The real vancomycin-resistant Staphylococcus aureus has arrived

Over the last decade, staphylococci with reduced susceptibility to vancomycin have been reported with increasing frequency. The appearance of Staphylococcus aureus isolates with thickened cell walls for which the vancomycin MICs were 8 or 16 μg/ml (vancomycin-intermediate S. aureus, or VISA) were followed by reports of isolates for which the vancomycin MICs were ≥ 32 pg/ml (vancomycin-resistant S. aureus, or VRSA). The latter isolates harbor the vanA vancomycin resistance gene from enterococci. Both VISA and VRSA isolates may escape detection in the clinical microbiology laboratory, depending on the method of susceptibility testing in use. The use of a brain heart infusion agar plate containing 6 pg of vancomycin/ml can aid in detecting these clinically significant isolates. The clinical significance and optimal methods for detecting heterogeneously vancomycin-intermediate S. aureus (hVISA) isolates remains controversial.