Surveillance of antibiotic resistance in clinical isolates of Staphylococcus aureus from Changsha

Objective To investigate the characteristics of drug resistance and current situation of methicillin-resistance in clinical isolates of Staphylococcus aureus(S.aureus) in Changsha.Method Totally 279 non-duplicate clinical isolates of S.aureus were collected during November 2009 to November 2010 from 11 hospitals in Changsha and then identified by Vitek-2 system.K-B disk method was used to test drug sensitivity to 24 commonly used antibiotics.Chromogenic cephalosporin spot test was applied to detect β-lactamase and D-test were used to check inducible resistance to clindamycin.MRSA was screened by oxacillin and cefoxitin disk diffusion methods.Agar dilution method was used to determine the minimal inhibitory concentrations(MICs) of oxacillin and cefoxitin to isolates of S.aureus.Result Of the 279 S.aureus isolates,the sensitive rates to 9 of the 24 antibiotics tested were higher than 50%.All the isolates were susceptible to tecoplanin,vancomycin and linezolid.The resistance rates to 11 of the 24 antibiotics tested were higher than 50%;resistance rates to penicillin and ampicillin were the highest(both 97.1%).Among the 279 strains,MRSA accounted for 54.5%,which were high sensitive to vancomycin,tecoplanin and linezolid.The resistance rates of MRSA to the other 16 antibiotics were higher than that of MSSA.Of the 279 strains,the positive rate of β-lactamase was 89.6%(250/279).Of the 30 isolates resistant to erythromycin but susceptible or intermediate to clindamycin,22(73.3%) showed a positive result of D-test.The MIC range of oxacillin and cefoxitin on S.aureus was 0.125-256 μg/mL and 2-256 μg/mL respectively.The MIC50 and MIC90 of oxacillin and cefoxitin were 128 μg/mL and 256 μg/mL,64 μg/mL and 256 μg/mL respectively.Conclusion Clinical isolates of S.aureus in Changsha are multiple resistant to commonly used antimicrobial agents.Rate of MRSA and level of resistance to methicillin are very high.