Purpose: To evaluate the Most Probable Number technique for the simultaneous quantification of Salmonella spp., Shigella spp. and Listeria monocytogenes on saladette tomato and serrano peppers.
Methods: A cocktail of three Salmonella spp., three Shigella spp. and three Listeria monocytogenes strains were used to inoculate the surface of saladette tomatoes and serrano peppers. Two levels of inoculum were evaluated: approximately 0-1 (low) and 1-2 (high) log CFU/ml. Peppers were inoculated with 50 µl each, and tomato with 100 µl. Composite samples (8 peppers or 4 tomatoes) were rinsed and massaged for 2 min in 400 ml of universal pre-enrichment broth. Serial decimal dilutions were prepared for MPN: 100, 10, 1 and 0.1 ml. The isolation of pathogens was performed according to FDA-BAM for Salmonella and Shigella and the USDA-FSIS for L. monocytogenes. Experiments were conducted in triplicate.
Results: The three pathogenic bacteria were simultaneously recovered from the tomato surface in concentrations ranging from 0.0036 to 0.2300 MPN/ml for Salmonella, <0.003-11 MPN/ml, for Shigella and <0.003-0.15 MPN/ml for Listeria monocytogenes, while these pathogens were recovered from serrano peppers at concentrations ranging from <0.003 to 0.21 MPN/ml, <0.003 to 0.15 MPN/ml and <0.003-0.036 MPN/ml, respectively.
Significance: The Most Probable Number technique tested is useful for the simultaneous quantification of Salmonella spp., Shigella spp. and Listeria monocytogenes in saladette tomatoes and serrano pepper.