Vancomycin MIC and agr dysfunction in invasive MRSA infections in southern Brazil

In methicillin-resistant Staphylococcus aureus (MRSA) treatment, the vancomycin minimum inhibitory concentration (MIC) increase, vancomycin heteroresistance (hVISA) presence, and accessory gene regulator (agr) dysfunction are predictors of vancomycin therapy failure. This study evaluated the association between vancomycin MIC (≥ 1.0 μg/mL) and agr dysfunction in invasive MRSA isolates. Vancomycin MIC, hVISA phenotype, agr group, and function were determined in 171 MRSA isolates obtained between 2014 and 2019 from hospitals in Porto Alegre, Brazil. All MRSA were susceptible to vancomycin; 16.4% of these had MIC ≥ 1.0 μg/mL. Seventeen MRSA isolates expressed the hVISA phenotype; 35.3% of them had MIC of 1.5 μg/mL. agr groups I (40.9%) and II (47.1%) were the most found groups for MRSA and hVISA isolates, respectively. The proportion of MRSA with vancomycin MIC ≥ 1.0 μg/mL in agr group II was significantly higher than in agr groups I and III (p = 0.002). agr dysfunction was observed in 4.7% (8/171) of MRSA, especially those with vancomycin MIC ≥ 1.0 μg/mL (p < 0.001). In addition, six isolates (35.3%; 6/17) with hVISA phenotype presented agr dysfunction, which was significantly higher than that in non-hVISA phenotype (p < 0.001). In conclusion, agr dysfunction in MRSA is associated with vancomycin MIC ≥ 1.0 μg/mL and hVISA phenotype, which suggests that agr dysfunction might confer potential advantages on MRSA to survive in invasive infections.