Prevalence And Mutant Prevention Concentration Of Heterogeneous Vancomycin-intermediate Staphylococcus Aureus Among Methicillin-resistant Isolates In Anhui Province

ObjectiveTo investigate the prevalence of heterogeneous vancomycin-intermediateStaphylococcus aureus (hVISA) in Anhui district.Mutant prevention concentrations (MPC) of vancomycin against10hVISA and30vancomycin-sensitive Staphylococcus aureus (VSSA) strains were evaluated to assesstheir abilities to restrict the selection of resistant mutants.To evaluate the MPCs of vancomycin in combination with rifampicin, fosfomycin orfusidic acid against hVISA in vitro, which provide evidence for new strategies to restrictthe resistance development of hVISA.Materials and Methods256methicillin-resistant Staphylococcus aureus (MRSA) were collected in eachSeptember from2005to2010as part of the Anhui Antimicrobial ResistanceInvestigation Net program. There were no duplicate isolate from the same patient.MRSA were identified by the Microscan WalkAway automated system-40andconfirmed by additional polymerase chain reaction method.All isolates were preliminary screened by brain heart infusion agar containing4μg/mL vancomcyin and16g/L casein (BHIV4with casein) using0.5McFarlandinocula. Subsequently, positive strains through BHIV4with casein were confirmed bypopulation analysis profile-area under the curve. The minimal inhibitory concentrationsof17antimicrobial agents against9hVISA and Mu3were determined by agar dilutionmethod. The cells of1010colony forming units per milliliter coccus were enriched in broth.MPCs of vancomycin against10strains of hVISA and30strains of VSSA were detectedby agar dilution method. MPC values were combined with pharmacokinetic parametersto assess their abilities to restrict the selection of resistant mutants.MPCs of vancomycin in combination with rifampicin, fosfomycin or fusidic acidagainst10strains of hVISA were also detected by agar dilution method and the resultwere compared with the MPCs when vancomycin were used alone.ResultsOf256strains clinical MRSA, the numbers of preliminary screened strains were12.Only9strains of12positive strains were confirmed as hVISA by population analysisprofile-area under the curve finally. Then, the isolating rate of hVISA was3.5%. Theprevalence rates of hVISA by year were2.5%(1/40) in2005,2.6%(1/39) in2006,3.8%(2/52) in2007,8.3%(4/48) in2008,0%(0/31) in2009,2.2%(1/46) in2010. Novancomycin-resistant Staphylococcus aureus or vancomycin-intermediateStaphylococcus aureus isolates were detected in this study.The results of drug-susceptibility showed that the sensitive rates of17kinds ofantibiotics against10hVISA were follows: linezolid100%, rifampincin90%,fosfomycin70%and fusidic acid70%. Other antibiotics were low-sensitive to10strainsof hVISA.The MPCrangeof vancomycin alone against10strains of hVISA were16~64μg/ml,MPC50, MPC90and MPCrangeof the vancomycin alone were16μg/ml,32μg/ml and4~32μg/ml, respectively, levels lower than hVISA (P<0.5). The MPCrangeof thecombination application with rifampicin, fosfomycin and fusidic acid were16~32μg/ml,2~32μg/ml and4~8μg/ml, respectively, and the MPCs for the combination applicationdecreased2to64-fold.ConclusionThe prevalence rate of hVISA was3.5%in Anhui district and lower than the majority of other district in China. Geographically diverse collection of Staphylococcusaureus strains and the inconsistencies of detection methods are responsible for thisdiversity. There is no yearly trend of prevalence rates was observed. All hVISA strainswere multidrug-resistant, but potential agents with low resistance rates such as linezolid,rifampicin, fosfomycin and fusidic acid could be used for hVISA infection.MPC values of vancomycin against hVISA were much higher than VSSA andvancomycin concentrations have fell into the mutant selection window for most time ofthe entire dosing period when this compound was used to treat infection due to hVISAwith conventional-dose. Resistant mutants could be selected if vancomycin wascontinued to be administered as monotherapy for the hVISA infection.Fortunately, vancomycin combined with the other antimicrobial agents coulddecrease the MPC, and narrower mutant selection window. Combination therapy ofhVISA infections where vancomycin is being used may be appropriate in an attempt toavoid the selection of further resistant mutants.