Vancomycin and ampicillin resistance among Enterococcus species, particularly E. faecium have been on a steady increase.
- The disk diffusion screening test is used in many laboratories to detect vancomycin resistant strains. Note in the upper image that 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 phenotypes - Van A, Van B, and Van C.
- Vancomycin-resistant strains of E. faecalis and E. faecium are commonly of the Van A phenotype, demonstrating high level resistance (MIC's higher than 64 ug/mL), as illustrated by total resistance of the test strain in the E test and the VA disk, as illustrated in the lower image.
- The strain shown in the lower image, however, is ampicillin susceptible at the level of 1 ug/mL (see lower set of yellow arrows), indicating that this drug may be effective in treating the urinary tract infection.
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 prior diagnosis of C. difficle
enteritis, individuals over the age of 60, and the presence of a hematologic malignancy.
Reference: Garbutt JM. Littenberg B. Evanoff BA. Sahm D. Mundy LM. Enteric carriage of vancomycin-resistant Enterococcus faecium in patients tested for Clostridium difficile. Infection Control & Hospital Epidemiology. 20(10):664-70, 1999.