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

The study objective was to screen both methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive S. aureus (MSSA) isolates from blood cultures for reduced suscept-ibility 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 teico-planin 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 b