Rapid and automated detection of salmonella by electrical measurements

A rapid method for determining the presence of salmonella in food is described. It consists of pre-enrichment in buffered peptone water modified by the addition of dulcitol and trimethylamine oxide, followed by selective enrichment in a selenite-cystine broth with similar modifications. Changes in the conductance of the selective enrichment broth are monitored continuously using a suitable impediometric instrument. Most of the Salmonella spp. tested gave a fast (approximately 100 microS/h) and large (greater than 600 microS) change in conductance, other enteric bacteria much less or no change. The assay is usually complete within 24 h. Samples of foodstuffs, naturally and artificially contaminated with Salmonella spp., were all correctly classified. Some strains of Citrobacter freundii produced a false positive conductance response, and they could not be selectively eliminated using antibiotics or cyanide. The conductance method is simple and easy to use, gives rapid results and involves less media and subculturing than is required for traditional methods.