[Evaluation of the Utility of 72-hour Cultivation in Vancomycin-resistant Enterococci (VRE) Screening Medium]

Vancomycin-resistant Enterococcus (VRE) infections are classified as a Category V infectious disease under the Infectious Diseases Control Law. The reporting criteria are VCM MIC ≧16 µg/mL, and the presence of van is not included, but vanA/B are important resistance genes for infection control. Since some VRE strains with van genes have low VCM MIC, screening medium are useful for detection; however, culture time varies depending on the medium, and the content of VCM and selective agents is not clear. In this study, we evaluated the optimal culture time for CHROMagar VRE blue medium (CH-blue, Kanto Chemical) and BD BBL VRE medium (BD-VRE, BD Japan). A total of 29 strains were tested, including 24 Enterococcus faecium strain with vanA/B/D (VCM MIC 8->16 µg/mL) and one Enterococcus faecalis strain with the vanA (VCM MIC 8 µg/mL) detected in our hospital, and 4 standard strains. According to the Misra method, each medium was inoculated with 10⁶-10 cfu/spot. Cultures were incubated at 35°C under aerobic conditions for 72 hours, with growth checked every 24 hours. CH-blue showed no concentration with a 100% detection rate in the specified 24-hour incubation, whereas BD-VRE showed a 100% detection rate only at 10⁶ cfu/spot in the specified 48-hour incubation. Both media showed a 100% detection rate at 10⁶ and 10⁵ cfu/spot in the 72-hour incubation. The detection sensitivity was improved with a 72-hour incubation for both media, potentially reducing the risk of missing VRE in routine examinations.