Occult Vancomycin-Resistant Enterococcus faecium ST117 Displaying a Highly Mutated vanB₂ Operon

Area of Science:

• Medical Microbiology
• Infectious Diseases
• Antimicrobial Resistance

Background:

• Limited data exists on clinical Enterococcus faecium infections in Sardinia, Italy.
• Enterococcus faecium is a significant nosocomial pathogen, often developing vancomycin resistance.

Purpose of the Study:

• To investigate the antimicrobial susceptibility and genotypic characteristics of Enterococcus faecium isolates from Sassari, Italy.
• To evaluate the performance of Vitek2 system in detecting vancomycin resistance in Enterococcus faecium.

Main Methods:

• Antimicrobial susceptibility testing using Vitek2 system.
• Genotypic analysis including PCR, MLST, and Whole Genome Sequencing (WGS).
• Phenotypic confirmation of vancomycin resistance.

Main Results:

• Two vanB-type vancomycin-resistant Enterococcus faecium (VREfm) isolates were identified.
• Vitek2 system failed to detect vancomycin resistance in one vanB-positive isolate.
• All identified isolates possessed mutated vanB2 operons within a Tn1549 transposon.
• Molecular typing revealed a polyclonal population, with ST117 being common among VREfm and vancomycin-susceptible E. faecium (VSEfm).

Conclusions:

• The Vitek2 system may not reliably detect all vanB-carrying VREfm isolates.
• Mutations in the vanB2 operon contribute to vancomycin resistance.
• Alternative diagnostic methods are essential for accurate VREfm detection to prevent treatment failure and transmission.