Characterization of heterogeneous vancomycin-intermediate resistance, MIC and accessory gene regulator (agr) dysfunction among clinical bloodstream isolates of staphyloccocus aureus.

Background: The development of hVISA has been associated with vancomycin clinical failures and is commonly misidentified in clinical microbiology laboratories. Therefore, the objectives of this present study was to improve the reliability of methodologies and criteria for identifying hVISA, evaluate the prevalence of hVISA among clinical bloodstream isolates of S. aureus and determine if there exists a relationship between accessory gene regulator (agr) dysfunction and the hVISA phenotype.

Methods: The presence of hVISA in 220 clinical S. aureus isolates (121 MSSA, 99 MRSA) from bloodstream infections was examined by CLSI broth microdilution, Macro & Standard Etest. Isolates which were classified as hVISA by Macro Etest, were additionally evaluated using a modified PAP-AUC method using a modified starting inoculum of 10(10) CFU/mL, and growth on brain heart infusion agar with 4 mg/L vancomycin (BHIV4) at 10(8) and 10(10) CFU/mL, and agr function was assessed by delta-hemolysin production.

Results: Broth microdilution MIC(50/90) of S.aureus and hVISA was 1.0/2.0 and 1.5/2.0 mg/L (p= 0.02), respectively. Macro Etest identified 12 (5.5%) hVISA isolates; higher among MRSA (9.1%) versus MSSA (2.5%) (p = 0.03). The mean modified PAP-AUC ratios (> 0.8) of 7 MRSA strains and 3 MSSA strains were significantly different (p = 0.001). 58% of hVISA strains were found to be agr dysfunctional when 21% of MRSA strains were agr dysfunctional. hVISA was detected among S. aureus bloodstream isolates, which were classified as susceptible among clinical microbiology laboratories.

Conclusions: Evaluating the correlation between Etest MICs and modified PAP-AUC ratio values will add further improvement of discriminating hVISA, and agr dysfunction may be predictive of strains which display a greater predilection to display the hVISA phenotype.