The detection of Salmonella enteritidis and S. typhimurium using immunomagnetic separation and conductance microbiology.

A rapid method for detection of Salmonella in milk powder is described. The technique involves immunomagnetic separation of Salmonella from pre-enrichment broths using new commercially-available materials, and detection using conductance measurements. Salmonella detection was enhanced by reducing the number and types of competing bacteria present and concentrating the number of Salmonella in the final assay. After a 6 h pre-incubation period Salmonella enteritidis, from an initial inoculum size of 20 cells/ml, were detected in 7.5 h by conductance.

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.

Modelling the effect of pH, acidulant and temperature on the growth rate of Yersinia enterocolitica.

Growth of two pathogenic and one environmental serotype of Yersinia enterocolitica under acidic conditions and at 4 and 25 degrees C was investigated. At both temperatures the maximum growth inhibitory pH depended on the acidulant used and was in the order acetic greater than lactic greater than citric greater than sulphuric. At the lower temperature the maximum growth inhibitory pH was 0.3-0.5 pH units higher than at 25 degrees C. No difference was observed between the behaviour of pathogenic and environmental serotypes in this respect. Measurement of growth at a number of sub-optimal temperatures and pH values showed that the variation of growth rate with temperature could be represented by a square root plot. The effect of different pH values could be incorporated into the model by replacing the regression coefficient b by its relationship with pH. Values of maximum growth inhibitory pH derived from the model were in good agreement with experimental values with the exception of acetic acid.