From DNA sequence to application: possibilities and complications

The development of sophisticated genetic tools during the past 15 years have facilitated a tremendous increase of fundamental and application-oriented knowledge of lactic acid bacteria (LAB) and their bacteriophages. This knowledge relates both to the assignments of open reading frames (ORF’s) and the function of non-coding DNA sequences. Comparison of the complete nucleotide sequences of several LAB bacteriophages has revealed that their chromosomes have a fixed, modular structure, each module having a set of genes involved in a specific phase of the bacteriophage life cycle. LAB bacteriophage genes and DNA sequences have been used for the construction of temperature-inducible gene expression systems, gene-integration systems, and bacteriophage defence systems. The function of several LAB open reading frames and transcriptional units have been identified and characterized in detail. Many of these could find practical applications, such as induced lysis of LAB to enhance cheese ripening and re-routing of carbon fluxes for the production of a specific amino acid enantiomer. More knowledge has also become available concerning the function and structure of non-coding DNA positioned at or in the vicinity of promoters. In several cases the mRNA produced from this DNA contains a transcriptional terminator-antiterminator pair, in which the antiterminator can be stabilized either by uncharged tRNA or by interaction with a regulatory protein, thus preventing formation of the terminator so that mRNA elongation can proceed. Evidence has accumulated showing that also in LAB carbon catabolite repression in LAB is mediated by specific DNA elements in the vicinity of promoters governing the transcription of catabolic operons. Although some biological barriers have yet to be solved, the vast body of scientific information presently available allows the construction of tailor-made genetically modified LAB. Today, it appears that societal constraints rather than biological hurdles impede the use of genetically modified LAB.

Effects of suspension in emulsified wiener or incubation in wiener packages, on the virulence of L. monocytogenes Scott A in intragastrically inoculated A/J mice

Several outbreaks of listeriosis have been associated with contamination of wieners and other ready-to-eat meat products. In this study, we addressed the question of whether emulsification in, or growth on, wieners triggers a response in the listerial cells that makes them more virulent or protects them against the harsh environment of the gastrointestinal tract in mice. Our results indicate that Listeria monocytogenes Scott A grows poorly, if at all, in one brand of commercially prepared wieners inoculated with 5 × 103 to 5 × 106 CFU per package and incubated at 15°C. Neither L. monocytogenes Scott A emulsified in a slurry of homogenized wieners nor recovered from wiener package fluid after a 7-day incubation at 15°C were more virulent when inoculated into the stomachs of A/J mice than L. monocytogenes Scott A grown in brain heart infusion broth. These findings suggest that the ability of L. monocytogenes Scott A to cause systemic infection following introduction into the gastrointestinal tract was not improved by incubation with wieners or suspension in a meat matrix

Validation of Cooking Times and Temperatures for Thermal Inactivation of Yersinia pestis Strains KIM5 and CDC-A1122 in Irradiated Ground Beef

Irradiated ground beef samples (ca. 3-g portions with ca. 25% fat) inoculated with Yersina pestis strain KIM5 (ca. 6.7 log CFU/g) were heated in a circulating water bath stabilized at 48.9, 50, 52.5, 55, 57.5, or 60°C (120, 122, 126.5, 131, 135.5, and 140°F, respectively). Average D-values were 192.17, 34.38, 17.11, 3.87, 1.32, and 0.56 min, respectively, with a corresponding z-value of 4.67°C (8.41°F). In related experiments, irradiated ground beef patties (ca. 95 g per patty with ca. 25% fat) were inoculated with Y. pestis strains KIM5 or CDC-A1122 (ca. 6.0 log CFU/g) and cooked on an open-flame gas grill or on a clam-shell type electric grill to internal target temperatures of 48.9, 60, and 71.1°C (120, 140, and 160°F, respectively). For patties cooked on the gas grill, strain KIM5 populations decreased from ca. 6.24 to 4.32, 3.51, and ≤0.7 log CFU/g at 48.9, 60, and 71.1°C, respectively, and strain CDC-A1122 populations decreased to 3.46 log CFU/g at 48.9°C and to ≤0.7 log CFU/g at both 60 and 71.1°C. For patties cooked on the clam-shell grill, strain KIM5 populations decreased from ca. 5.96 to 2.53 log CFU/g at 48.9°C and to ≤0.7 log CFU/g at 60 or 71.1°C, and strain CDC-A1122 populations decreased from ca. 5.98 to ≤0.7 log CFU/g at all three cooking temperatures. These data confirm that cooking ground beef on an open-flame gas grill or on a clam-shell type electric grill to the temperatures and times recommended by the U.S. Department of Agriculture and the U.S. Food and Drug Administration Food Code, appreciably lessens the likelihood, severity, and/or magnitude of consumer illness if the ground beef were purposefully contaminated even with relatively high levels of Y. pestis