A conductance medium to distinguish between Salmonella and Citrobacter spp

Published methods for detecting Salmonella by conductance methods yield some Citrobacter spp. as false positives. A medium is described which, when used in conjunction with selenite-cystine/trimethylamine oxide/mannitol (SC/T/M) resolves this difficulty. It is based on the ability of Salmonella to decarboxylate lysine in the presence of selenite (4 g/l), after pre-enrichment in buffered peptone water supplemented with glucose and lysine for 7–17 h. Salmonella produced a conductance change of 250–400 μS in 24 h with a maximum rate of conductance change of 10–15 μS/10 min. 299 samples were tested and the only false positives were due to Hafnia alvei which gave no conductance response in the SC/T/M medium. Using both media in a two tube system would lead to a rapid clearance of Salmonella negative samples.