Mutations in the quinolone resistance determining region in Staphylococcus epidermidis recovered from conjunctiva and their association with susceptibility to various fluoroquinolones

Background: Staphylococcus epidermidis is one of the prominent pathogens in ocular infection. The prevalence of mutations in the quinolone resistance determining region (QRDR) area in S epidermidis isolated from the ocular surface and its association with fluoroquinolone resistance has not been fully elucidated. Methods: Mutations in the QRDR of gyrA, gyrB, parC, and parE genes of 138 isolates of S epidermidis recovered from the human conjunctival flora were analysed. 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 mu g/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. 19 different combinations of mutations were detected: 3 isolates ( 2.2%) had four mutations, 8 ( 5.8%) had three mutations, 43 ( 31.2%) had double mutations and 16 ( 11.6%) 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 the S 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.