NEOMYCIN BLOOD AGAR AS A SELECTIVE MEDIUM FOR VANCOMYCIN-RESISTANT ENTEROCOCCUS-FAECIUM

Neomycin blood agar is commonly used as a selective medium for the iso lation of vancomycin resistant enterococci from faeces; however, not a ll isolates are recovered using this medium, perhaps because the neomy cin concentrations are too high. To test this hypothesis, the neomycin minimum inhibitory concentration (MIC) was determined for 27 vancomyc in resistant Enterococcus faecium isolates, 14 from patients with leuk aemia and 13 from patients on the renal unit, A further eight isolates that had been recovered from the faeces of patients on the renal unit on neomycin agar were also studied. Eleven of the 14 isolates from th e patients with leukaemia showed equal recovery on neomycin agar and b lood agar and had MICs > 64 mg/l. In three other isolates there was a 4 log(10) reduction in recovery on neomycin agar and the MIC was 8 mg/ l. Two of the non-selected isolates from the renal unit were recovered equally on the two media, the other 11 isolates showed a 4-5 log(10) reduction in recovery. All eight faecal isolates recovered from patien ts on the renal unit on neomycin agar were highly resistant to neomyci n (MIC > 64 mg/l). Comparative studies of screening media are urgently needed as vancomycin resistant enterococci become more prevalent noso comial pathogens.