Reduced susceptibility to vancomycin and biofilm formation in methicillin-resistant Staphylococcus epidermidis isolated from blood cultures

This study aimed to correlate the presence of ica genes, biofilmformation and antimicrobial resistance in 107 strains of Staphylococcusepidermidis isolated from blood cultures. The isolates were analysed todetermine their methicillin resistance, staphylococcal cassette chromosomemec (SCCmec) type, ica genesand biofilm formation and the vancomycin minimum inhibitory concentration (MIC) wasmeasured for isolates and subpopulations growing on vancomycin screen agar. ThemecA gene was detected in 81.3% of the S.epidermidis isolated and 48.2% carried SCCmec type III.The complete icaADBC operon was observed in 38.3% of the isolates;of these, 58.5% produced a biofilm. Furthermore, 47.7% of the isolates grew onvancomycin screen agar, with an increase in the MIC in 75.9% of the isolates.Determination of the MIC of subpopulations revealed that 64.7% had an MIC ≥ 4 μgmL^-1, including 15.7% with an MIC of 8 μg mL^-1 and 2% withan MIC of 16 μg mL^-1. The presence of the icaADBC operon,biofilm production and reduced susceptibility to vancomycin were associated withmethicillin resistance. This study reveals a high level of methicillin resistance,biofilm formation and reduced susceptibility to vancomycin in subpopulations ofS. epidermidis. These findings may explain the selection ofmultidrug-resistant isolates in hospital settings and the consequent failure ofantimicrobial treatment.