Vancomycin intermediate and high level vancomycin resistant Staphylococcus aureus clinical isolates in Osogbo, Nigeria

The decreased vancomycin susceptibility and subsequent emergence of vancomycin resistant Staphylococcus aureus (VRSA) strains already multi-resistant to antibiotics is a major public health problem. In 2009, the Clinical and Laboratory Standards Institute (CLSI) altered its guidelines for vancomycin susceptibility testing in S. aureus and recent data suggests the possibility that VRSA may emerge more frequently than previously expected. Against this background, we conducted a study to ascertain the susceptibility status of clinical S. aureus isolates to vancomycin in our environment using vancomycin agar screen, disk diffusion and broth dilution methods. Of the total 49 S. aureus invasive strains isolated, 25 (51.0%) had vancomycin MIC of ≤2µg/ml by the CLSI standard broth dilution method and are classed as vancomycin susceptible; 18 (36.7%) had MIC of 4-8µg/ml (vancomycin intermediate resistant) and 6 (12.2%) had MIC of >256µg/ml (high level vancomycin resistant). Vancomycin agar screen with Mueller-Hinton agar containing 3µg/ml vancomycin (MHA-V3) correctly identified 20 of 25 (80%) vancomycin susceptible isolates; detected all 6 vancomycin resistant isolates and 16 of 18 (88.9%) vancomycin intermediate strains. Similarly, Mueller-Hinton agar containing 6µg/ml vancomycin (MHA-V6) correctly identified 23 of 25 (92%) vancomycin susceptible isolates and all 6 vancomycin resistant isolates but detected 14 (77.8%) of 18 vancomycin intermediate strains. Vancomycin disk diffusion test correctly identified all the 25 vancomycin susceptible S. aureus isolates giving 100% specificity but detected only 1 of 18 (5.6%) vancomycin intermediate and none (0%) of vancomycin resistant isolates. This result shows the occurrence of VISA and high level VRSA isolates in our environment, which contrary to current belief, may indicate widespread dissemination of VRSA. MHA-V3 agar is a useful alternative screening medium for vancomycin non-susceptibility detection in clinical S. aureus isolates but vancomycin disk diffusion is not useful in this regard.