Identification and characterization of teicoplanin-intermediate Staphylococcus aureus blood culture isolates in NE Scotland

Abstract

The study objective was to screen both methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive S. aureus (MSSA) isolates from blood cultures for reduced susceptibility to vancomycin and teicoplanin. A total of 72 MRSA and 143 MSSA isolates were screened on brain-heart infusion agar containing either 4 mg/L vancomycin or 8 mg/L teicoplanin, using an inoculum of ∼106 organisms. MICs were determined by Etest, broth microdilution and agar incorporation. Isolates were characterized by PFGE, mecA and nuc PCR, transmission electron microscopy (TEM) and analysis of cell proteins (proteomics). Based on British Society for Antimicrobial Chemotherapy (BSAC) breakpoints, seven MRSAs and seven MSSAs were teicoplanin resistant, with MICs of up to 16 and 24 mg/L respectively, but were vancomycin sensitive. Based on higher NCCLS breakpoints, five MRSAs and six MSSAs were teicoplanin intermediate, vancomycin sensitive. All the MRSAs belonged to the EMRSA-16 clone and subdivided into two groups. The MSSAs belonged to five different clones. TEM showed the resistant variants to have slightly thicker cell walls than sensitive variants. Most notably, the resistant variants possessed characteristic dark, granular material concentrated in the middle of the cells, believed to be chromosome. Proteomics showed the resistant variants to overexpress phosphoglycerate kinase. Both MRSA and MSSA with reduced teicoplanin susceptibility may remain vancomycin sensitive by NCCLS and BSAC criteria and it is important to screen clinical isolates of MRSA and MSSA for reduced susceptibility to both agents.