Aerobio Growth of Listeria monocytogenes on Beef Lean and Fatty Tissue: Equations Describing the Effects of Temperature and pH.

The aerobio growth rate and the duration of the lag period were determined for Listeria monocytogenes strain Murray B growing on ground beef lean and on pieces of fatty tissue. The organism grew at 0°C on lean tissue at pH ≥ 6 and on fatty tissue. It failed to grow at 0°C on lean at pH 5.6 but did grow at 2.5°C. The effect of temperature, between 0 and 30°C, on the growth rate on fatty tissue can be described by a modified Arrhenius equation Ln (gen/h) = -205.73 + 1.2939 × 105/K -2.0298 × 107/K2, where K = °Kelvin. This equation accounted for 99.7% of the variance. The combined effect of temperature and pH on the growth rate on beef lean was described by Ln (gen/h) = - 232.64 + 1.4041 × 105/K - 2.1908 × 107K2 + 1.1586 × 102/pH - 4.0952 × 102/pH2 (variance accounted for 99.5%). For lean at about pH 5.5-5.6, this equation applied between about 2.5 and 35°C; for lean of pH 6-7, it applied between about 0 and 35°C. Though the lag period increased with decrease in temperature and pH, measured lag times were more variable than generation times, and the goodness of fit of modified Arrhenius equations to lag times was relatively poor (variance accounted for 83-92%).

Occurrence, Numbers, and Growth of Listeria monocytogenes on some Vacuum-Packaged Processed Meats.

Listeriae were detected on 93 of 175 samples of vacuum-packed processed meats obtained from retail stores. More than 1000 colony forming units of listeriae per g were found on seven of 130 samples in which the numbers of listeriae were estimated. When sliced corned-beef and ham, from four manufacturers, were inoculated with Listeria monocytogenes and vacuum-packed, the growth rate of the organism varied with the composition of the product. High residual nitrite or lowered aw reduced growth, particularly when products were stored at 0 to 5°C. As the storage temperature increased from 0 to 15°C, the growth rate of L. monocytogenes increased more rapidly than that of the other flora (lactic acid bacteria and Brochothrix thermosphacta ). Growth rates on inoculated packs were similar to rates observed for packs contaminated with L. monocytogenes during commercial production.

Detection of Listeria spp. in Meat and Environmental Samples by an Enzyme-linked Immunosorbent Assay (ELISA).

An ELISA kit (TECRA™) for the detection of Listeria spp. was evaluated for its ability to detect these organisms in naturally contaminated meat and in environmental samples from meat processing plants. Of the 170 samples examined, Listeria monocytogenes and/or L. innocua were detected in 74 by enrichment and selective plating. Testing of the enrichment broths with the ELISA kit detected 72 of these positive samples and gave 2 false-negative and 2 false-positive reactions.

Growth of Listeria monocytogenes on Vacuum-packaged Beef.

Pieces of beef striploin (400 g) were inoculated with Listeria monocytogenes strain Murray B, vacuum packaged, and stored at either 0°C or 5.3°C. Growth of the organism on the beef depended on the temperature of storage, the pH of the lean, and the type of tissue. Growth was more rapid at 5.3°C than at 0°C, and faster on striploins of high pH (6.0-6.1) than on striploins of low pH (5.5-5.7). During storage, the population of L. monocytogenes was higher on fatty tissue than on lean principally because growth occurred earlier on the fat. When low pH striploins were held at 5.3°C, listeria grew from an initial count of 2-5×103 CFU/cm2 to 3×107 CFU/cm2 in 16 d on the fat, and in 20 d, to 106 CFU/cm2 on the lean and to 5×107 CFU/ml in the purge fluid. After storage at 0°C for 76 d, the populations reached were 106 CFU/cm2 on the fat, 104 CFU/cm2 on the lean, and 3×105 CFU/ml in the purge fluid. When high pH striploins were held at 0°C for 10 weeks, listeria grew from an initial population of 150-400 CFU/cm2 to just over 106 CFU/cm2 on the fat, 2×105 CFU/cm2 on the lean, and 4×106 CFU/ml in purge fluid.

Campylobacter Jejuni and Campylobacter Hyointestinalis in the Intestinal Tract and on the Carcasses of Calves and Cattle.

Campylobacter jejuni was commonly detected in both rumen (74%) and faecal (54%) samples obtained at slaughter from calves which were about 4-weeks old but was much less frequently found in adult cattle. However there was a high incidence (47-88%) of C. hyointestinalis in intestinal tract samples from both calves and cattle. While C. jejuni occurred in ruminal material at only low counts (less than 100/g), C. hyointestinalis was often present at high counts (above 105/g). Lot-fed cattle were more likely to have both organisms in their intestinal tracts and on their carcasses than were pasture-fed cattle. Chilling cattle carcasses for 20-24 h significantly reduced the incidence of both Campylobacter species on the carcasses. In contrast campylobacters were found on almost all calf carcasses examined after chilling. The mean count of C. jejuni on calf carcasses before chilling was about 16/cm2 and decreased by about 10-fold after 20 h of chilling.

Substrates Used by Brochothrix thermosphacta When Growing on Meat.

Substrates used by Brochothrix thermosphacta when growing aerobically on meat include glucose, ribose, glycerol, glycerol-3-phosphate and inosine. Glycogen and inosine-monophosphate are not used. Of these substrates, only glucose and ribose are metabolized during anaerobic growth. Ribose is probably the major energy source for anaerobic growth on high pH meat.