In vitro detection of enterococcal vancomycin resistance

Area of Science:

• Microbiology
• Clinical Diagnostics
• Antimicrobial Resistance

Background:

• Enterococci are a significant cause of nosocomial infections.
• Vancomycin resistance in enterococci (VRE) is a growing public health concern.
• Accurate detection of VRE is crucial for effective treatment and infection control.

Purpose of the Study:

• To evaluate the performance of different methods in detecting varying levels of vancomycin resistance in enterococcal isolates.
• To compare agar dilution, broth microdilution, disk diffusion, and an automated system (AMS Vitek) for VRE detection.

Main Methods:

• Agar dilution was used as the gold standard method.
• Broth microdilution, disk diffusion, and AMS Vitek were tested against six enterococcal isolates with known vancomycin resistance levels.
• Standard and modified (increased inoculum or prolonged incubation) protocols were employed.

Main Results:

• Low-level vancomycin resistance (MIC 16-32 µg/mL) was only detected by agar and broth dilution.
• Moderate-to-high level resistance (MIC 128-256 µg/mL) was missed by disk diffusion and AMS Vitek under standard conditions.
• Modified protocols (10-fold increased inoculum) improved detection for disk diffusion and AMS Vitek for moderate-to-high resistance.
• High-level resistance (MIC 2048 µg/mL) was detected by all methods.

Conclusions:

• The ability of different methods to detect vancomycin resistance in enterococci varies significantly.
• Standard disk diffusion and automated systems may lead to underestimation of VRE, especially for low-to-moderate resistance.
• Variations in detection capabilities can complicate VRE incidence monitoring and lead to critical susceptibility reporting errors.