Which Enterococcal Species Are Capable of VRE
The two primary enterococcal species capable of vancomycin resistance are Enterococcus faecium and Enterococcus faecalis, with E. faecium being far more commonly resistant (up to 95% of VRE isolates) while only approximately 5% of E. faecalis strains develop vancomycin resistance. 1, 2
Primary VRE Species
Enterococcus faecium
- Accounts for 10-15% of all enterococcal isolates but represents the majority of VRE cases 2
- Up to 95% of vancomycin-resistant enterococci are E. faecium, expressing multiple resistance patterns including vancomycin, aminoglycosides, and penicillins 3
- More likely to be multidrug-resistant compared to E. faecalis 3
- The VanA and VanB resistance genes are found primarily in this species 1, 3
Enterococcus faecalis
- Most prevalent enterococcal species overall (85-89% of enterococcal isolates) but only 5% develop vancomycin resistance 2
- When resistance occurs, it typically involves the VanA or VanB phenotypes 1
- Less commonly multidrug-resistant than E. faecium 3
Intrinsically Resistant Species (VanC)
Two enterococcal species possess intrinsic low-level vancomycin resistance through the VanC gene:
- E. casseliflavus - causes low to intermediate-level resistance with different clinical implications than acquired VanA/B resistance 1
- E. gallinarum - similarly expresses intrinsic VanC-mediated resistance 1
These species are clinically less significant and their intrinsic resistance does not carry the same transmission or public health concerns as acquired resistance in E. faecium and E. faecalis 1
Resistance Mechanisms
The VanA phenotype confers high-level resistance (MIC ≥64 μg/mL) to both vancomycin and teicoplanin, and is frequently plasmid-borne, allowing potential transfer to other gram-positive bacteria including Staphylococcus aureus 1, 4
The VanB phenotype also causes clinically significant resistance but may be more difficult to detect with automated laboratory methods 1
Clinical Pitfall
Automated laboratory methods frequently fail to detect vancomycin resistance, particularly the VanB phenotype with moderate resistance levels, potentially leading to significant underreporting and delayed implementation of infection control measures 1, 5