Validation of cooking times and temperatures for thermal inactivation of Yersinia pestis strains KIM5 and CDC-A1122 in irradiated ground beef

Inactivation of Avian Influenza Virus Inoculated into Ground Beef Patties Cooked on a Commercial Open-Flame Gas Grill

With the emergence of clade 2.3.4.4b H5N1 highly pathogenic avian influenza virus (AIV) infection of dairy cattle and its subsequent detection in raw milk, coupled with recent AIV infections affecting dairy farm workers, experiments were conducted to affirm the safety of cooked ground beef related to AIV because such meat is often derived from cull dairy cows. Specifically, retail ground beef (percent lean:fat = ca. 80:20) was inoculated with a low pathogenic AIV (LPAIV) isolate to an initial level of 5.6 log10 50% egg infectious doses (EID50)  per 300 g patty. The inoculated meat was pressed into patties (ca. 2.54 cm thick, ca. 300 g each) and then held at 4 °C for up to 60 min. In each of the two trials, two patties for each of the following three treatments were cooked on a commercial open-flame gas grill to internal instantaneous temperatures of 48.9 °C (120°F), 62.8 °C (145°F), or 71.1 °C (160°F), but without any dwell time. Cooking inoculated ground beef patties to 48.9 °C (ave. cooking time of ca. 15 min) resulted in a mean reduction of ≥2.5 ± 0.9 log10 EID50 per 300 g of ground beef as assessed via quantification of virus in embryonating chicken eggs (ECEs). Likewise, cooking patties on a gas grill to 62.8 °C (ave. cooking time of ca. 21 min) or to the USDA FSIS recommended minimum internal temperature for ground beef of 71.1 °C (ave. cooking time of ca. 24 min) resulted in a reduction to nondetectable levels from initial levels of ≥5.6 log10 EID50 per 300 g. These data establish that levels of infectious AIV are substantially reduced within inoculated ground beef patties (20% fat) using recommended cooking procedures.

Two fatal cases of plague after consumption of raw marmot organs

Marmots are an important reservoir of Yersinia pestis and a source of human plague in Mongolia. We present two fatal cases of plague after consumption of raw marmot organs and discuss the distribution of natural foci of Y. pestis in Mongolia.

Yersinia pestis: the Natural History of Plague

The Gram-negative bacterium Yersinia pestis is responsible for deadly plague, a zoonotic disease established in stable foci in the Americas, Africa, and Eurasia. Its persistence in the environment relies on the subtle balance between Y. pestis-contaminated soils, burrowing and nonburrowing mammals exhibiting variable degrees of plague susceptibility, and their associated fleas. Transmission from one host to another relies mainly on infected flea bites, inducing typical painful, enlarged lymph nodes referred to as buboes, followed by septicemic dissemination of the pathogen. In contrast, droplet inhalation after close contact with infected mammals induces primary pneumonic plague. Finally, the rarely reported consumption of contaminated raw meat causes pharyngeal and gastrointestinal plague. Point-of-care diagnosis, early antibiotic treatment, and confinement measures contribute to outbreak control despite residual mortality. Mandatory primary prevention relies on the active surveillance of established plague foci and ectoparasite control. Plague is acknowledged to have infected human populations for at least 5,000 years in Eurasia. Y. pestis genomes recovered from affected archaeological sites have suggested clonal evolution from a common ancestor shared with the closely related enteric pathogen Yersinia pseudotuberculosis and have indicated that ymt gene acquisition during the Bronze Age conferred Y. pestis with ectoparasite transmissibility while maintaining its enteric transmissibility. Three historic pandemics, starting in 541 AD and continuing until today, have been described. At present, the third pandemic has become largely quiescent, with hundreds of human cases being reported mainly in a few impoverished African countries, where zoonotic plague is mostly transmitted to people by rodent-associated flea bites.

