An ion-exchange based extraction method for the detection of salmonellas in soil

A method that uses a cation-exchange resin (Chelex 100) and differential centrifugation for the extraction and detection of salmonellas in soil was developed. The extraction efficiencies of a range of materials were examined and Chelex plus polyethylene glycol was identified as the best combination. Shake speeds, shake times and differential centrifugation speeds were selected to give an optimum salmonella recovery. The Chelex method accurately enumerated 1 cell per 10 g of nonsterile soil within 24 h. Addition of glycerol to soil samples enabled storage at — 70°C for 85 d without significant decreases in salmonella numbers. The Oxoid Salmonella Rapid Test (SRT) could be used to pre-screen large numbers of soil samples for the presence of salmonellas, prior to analysis by the Chelex method. The SRT method detected Salmonella typhimurium at levels as low as 2·5 cells per 10 g of nonsterile soil.