Monoclonal antibody-based dot-blot ELISA for the detection of Salmonella in foods

Monoclonal antibody (MAb) produced to polysaccharides in the LPS molecule of salmonellae was used in a dot-blot ELISA for detecting Salmonella in 873 food samples, ie 100 fresh chicken, 261 frozen chicken, 78 pork, 84 beef, 100 hen eggs, 100 duck eggs, 50 sea-mussels, 50 shrimps and 50 squids in comparison with the conventional culture method. Salmonella culture from foods involved the following steps: pre-enrichment, enrichment in selective medium, isolation on selective and indicator media, followed by biochemical and serological identification of appropriate colonies, respectively. The whole culture procedure took 5 days. Food samples from the selective enrichment medium were also subjected to the MAb-based dot-blot ELISA. The whole procedure of dot-blot ELISA took less than 2 hours. Among 873 food samples, salmonellae could be recovered from 7.4% of the samples by the bacterial isolation method (16% of fresh chicken, 8.8% of frozen chicken, 24.4% of pork, 3.6% of beef and 2% each of hen eggs and duck eggs, respectively). Salmonella derby were predominant among pork samples while S.paratyphi B biovar java predominated in chicken. The MAb-based dot-blot ELISA were positive in 19.5% of the food samples, i.e. 30% of fresh chicken, 27.6% of frozen chicken, 34.6% of pork, 21.4% of beef, 20% of shrimp, 16% of sea-mussels, 2% of hen eggs and 4% of duck eggs. The sensitivity and specificity of the MAb-based dot-blot ELISA compared to the bacterial culture method were 81.5% and 85%, respectively. The discrepancy of the data between the culture method and the dot-blot ELISA might be due to the fact that the culture method could detect only living cells at numbers that gave at least one isolated colony on the selective/differential plate while the dot-blot ELISA detects any form of Salmonella antigen. The monoclonal antibody-based dot-blot ELISA offers several advantages over the conventional bacterial culture method when it is used for screening of Salmonella contamination in foods, especially export foods. These include rapidity, cost-effectiveness and simplicity (the dot-blot ELISA does not need highly trained personnel or equipment, in contrast to the culture method). The test can be performed in field conditions and the result can be read visually. It also offers multisample analysis at one time which renders more samples of food for screening possible, thus false negative results are fewer which, in turn, assures the quality of the export food in a cost-saving, short time frame.