Thermal inactivation of Escherichia coli O157:H7 and non-O157 shiga toxin-producing Escherichia coli cells in mechanically tenderized veal

Preflattened veal cutlets (ca. 71.5 g, ca. 0.32 cm thick) were surface inoculated with ca. 6.8 log CFU/g of a multistrain cocktail of Escherichia coli O157:H7 (ECOH) or a cocktail made of single strains of serogroups O26, O45, O103, O104, O111, O121, and O145 of Shiga toxin-producing E. coli (STEC) cells and then were mechanically tenderized by passing once through a "Sir Steak" tenderizer. For each cooking time, in each of at least three trials, three inoculated and tenderized cutlets, with and without breading, were individually cooked in 15 or 30 ml of canola oil for 0.0, 0.75, 1.0, 1.25, 1.5, 1.75, or 2.25 min per side on an electric skillet set at 191.5°C. The temperatures of the meat and of the skillet were monitored and recorded using a type J thermocouple. Regardless of the breading or volume of oil used to cook the meat, the longer the cooking times, the higher was the internal temperature of the meat, along with a greater reduction of both ECOH and STEC. The average final internal temperature of the meat at the approximate geometric center ranged from 56.8 to 93.1°C. Microbial reductions of ca. 2.0 to 6.7 log CFU/g and ca. 2.6 to 6.2 log CFU/g were achieved for ECOH and STEC, respectively. Our data also revealed no differences in thermal inactivation of ECOH relative to the volume of oil used to cook nonbreaded cutlets. However, when cooking breaded cutlets, the use of more (30 ml) compared with less (15 ml) cooking oil resulted in greater reductions in pathogen numbers. To deliver about a 5.0-log reduction of ECOH and STEC, and to achieve the recommended internal temperature of 71.1°C, it was necessary to cook mechanically tenderized veal cutlets for at least 1.5 min per side on a preheated electric skillet set at 191.5°C and containing 15 ml of cooking oil. These data also established that cooking times and temperatures effective for inactivating serotype O157:H7 strains of E. coli in tenderized veal are equally effective against the additional six non-O157 Shiga toxin-producing strains investigated herein.

Customizable PCR-microplate array for differential identification of multiple pathogens

Customizable PCR-microplate arrays were developed for the rapid identification of Salmonella Typhimurium, Salmonella Saintpaul, Salmonella Typhi, Shigella dysenteriae, Escherichia coli O157:H7, Francisella tularensis subsp. tularensis, Francisella tularensis subsp. novicida, Vibrio cholerae, Vibrio parahaemolyticus, Yersinia pestis, and Yersinia pseudotuberculosis. Previously, we identified highly specific primers targeting each of these pathogens. Here, we report the development of customizable PCR-microplate arrays for simultaneous identification of the pathogens using the primers identified. A mixed aliquot of genomic DNA from 38 strains was used to validate three PCR-microplate array formats. Identical PCR conditions were used to run all the samples on the three formats. Specific amplifications were obtained on all three custom plates. In preliminary tests performed to evaluate the sensitivity of these assays in samples inoculated in the laboratory with Salmonella Typhimurium, amplifications were obtained from 1 g of beef hot dog inoculated at as low as 9 CFU/ml or from milk inoculated at as low as 78 CFU/ml. Such microplate arrays could be valuable tools for initial identification or secondary confirmation of contamination by these pathogens.

Fate of Shiga toxin-producing O157:H7 and non-O157:H7 Escherichia coli cells within refrigerated, frozen, or frozen then thawed ground beef patties cooked on a commercial open-flame gas or a clamshell electric grill

Both high-fat and low-fat ground beef (percent lean:fat = ca. 70:30 and 93:7, respectively) were inoculated with a 6-strain cocktail of non-O157:H7 Shiga toxin-producing Escherichia coli (STEC) or a five-strain cocktail of E. coli O157:H7 (ca. 7.0 log CFU/g). Patties were pressed (ca. 2.54 cm thick, ca. 300 g each) and then refrigerated (4°C, 18 to 24 h), or frozen (-18°C, 3 weeks), or frozen (-18°C, 3 weeks) and then thawed (4°C for 18 h or 21°C for 10 h) before being cooked on commercial gas or electric grills to internal temperatures of 60 to 76.6°C. For E. coli O157:H7, regardless of grill type or fat level, cooking refrigerated patties to 71.1 or 76.6°C decreased E. coli O157:H7 numbers from an initial level of ca. 7.0 log CFU/g to a final level of ≤1.0 log CFU/g, whereas decreases to ca. 1.1 to 3.1 log CFU/g were observed when refrigerated patties were cooked to 60.0 or 65.5°C. For patties that were frozen or freeze-thawed and cooked to 71.1 or 76.6°C, E. coli O157:H7 numbers decreased to ca. 1.7 or ≤0.7 log CFU/g. Likewise, pathogen numbers decreased to ca. 0.7 to 3.7 log CFU/g in patties that were frozen or freeze-thawed and cooked to 60.0 or 65.5°C. For STEC, regardless of grill type or fat level, cooking refrigerated patties to 71.1 or 76.6°C decreased pathogen numbers from ca. 7.0 to ≤0.7 log CFU/g, whereas decreases to ca. 0.7 to 3.6 log CFU/g were observed when refrigerated patties were cooked to 60.0 or 65.5°C. For patties that were frozen or freeze-thawed and cooked to 71.1 or 76.6°C, STEC numbers decreased to a final level of ca. 1.5 to ≤0.7 log CFU/g. Likewise, pathogen numbers decreased from ca. 7.0 to ca. 0.8 to 4.3 log CFU/g in patties that were frozen or freeze-thawed and cooked to 60.0 or 65.5°C. Thus, cooking ground beef patties that were refrigerated, frozen, or freeze-thawed to internal temperatures of 71.1 and 76.6°C was effective for eliminating ca. 5.1 to 7.0 log CFU of E. coli O157:H7 and STEC per g.

