Detection and Characterisation of Heteroresistant Vancomycin Intermediate Staphylococcus aureus (hVISA) using Phenotypic and Genotypic Methods

Introduction: Vancomycin remains the drug of choice to treat infections caused by Methicillin Resistant Staphylococcus aureus (MRSA). Frequent therapeutic failures arise due to the emergence of strains with reduced susceptibility to vancomycin and difficulty in detecting these isolates. Therefore, improvements in screening and confirmatory tests to detect these isolates are required. Aim: To detect heteroresistant Vancomycin Intermediate Staphylococcus aureus (hVISA) isolates among Methicillin Resistant Staphylococcus aureus (MRSA) and Methicillin Sensitive Staphylococcus aureus (MSSA) obtained from various clinical samples and to study the phenotypic and genotypic charecteristics of these strains. Materials and Methods: This study was carried out in the Department of Microbiology, Chettinad Hospital and Research Institute, Kelambakkam, Chennai, Tamil Nadu, India. A total of 468 non-duplicate S.aureus strains obtained from various clinical samples during a period of two years from September 2014 to November 2016 were included in the study. MRSA isolates and hVISA/VISA isolates were detected by screening and confirmatory methods (MRSA screen by cefoxitin disc, hVISA/VISA screen by brain heart infusion agar with 6 mu g vancomycin and hVISA/VISA confirmation by PAP-AUC method). Agr polymorphism in hVISA/VISA was analysed by duplex PCR. Accessory gene regulator (Agr) dysfunction in these strains was studied by their colony spreading property in soft agar and autolytic property by Triton X-100 induced lysis. The results were analysed using IBM SPSS (version 21.0) software. Results: Out of 468 S.aureus strains, 114 (24%) strains were detected as MRSA and 33 (7.0%) were hVISA/VISA strains. Among MRSA detected, 11 (9.5%) strains were confirmed as hVISA/VISA. All hVISA/VISA isolates showed reduction in autolytic activity. Colony spreading property was appreciated in 13 (39%) of these isolates. Out of the 33 hVISA/VISA strains, majority belonged to agr I, III and II and the least number of isolates belonged to agr IV. Conclusion: It was concluded that hVISA/VISA strains have diverse virulence properties and are detected in both MRSA and MSSA strains. Therefore. surveillance of these strains among S.aureus isolates having vancomycin MIC's <= 1.5 mu g/mL in laboratory settings is needed.