A large outbreak of Salmonella Saintpaul associated with raw jalapeño peppers, serrano peppers, and possibly tomatoes was reported in the United States in 2008. During the outbreak, two clusters of illness investigated among restaurant patrons were significantly associated with eating salsa. Experiments were performed to determine the survival and growth characteristics of Salmonella in salsa and related major ingredients, i.e., tomatoes, jalapeño peppers, and cilantro. Intact and chopped vegetables and different formulations of salsas were inoculated with a five-strain mixture of Salmonella and then stored at 4, 12, and 21 degrees C for up to 7 days. Salmonella populations were monitored during storage. Salmonella did not grow, but survived on intact tomatoes and jalapeño peppers, whereas significant growth at 12 and 21 degrees C was observed on intact cilantro. In general, growth of Salmonella occurred in all chopped vegetables when stored at 12 and 21 degrees C, with chopped jalapeño peppers being the most supportive of Salmonella growth. Regardless of differences in salsa formulation, no growth of Salmonella (initial inoculation ca. 3 log CFU/g) was observed in salsa held at 4 degrees C; however, rapid or gradual decreases in Salmonella populations were only observed in formulations that contained both fresh garlic and lime juice. Salmonella grew at 12 and 21 degrees C in salsas, except for those formulations that contained both fresh garlic and lime juice, in which salmonellae were rapidly or gradually inactivated, depending on salsa formulation. These results highlight the importance of preharvest pathogen contamination control of fresh produce and proper formulation and storage of salsa.

In search of a suitable surrogate microorganism for in-plant critical control point validation, we compared the rates of thermal inactivation of three bacteria, Enterococcus faecium B2354, Pediococcus parvulus HP, and Pediococcus acidilactici LP, to those of Listeria monocytogenes and Salmonella. Ground beef samples containing 4 and 12% fat were inoculated with E. faecium, L. monocytogenes, and Salmonella Senftenberg 775W and heated at 58, 62, 65, or 68 degrees C. The decimal reduction times (D-values) for E. faecium B2354 in 4 and 12% fat ground beef were 4.4 to 17.7 and 3.6 to 14.6 times greater, respectively, than those for L. monocytogenes or Salmonella Senftenberg 775W at all temperatures tested, with the greatest differences in D-values occurring at 58 and 62 degrees C. Higher fat content protected bacteria from thermal inactivation in general, especially at temperatures lower than 68 degrees C. The heat resistance in a broth medium at 62degrees C of two food-grade bacteria, P. parvulus HP and P. acidilactici LP, was compared with that of the three strains under study. The D-values of P. parvulus HP and P. acidilactici LP were lower than those of E. faecium B2354 but 4.1 and 2.5 times greater, respectively, than those of Salmonella Senftenberg 775W, the most resistant pathogen. These results indicate that thermal treatments of ground beef at 58 to 68 degrees C that kill E. faecium B2354 will also kill Salmonella and L. monocytogenes, and the two Pediococcus isolates may serve as alternate surrogates for validation studies when a less heat-resistant surrogate is desired. However, additional studies in ground beef are needed with the Pediococcus strains in the desired temperature range intended for validation purposes.

Aerosol studies of Listeria monocytogenes in food processing plants have been limited by lack of a suitable surrogate microorganism. The objective of this study was to investigate the potential of using green fluorescent protein-labeled strains of Listeria innocua as a surrogate for L. monocytogenes for aerosol studies. These studies were conducted in a laboratory bioaerosol chamber and a pilot food-processing facility. Four strains of L. innocua and five strains of L. monocytogenes were used. In the laboratory chamber study, Listeria cells were released into the environment at two different cell numbers and under two airflow conditions. Trypticase soy agar (TSA) plates and oven-roasted breasts of chicken and turkey were placed in the chamber to monitor Listeria cell numbers deposited from aerosols. A similar experimental design was used in the pilot plant study; however, only L. innocua was used. Results showed that L. monocytogenes and L. innocua survived equally well on chicken and turkey breast meats and TSA plates. No-fan and continuous fan applications, which affected airflow, had no significant effect on settling rates of aerosolized L. monocytogenes and L. innocua in the bioaerosol chamber or L. innocua in the pilot plant study. Listeriae cell numbers in the air decreased rapidly during the first 1.5 h following release, with few to no listeriae detected in the air at 3 h. Aerosol particles with diameters of 1 and 2 microM correlated directly with the number of Listeria cells in the aerosol but not with particles that were 0.3, 0.5, and 5 microM in diameter. Results indicate that L. innocua can be used as a surrogate for L. monocytogenes in an aerosol study.

Rapid and sensitive detection of Salmonella is a critical step in routine food quality control, outbreak investigation, and food recalls. Although various genes have been the targets in the design of rapid molecular detection methods for Salmonella, there is limited information on the diversity of these target genes at the level of DNA sequence and the encoded protein structures. In this study, we investigated the diversity of ten target genes (invA, fimA, phoP, spvC, and agfA; ttrRSBCA operon including 5 genes) commonly used in the detection and identification of Salmonella. To this end, we performed whole genome sequencing of 143 isolates of Salmonella serotypes (Enteritidis, Typhimurium, and Heidelberg) obtained from poultry (eggs and chicken). Phylogenetic analysis showed that Salmonella ser. Typhimurium was more diverse than either Enteritidis or Heidelberg. Forty-five non-synonymous mutations were identified in the target genes from the 143 isolates, with the two most common mutations as T ↔ C (15 times) and A ↔ G (13 times). The gene spvC was primarily present in Salmonella ser. Enteritidis isolates and absent from Heidelberg isolates, whereas ttrR was more conserved (0 non-synonymous mutations) than ttrS, ttrB, ttrC, and ttrA (7, 2, 2, and 7 non-synonymous mutations, respectively). Notably, we found one non-synonymous mutation (fimA-Mut.6) across all Salmonella ser. Enteritidis and Salmonella ser. Heidelberg, C → T (496 nt postion), resulting in the change at AA 166 position, Glutamine (Q) → Stop condon (TAG), suggesting that the fimA gene has questionable sites as a target for detection. Using Phyre2 and SWISS-MODEL software, we predicted the structures of the proteins encoded by some of the target genes, illustrating the positions of these non-synonymous mutations that mainly located on the α-helix and β-sheet which are key elements for maintaining the conformation of proteins. These results will facilitate the development of sensitive molecular detection methods for Salmonella.