Development of a selective medium to detect both vancomycin-resistant enterococci (VRE) and gram-negative bacilli (GNB) containing extended-spectrum beta-lactamases (ESBLs)

Antimicrobial drug resistance has become a global problem, and organisms such as VRE and GNB containing ESBLs are isolated with increasing frequency. Surveillance rectal cultures from high-risk pts facilitate early detection of these organisms, allowing the implementation of infection control procedures which may decrease nosocomial transmission. Currently, surveillance rectal swabs are obtained from high-risk patients at NMH on a weekly basis. These swabs are plated on media containing trypticase soy agar with 5% sheep blood supplemented with 20 μg/ml gentamicin (G) and 10 μg/ml vancomycin (V) [GV plates] to screen for VRE. We have modified this media to contain 2 μg/ml G, 10 μg/ml V, 2 μg/ml amphotericin (A), and 2 μg/ml ceftazidime (C) [GVAC plates] to detect both VRE and ESBL-containing GNB while limiting the overgrowth of other organisms. Results of 135 surveillance cultures have been compared using both the GV and GVAC plates. Of the 135 cultures, 11 (8.1%) had VRE and 9 (6.7%) had GNB containing ESBLs detected. The GV plates detected 8 and the GVAC 9 (73% vs. 82%; P=NS) of the VRE. There was one isolate that was detected on one plate but not the other. For the ESBLs, the GV plates detected 5 and the GVAC 9 (56% vs. 100%; P=.04). In summary, this new selective medium is equally efficacious at detecting colonization with VRE and is significantly more efficacious in detecting colonization with GNB containing ESBLs than our existing surveillance system. It represents a quick, simple, cost effective method for detecting both VRE and ESBL-containing GNB in hospitalized patients.