Detection of Vancomycin Resistance among Enterococcus faecalis and Staphylococcus aureus

Area of Science:

• Clinical Microbiology
• Infectious Diseases
• Antimicrobial Resistance

Background:

• Vancomycin is crucial for treating resistant Gram-positive infections, including MRSA and VRE.
• Increasing vancomycin use has led to reduced susceptibility (MIC creep) and the emergence of VISA, VRSA, and VRE.
• These resistant strains are significant nosocomial pathogens causing treatment failures.

Purpose of the Study:

• To detect vancomycin resistance in clinical isolates of Staphylococcus aureus and Enterococcus faecalis.
• To compare phenotypic (disk diffusion, MIC, D test) and genotypic (PCR) methods for resistance detection.
• To identify the presence of van A and van B genes associated with vancomycin resistance.

Main Methods:

• Phenotypic identification and antibiotic susceptibility testing of 109 Staphylococcus aureus and 124 Enterococcus faecalis isolates.
• Determination of vancomycin Minimum Inhibitory Concentration (MIC) using agar dilution.
• Genotypic detection of van A and van B genes using Polymerase Chain Reaction (PCR).

Main Results:

• No vancomycin resistance was observed in Staphylococcus aureus isolates by MIC testing; however, MIC creep was noted (MIC50 = 1 μg/mL).
• None of the Staphylococcus aureus isolates harbored van A or van B genes.
• 12.9% of Enterococcus faecalis isolates showed vancomycin resistance by disk diffusion.
• Van A and van B genes were detected in 8% of Enterococcus faecalis isolates, with one isolate having both genes.

Conclusions:

• PCR is the gold standard for diagnosing vancomycin resistance.
• While vancomycin resistance was absent in Staphylococcus aureus, MIC creep warrants monitoring.
• A significant proportion (8%) of Enterococcus faecalis isolates were confirmed as vancomycin-resistant via genotypic analysis.