Background: Staphylococcus epidermidis is one of the prominent pathogens in ocular infection. However, the prevalence of mutations in the quinolone resistance determining region (QRDR) area in Staphylococcus epidermidis isolated from the ocular surface, and its association with fluoroquinolone resistance has not yet been fully elucidated.
Methods: Mutations in the QRDR of gyrA, gyrB, parC, and parE genes of 138 isolates of Staphylococcus epidermidis recovered from the human conjunctival flora were analyzed. The minimal inhibitory concentrations (MICs) of four fluoroquinolones (levofloxacin, gatifloxacin, moxifloxacin, and tosufloxacin) against these isolates were also determined using agar dilution methods.
Results: The MIC90 values of levofloxacin, gatifloxacin, moxifloxacin, and tosufloxacin were 3.13, 1.56, 0.78, and 3.13 μg/ml, respectively. The MIC values of all fluoroquinolones showed a bimodal distribution; susceptible strain and less susceptible strain. Mutations with amino acid substitution in the QRDR were present in 70 (50.7%) isolates. Nineteen different combinations of mutations were detected: 3 (2.2%) isolates had four mutations, 8 (5.8%) of the isolates had three mutations, 43 (31.2%) of the isolates had double mutations and 16 (11.6%) of the isolates had single mutations. Isolates with mutations in the QRDR of both gyrA and parC (n=53) were less susceptible to fluoroquinolones.
Conclusions: The present findings show that approximately half of the Staphylococcus epidermidis isolates from the normal human conjunctiva have mutation(s) in the QRDR. The presence of mutations in both gyrA and parC is strongly associated with reduced susceptibility to fluoroquinolones.