Role of Enrichment Period for PCR Detection of Salmonella in Seafood

This communication presents the results of a study initiated to understand the role of enrichment period for detection of Salmonella by PCR assay. A total of 159 fresh and raw seafood (fish, shrimp, clam, crab, mussel, edible oyster, cuttlefish, and lobster) samples were analyzed for presence of Salmonella with the 0, 6, 12, 24, 36, and 48 h enrichment periods prior to PCR assay. The assay was performed with Salmonella specific PCR that amplifies a 284 bp invA gene fragment. Results obtained from PCR assay were compared with conventional culture method. The sensitivity of PCR assay was confirmed in spiked seafood samples with Salmonella cells in the range 2 to 2x106cfu/25 g. Inhibition of seafood matrix on PCR was also determined. All seafood samples detected negative for Salmonella at 0 h PCR, whereas, 35.2% seafood samples found to be positive for Salmonella by 24 h PCR assay. Thereafter, incremental increase in enrichment period did not significantly enhance the detection efficiency of PCR assay. A total of 57/159 seafood samples were positive by PCR assay, whereas, 37/159 were positive by culture method. The sensitivity of PCR assay was found to be at 2 cfu/25 g level with the exception of cuttlefish samples. Detection limit of Salmonella dead cells was 3x106cfu/250 ml in fish homogenate by 24 h enrichment followed by PCR assay. The 24 h enrichment preceeding the PCR assay can be used as an alternative and rapid technique for detection of Salmonella that gives greater efficiency and sensitivity as compared to the conventional method. Present study highlighted the remote possibility of Salmonella dead cells encountering as false positive in natural contaminated seafood samples. This PCR assay can be used in the routine analysis of seafood samples and would be an ideal step for implementation of seafood safety measure at harvest and post-harvest level.