In vitro susceptibilities and molecular analysis of vancomycin-intermediate and vancomycin-resistant Staphylococcus aureus isolates

Abstract

Background: There is increasing frequency of vancomycin-intermediate and -resistant Staphylococcus aureus (VISA and VRSA) isolates identified in clinical practice. There are limited reports evaluating susceptibility patterns and molecular characteristics of these strains.

Methods: Laboratory analysis was performed on 13 VRSA and 33 VISA isolates, including susceptibility testing by broth microdilution, detection of Panton-Valentine leukocidin (PVL) genes, arginine catabolic mobile element (ACME), and staphylococcal cassette chromosome mec typing using polymerase chain reaction. Strain typing using pulsed-field gel electrophoresis (PFGE) was performed on VRSA isolates.

Results: Telavancin, linezolid, tigecycline, and minocycline were active against >90% of VISA isolates, while >90% of VRSA isolates were susceptible to ceftaroline, daptomycin, linezolid, minocyline, tigecycline, rifampin, and trimethoprim/sulfamethoxazole. There were no VISA or VRSA isolates that carried PVL genes or ACME, and most strains (69.8%) were staphylococcal cassette chromosome mec type II. VRSA isolates were predominantly related to USA100 (53.8%) and none were related to USA300 or USA400.

Conclusions: A large number of available antimicrobial agents retain very good in vitro activity against VRSA and VISA isolates. The present isolates appear to be derived from healthcare-associated strains based on the absence of features associated with community-associated strains, and VRSA isolates are polyclonal by PFGE.