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Detection of salmonellas in animal feeds by electrical conductance

Smith, P.J., Boardman, A., Shutt, P.C.
Journal of applied microbiology 1989 v.67 no.6 pp. 575-588
Salmonella, cystine, electrical conductivity, feeds, glucose, lysine, lysine decarboxylase, mannitol, monitoring, operating costs, trimethylamine
A comparison was made between standard culture methods and electrical conductance using a Malthus AT Microbiological Analyser for the examination of animal feeds for salmonella. Conductance testing with a selenite cystine/trimethylamine-N-oxide/dulcitol medium resulted in the detection of salmonellas in 49 of 55 known positive animal feeds, 13 of 19 spiked feed samples and 36 of 47 salmonella cultures. Testing with a lysine decarboxylase/glucose medium gave significantly better results (P < 0 05) than with selenite cystine medium but five lysine decarboxylase negative strains of salmonella were undetected. When both media were used in parallel all salmonella positive samples were detected. No difference was found between pre-enrichment in buffered peptone water containing trimethylamine/mannitol and that containing lysine/glucose. Positive detection criteria for selenite medium of conductance peak at ≥ 500 microsiemens (μS) with a rate of change of ≥60 μS/h or 400-499 μS with a rate of change of ≥40 μS/h and for lysine medium with a peak of ≥100 μS have been established. The method offers savings in media and operating costs over conventional standard culture methods, provides results within 48 h and is recommended for statutory feed monitoring purposes.