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A strain of Shigella boydii 18 involved in a bean salad outbreak and S. boydii 18 ATCC 35966 were used to inoculate bean salad. Bean salad samples were stored at 4 or 23 degrees C. At 4 degrees C, the S. boydii survived for the duration of the shelf life of the salad but did not grow. At 23 degrees C, both strains increased by two orders of magnitude by day 2 and decreased rapidly thereafter. This demonstrates the importance of proper storage in preventing the outgrowth of foodborne pathogens.

Shigella boydii CDPH (Chicago Department of Public Health) serotype 18 was implicated in an outbreak of foodborne illness in 1998. The suspected food vehicles were parsley and cilantro imported from Mexico used to prepare bean salad. Previous studies revealed that S. boydii CDPH serotype 18 can survive in bean salad, which contains organic acids and whose pH decreases over time. Acid challenge assays in acidified tryptic soy broth at pH 4.5, acidified Luria-Bertani broth at pH 4.5, and acidified M9 minimal salts medium at pH 2.5 containing amino acids, arginine, or glutamic acid were performed using S. boydii CDPH, S. boydii ATCC 35966, S. flexneri 3136, Escherichia coli O157:H7 dd8872, and E. coli O157:H7 dd642 to compare differences in acid tolerance. Differences in survival of exponential-phase cells were detected in acidified tryptic soy broth and Luria-Bertani broth at pH 4.5. In acidified minimal medium containing arginine, S. boydii strains were able to survive at pH 2.5. The arginine decarboxylase gene (adiA) present in S. boydii is involved in survival at extremely low pH. The discovery of adiA expression in S. boydii serotype 18 by use of an acidified minimal medium challenge and arginine decarboxylase biochemical assay is significant because arginine decarboxylase activity was thought to be unique to E. coli. Sequencing of the rpoS gene from the S. boydii outbreak strain indicates that it is 99% conserved compared with the E. coli K-12 rpoS gene and plays a vital role in survival under acidic conditions.

A monoclonal antibody-based ELISA was developed to detect alpha-toxin present in Clostridium perfringens bacterial cell lysates and cell-free culture supernatants, Monoclonal antibodies against C. perfringens alpha-toxin were produced in hybridoma tissue culture supernatants and in BALB/c mice ascites fluid, The monoclonal antibodies obtained from hybridoma culture supernatant and ascites fluid showed identical antigen specificity, but the latter showed a higher titer, with a 50% endpoint at 1/4,000. The monoclonal antibodies were specific for phospholipase C produced by C. perfringens , but not by Bacillus cereus . The lower limit of phospholipase C detection was 16 ng/ml. The dose-dependent relationship between absorption at 490 nm and concentration of phospholipase C diluted in HEPES ( N -2-hydroxyethylpiperazine- N' -2-ethanesulfonic acid) or Trypticase glucose yeast broth fit a four-parameter and a quadratic model, respectively. The monoclonal antibody-based ELISA developed is a rapid, sensitive and specific detection method and can be used for quantitative characterization of C. perfringens alpha-toxin.

Precooked beef loin chunks were inoculated separately with three strains of Listeria monocytogenes (Scott A, 101M, and 103M). Uninoculated chunks served as controls. All chunks were vacuum packaged after inoculation. Half were not pasteurized and half were pasteurized in 85°C water for 16 min. All samples were stored at 4°C for up to 85 d and examined periodically. Pasteurization reduced all microflora and significantly reduced populations of three strains of L. monocytogenes on the surface and in the broth of the precooked beef chunks for 85 d of refrigerated storage as determined by direct plating procedures. All three strains of L. monocytogenes were recovered from the inoculated pasteurized beef using enrichment. Uninoculated chunks were positive for Listeria spp. which were primarily Listeria welshimeri . Without pasteurization, microflora increased 5- to 6-fold within 14 d of storage. Populations of Scott A and 101M strains of L. monocytogenes increased significantly within 7 d. After 14 d, populations of all three strains did not differ from initial levels and remained unchanged for the remainder of the storage period.