation at gyrA codon 84 (TCA-->TTA or GCA). Mutations at grlA codon 80 (TCC-->TAC or TTC) and codon 84 (GAA-->AAA)were observed in 10 (19.2%) and 14 strains(26.9%) respectively.Strains containing mutations in gyrA or both gyrA and grlA gene showed a higher level of ciprofloxacin resistance than those with alternation in grlA gene but with wild type gyrA or non-gyrA mutants (P < 0.01). Decreased MICs to ciprofloxacin, norfloxacin and levofloxacin in reserpine reverse test indicated the presence of norA phenotype.

Conclusions: It is clear that emergence of resistant SA strains will continue to be a problem, especially in MRSA which was resistant to most of the antibiotics. Fluoroquinolones are not the choice for MRSA now. The resistance to fluoroquinolones in clinical isolates of SA are due to the mutations of the gyrA and grlA gene encoding the target enzyme of fluoroquinolones and cell membrane resistance. Mutations of grlA gene may differ in different districts.