Thermal inactivation of a single strain each of serotype O26:H11, O45:H2, O103:H2, O104:H4, O111:H⁻, O121:H19, O145:NM, and O157:H7 cells of Shiga toxin-producing Escherichia coli in wafers of ground beef

For each of two trials, freshly ground beef of variable fat content (higher: 70:30 %lean:%fat; lower: 93:7 %lean:%fat) was separately inoculated with ca. 7.0 log CFU/g of a single strain of Escherichia coli serotypes O26:H11, O45:H2, O103:H2, O104:H4, O111:H⁻, O121:H19, O145:NM, and O157:H7. Next, ca. 3-g samples of inoculated beef were transferred into sterile filter bags and then flattened (ca. 1.0 mm thick) and vacuum sealed. For each temperature and sampling time, three bags of the inoculated wafers of beef were submerged in a thermostatically controlled water bath and heated to an internal temperature of 54.4°C (130°F) for up to 90 min, to 60°C (140°F) for up to 4 min, or to 65.6°C (150°F) for up to 0.26 min. In lower fat wafers, D-values ranged from 13.5 to 23.6 min, 0.6 to 1.2 min, and 0.05 to 0.08 min at 54.4, 60.0, and 65.6°C, respectively. Heating higher fat wafers to 54.4, 60.0, and 65.6°C generated D-values of 18.7 to 32.6, 0.7 to 1.1, and 0.05 to 0.2 min, respectively. In addition, we observed reductions of ca. 0.7 to 6.7 log CFU/g at 54.4°C after 90 min, ca. 1.1 to 6.1 log CFU/g at 60.0°C after 4 min, and 0.8 to 5.8 log CFU/g at 65.6°C after 0.26 min. Thus, cooking times and temperatures effective for inactivating a serotype O157:H7 strain of E. coli in ground beef were equally effective against the seven non-O157:H7 Shiga toxin-producing strains investigated herein.

Effect of storage and subsequent reheating on viability of Listeria monocytogenes on pork scrapple

We evaluated the fate of Listeria monocytogenes on commercial pork scrapple, a regionally popular, ready-to-eat (RTE) meat. We also conducted an informal survey to address consumer practices for storing and reheating scrapple. Of the 129 consumers who responded to at least one of the eight questions posed in the survey, about half (46.4%; 52 of 112) considered scrapple RTE, the majority (69.7%; 76 of 109) stored it in the refrigerator, and all (100%; 112 of 112) preferred to reheat it prior to consumption. Most respondents (83.9%; 94 of 112) reheated the scrapple by pan frying for 1 to 10 min at medium to high temperature. To study pathogen behavior, slices of pork scrapple were surface inoculated with a five-strain cocktail of L. monocytogenes (ca. 2.0 log CFU/g), vacuum sealed, and stored for up to 60 days. Pathogen levels increased to 8.9, 9.5, and 9.9 log CFU/g after 44 (4 degrees C), 21 (10 degrees C), and 5 (21 degrees C) days, respectively. When slices 1.3 cm (ca. 55 g) and 1.9 cm (ca. 85 g) thick were surface inoculated with L. monocytogenes (ca. 7.0 log CFU/g) and then reheated in a skillet (191 degrees C) for 0.5 to 4 min per side or to target instantaneous internal temperatures of 48.9 to 71.1 degrees C, it was possible to achieve pathogen reductions ranging from ca. 2.2 to 6.5 log CFU/g. These data confirm that in the unlikely event of postprocessing contamination of pork scrapple by L. monocytogenes, proper reheating can appreciably reduce levels of the pathogen before consumption.