Plasmids have an important role in the pathogenicity of certain bacterial species, and Escherichia coli provides the most complete example of the relationship involved. Enterotoxigenic strains of E. coli, in addition to producing heat-stable and/or heat-labile enterotoxins, may also produce a haemolysin and fimbriate cell surface antigens which facilitate the adherence of the bacterial cell to the mucosa of the small bowel. Numerous studies have shown that these properties are plasmid-mediated and that the plasmids act in concert to confer on the host bacterium the ability to produce enteric disease in man and in animals. Moreover, studies with invasive strains of E. coli have shown that the Col V plasmid, which codes for the synthesis of colicin V, significantly enhances the pathogenicity of its host bacterium. Although the relationship between Col V plasmids and virulence is unclear, reports indicate that Col V-containing strains of E. coli are better able to survive in the alimentary tract and that colicine V itself inhibits macrophage function. It is probable that bacterial virulence is a complex phenomenon involving both chromosomal and plasmid genes. We describe here a virulence plasmid which mediates tissue invasiveness in human pathogenic strains of Yersinia enterocolitica.
Cooked, vacuum packaged beef top rounds injected with 0, 1, 2, 3 or 4% sodium lactate were stored up to 84 days at 0°C. Aerobic plate counts (APC), water activity, pH and lactic acid content were determined at 14.day intervals. Microbial distributions were characterized following 0.56 and 84 days storage. Increasing sodium lactate resulted in reduction in APC. Water activity was not affected by sodium lactate level. Decreases in pH were minimized at sodium lactate >3%. Initially, the microflora of roasts consisted primarily of Micrococcus and coagulase-negative Staphylococcus spp. After 84 days the microflora consisted primarily of hetero-and homofermentative Lactobacilh spp. No differences in APC, lactic acid content, pH or water activity occurred between top rounds with 3 and 4% sodium lactate. Cooked, refrigerated roast beef injected with up to 3 or 4% sodium lactate had microbial shelf-life up to 84 days.
Test strains of C. fetus subsp. jejuni and C. fetus subsp. intestinalis failed to survive heating in skimmilk at 60 C for 1 min. A few strains survived heating in skimmilk at 55 C for 1–3 min. D50C values for C. fetus subsp. jejuni and C. fetus subsp. intestinalis in skimmilk ranged from 1.3–4.5 and from 1.0–3.7, respectively. No survivors of C. fetus subsp. jejuni and C. fetus subsp. intestinalis were detected in beef roasts inoculated at levels of 106–107 viable cells per g when the final temperature in the center was 57 and 55 C, respectively. At an internal temperature of 50–53 C, survivors of C. fetus were detected in beef roasts. Storage of skimmilk, beef and ground beef inoculated with C. fetus at −20, 1, 10, 20, 30 or 40 C resulted in decreases in C. fetus count. Survival of C. fetus was best at 1 and 10 C. Rapid increases in C. fetus counts occurred at 37 C in Brucella broth adjusted to pH 6–8. At pH 5, no survivors were detected after 24 h. At pH 9, counts of C. fetus subsp. jejuni decreased rapidly while those of C. fetus subsp. intestinalis increased slightly.
A microbiological examination of vacuum-packaged strip loin steaks that were defective (gassy packages, hydrogen sulfide odor) revealed high total counts (107–108/cm2) with Hafnia alvei, Lactobacillus and Pseudomonas spp. as major isolates. Re-inoculation experiments indicated that H. alvei was the likely cause of the hydrogen sulfide odor. Gas formation resulted from the activity of heterofermentative lactobacilli and H. alvei. Improvements in plant practices and temperature control eliminated the problem.
Swordfish (Xiphias gladius) steaks were held in retail packages containing 100% CO2 and in mixtures of 40% and 70% CO2 in combination with either oxygen or nitrogen. Controls were stored in air. Samples were removed for chemical and microbiological analyses after 2–22 d of storage at 3.5°C. The inhibitory effect of CO2 on psychrotrophic, aerobic gram-negative spoilage bacteria was proportional to the CO2 tension in the packages. Maximum inhibition of growth was achieved with 100% CO2. Except for steaks stored in 40% CO2:60% O2 heterofermentative Lactobacillus spp. became a dominant part of the microflora of steaks stored in CO2-enriched atmospheres. Pseudomonas spp. continued to be a major part of the microflora of steaks stored in 40% CO2:60% O2. During the first 2 d of storage, there was a decrease in the surface pH of the swordfish steaks proportional to the CO2 tension in the packages. Swordfish steaks stored in CO2-enriched atmospheres had lower total volatile nitrogen (TVN), trimethylamine (TMA) and total volatile acid (TVA) values than steaks stored in air. Oxidative rancidity was not a flavor problem of fish in any of the atmospheres after 20 d of refrigerated storage.
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