Vancomycin MIC Distribution among Methicillin-Resistant Staphylococcus Aureus. Is Reduced Vancomycin Susceptibility...

Area of Science:

• Clinical Microbiology
• Infectious Diseases
• Antimicrobial Resistance

Background:

• Methicillin-Resistant Staphylococcus aureus (MRSA) poses a significant treatment challenge, often requiring vancomycin as a last resort.
• Reduced vancomycin susceptibility in MRSA, including vancomycin-intermediate S. aureus (VISA) and heterogeneous VISA (hVISA), complicates treatment and necessitates accurate detection methods.
• Current methods for detecting vancomycin resistance in MRSA may not identify all resistant strains, potentially leading to treatment failures.

Purpose of the Study:

• To determine the distribution of vancomycin Minimum Inhibitory Concentration (MIC) values among MRSA isolates.
• To evaluate the frequency of MRSA strains exhibiting reduced vancomycin susceptibility.
• To assess the efficacy of different methods, including vancomycin screening agar, for detecting vancomycin resistance in MRSA.

Main Methods:

• One hundred MRSA isolates were tested using broth microdilution (BMD) for vancomycin MIC determination.
• Vancomycin screening agar with varying vancomycin concentrations, with and without casein, was employed to detect reduced susceptibility.
• The Vitek 2 system was also utilized for comparison in identifying vancomycin resistance.

Main Results:

• BMD identified 22% VISA and 78% VSSA, but failed to detect 9% hVISA isolates with MIC ≤ 2 µg/ml.
• Vancomycin screening agar detected hVISA (MIC ≤ 2 µg/ml) and VISA (MIC 4-6 µg/ml), with a 95.45% overall detection rate for VISA using 6 µg/ml concentration.
• Adding casein to screening agar improved VISA detection by 4.5%, and probable 'pre-hVISA' isolates (17%) were identified using 2 µg/ml with casein; Vitek 2 failed to detect any VISA.

Conclusions:

• Vancomycin screening agar, particularly with specific concentrations (2 µg/ml with casein for pre-hVISA, 4-6 µg/ml for hVISA/VISA), accurately detects reduced vancomycin susceptibility in MRSA.
• The Vitek 2 system demonstrated limitations in detecting VISA and hVISA isolates.
• Observed decreases in vancomycin susceptibility may be linked to 'MIC creep,' warranting long-term monitoring to understand its impact on treatment outcomes.