Emergence of Vancomycin Intermediate Staphylococcus aureus (VISA) in Clinical Isolates of Methicillin Resistant S. aureus from South Western Region of Nigeria

Aim: Vancomycin has been widely used in the treatment of infections caused by methicillin-resistant Staphylococcus aureus (MRSA). The emergence of vancomycin- intermediate and -resistant Staphylococcus aureus (VISA and VRSA, respectively) in various parts of the world has been reported. The level of vancomycin resistance phenotypically and genotypically in clinical isolates of S. aureus with or without methicillin resistance from south western region of Nigeria was determined.

Methods: A total of 116 non-duplicate S. aureus previously obtained from various clinical specimens were subjected to susceptibility testing using disc and microbroth dilution including polymerase chain reaction amplification of mecA and van genes.

Results: The disc susceptibility testing results depict multiple drug resistance with 100% resistance to co-amoxyclav, erythromycin and gentamicin had intermediate of 39.1 and 65.2% respectively, no strain sensitive. Vancomycin showed 100% susceptibility. The minimum inhibitory concentrations, MICs of 116 S. aureus strains against vancomycin showed the isolates to have MIC50 of 1 µg/ml and MIC90 of 2 µg/ml. Five (4.3%) of the 116 clinical isolates had intermediate MIC of 4 µg/ml. These five strains were from methicillin resistant strains and were isolated from different clinical sites and hospitals. However, none of these strains demonstrated the presence of van genes, vanA; vanB; vanC and vanH by PCR.

Conclusion: There is high level of multiple antibiotic resistance in S. aureus with some MRSA also showing reduced susceptibility to vancomycin resulting in VISA. However, the VISA strains have shown no van gene as their mechanism of acquiring reduced susceptibility.