Rapid, sensitive, and validated method for detection of Salmonella in food by an enrichment broth culture – Nested PCR combination assay

A rapid nested PCR assay for detection of Salmonella from food was developed. The sensitivity of the assay developed was comparable to the traditional culture based methods with an advantage in reduction of assay time. The assay procedure with artificially contaminated samples was able to detect as low as 4 CFU Salmonella/25 g of food samples (sprout, carrot, cucumber and poultry meat). With two synthetic primers of 26 mer TS11 and 25 mer TS4, a 1.2 kb fragment was amplified which served as a template for amplification of final 375 bp product using TS11 and TS5 primers. No non-specific amplification from the native microbial flora of food samples was observed. The reaction generates a single band specific to Salmonella which allows the analyst to interpret data at ease and without any confusion. Enriched broth serves as template for the reaction which removes labour intensive DNA isolation procedures. In case of artificially contaminated samples, 6 h enriched lactose broth can serve as template. However, for market samples where the organisms are under environmental stress, it is desirable to use template from Rappaport Vasiliadis medium. The assay also employes internal amplification control, which is amplified into a 300 bp fragment and thus serves as positive control for the reaction and any possibility of false negative due to inhibitory action of food components on PCR reaction can be ruled out.