Molecular features of heterogeneous vancomycin-intermediate Staphylococcus aureus strains isolated from bacteremic patients

We assessed phenotypic and genomic changes of hVISA (n = 24), MRSA (n = 16) and MSSA (n = 17) isolates by PCR to determine staphylococcal chromosomal cassette (SCCmec) types, Panton-Valentine leukocidin (PVL) and the accessory gene regulator (agr) loci. Biofilm formation was quantified. Genetic relatedness was assessed by PFGE. PFGE analysis of isolates was diverse suggesting multiple sources of infection. 50% of hVISA isolates carried SCCmec type I, 21% type II; 25% type V; in 4% the SCCmec type could not be identified. Among MRSA isolates, 44% were SCCmec type I, 12.5% type II, 25% type V, 12.5% were non-typable, and 6% were SCCmec type IVd. Only one hVISA isolate and two MSSA isolates carried the PVL. Biofilm formation and agr patterns were diverse.hVISA isolates were diverse in all parameters tested. A considerable number of hVISA and MRSA strains carried the SCCmec type V cassette, which was not related to community acquisition.In recent years there have been growing numbers of reports from many countries of strains of Staphylococcus aureus showing heterogeneous intermediate resistance to vancomycin (hVISA). The frequency of heteroresistance among MRSA isolates has recently reached 6% to 11% [1-3]. In our institution there are approximately 200 S. aureus bacteremias each year. Of these, 50% are MRSA and 6% demonstrate hVISA resistance [2,3]. Molecular assessment of the clonal dissemination of hVISA isolates has yielded conflicting results. Several studies found genetic linkage between hVISA isolates, reflected by a single pulsed field gel electrophoresis (PFGE) clone [4-6], while others showed that hVISA isolates were genetically diverse [7,8].The mechanism by which hVISA occurs is still under investigation. The hVISA phenotype has a thickened cell wall, altered peptidoglycan cross-linking, altered penicillin-binding protein expression, and slower growth rate [1-3,7]. Several genes related to cell regulation pathways have been proposed as involved in the develo