Vancomycin and ampicillin resistance among Enterococcus species, particularly E. faecium, have steadily increased.
- The disk diffusion screening test is used in many laboratories to detect vancomycin-resistant strains. In the upper image, no zone of inhibition is seen around either the vancomycin or the ampicillin disk, indicating resistance to both drugs.
- Vancomycin-resistant enterococci (VRE) have been divided into three most commonly described phenotypes: VanA, VanB, and VanC. VanA and VanB are acquired resistance, while VanC is intrinsic resistance, as seen in E. gallinarum and E. cassiflavus.
- High-level aminoglycoside resistance (HLAR) screening is another significant resistance factor; increasing concentrations of gentamicin and/or streptomycin are used in a unique test protocol.
- Vancomycin-resistant strains of E. faecalis and E. faecium are common in the acquired VanA or VanB phenotype, demonstrating high-level resistance (MICs higher than 32 μg/mL), as illustrated by the total resistance of the test strain in the E test and the VA disk, shown in the lower image.
- The strain shown in the lower image, however, is ampicillin susceptible at 1 μg/mL (see lower set of yellow arrows), indicating that this drug may effectively treat urinary tract infection.
Note: Always refer to current CLSI guidelines before interpreting results.
Studies show that antimicrobial exposures are the most important modifiable independent risk factors for enteric carriage of vancomycin-resistant
E. faecium (VREF) in hospitalized patients tested for
C. difficile. Additional risk factors of VREF carriage that are less contributory than prior antibiotic usage include a previous diagnosis of
C. difficile enteritis, individuals over the age of 60, and the presence of a hematologic malignancy (Garbutt et al., 1999).
