Neomycin blood agar as a selective medium for vancomycin resistant Enterococcus faecium

Neomycin blood agar is commonly used as a selective medium for the isolation of vancomycin resistant enterococci from faeces; however, not all isolates are recovered using this medium, perhaps because the neomycin concentrations are too high. To test this hypothesis, the neomycin minimum inhibitory concentration (MIC) was determined for 27 vancomycin resistant Enterococcus faecium isolates, 14 from patients with leukaemia 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 the patients with leukaemia showed equal recovery on neomycin agar and blood agar and had MICs > 64 mg/l. In three other isolates there was a 4log10 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 log10 reduction in recovery. All eight faecal isolates recovered from patients on the renal unit on neomycin agar were highly resistant to neomycin (MIC > 64 mg/l). Comparative studies of screening media are urgently needed as vancomycin resistant enterococci become more prevalent nosocomial pathogens.