Heterogeneous vancomycin-intermediate among methicillin resistant Staphylococcus aureus

Area of Science:

• Clinical microbiology
• Infectious diseases
• Antimicrobial resistance

Background:

• Hetero-resistance vancomycin intermediate Staphylococcus aureus (hVISA) presents a challenge as it tests susceptible (VSSA) but harbors intermediate (VISA) populations, leading to vancomycin treatment failures.
• Population Analysis Profile-Area under Curve (PAP-AUC) is a gold standard for hVISA detection but is impractical for routine clinical laboratories.
• Alternative methods like Brain Heart Infusion Agar with 6 μg/ml vancomycin (BHIA6V), E test, and Macromethod E test (MET) have shown variable performance in identifying hVISA.

Purpose of the Study:

• To evaluate the performance of PAP-AUC, BHIA6V, MET, and E test in detecting hVISA among clinical Methicillin-resistant S. aureus (MRSA) isolates.
• To determine the prevalence of hVISA in a cohort of MRSA isolates.
• To assess the clinical utility of available screening methods for hVISA detection.

Main Methods:

• Analysis of 58 clinical MRSA isolates with vancomycin MIC >1 μg/ml using E test and agar dilution.
• Comparison of PAP-AUC, BHIA6V, and MET methods for hVISA detection.
• Correlation of PAP-AUC ratios with vancomycin MICs obtained from E test and MET.

Main Results:

• The prevalence of hVISA was determined to be 6.9% among the studied MRSA isolates.
• hVISA isolates exhibited vancomycin E test MICs >2 μg/ml.
• Sensitivities for BHIA6V, MET, and E test (MIC >2 μg/ml) were 0.75, 0.67, and 1.0, respectively, with corresponding positive predictive values (PPV) of 0.43, 0.4, and 0.27 when compared to PAP-AUC.

Conclusions:

• Screening of MRSA isolates with in-vitro vancomycin susceptibility ≤2 μg/ml by agar dilution is recommended for hVISA detection.
• PAP-AUC is not suitable for routine laboratory diagnostics due to its impracticality.
• BHIA6V, MET, and E tests can serve as screening tools for hVISA, but their low PPVs necessitate careful interpretation and further confirmatory testing.