| Listeria monocytogenes (L. monocytogenes) is one species of Listeria spp., a zoonotic pathogen and one of important foodborne pathogens. If beef was contaminated by L. monocytogenes, this may lead to serious food safety problems, and become a serious threat to human health. Epidemiological distribution of L. monocytogenes during slaughter process of beef cattle was studied in this article. Antibiotic sensitivity test, serotypes, RAPD types and virulence-associated genes of L. monocytogenes strains were investigated to analyze their antibiotic resistances and genetic relationship. This research can provide solutions for prevention and control this pathogen, and lay the foundation for the safety of beef in China. The study was divided into 3 parts.1. The prevalence of L. monocytogenes in the beef cattle processing plantsThe prevalence of L. monocytogenes was investigated during slaughter process of beef cattle at three beef abattoirs in Shandong province. The sample points included feces, hides, preevisceration carcasses, postevisceration carcasses, post-washing carcasses, chilled carcasses and beef cuts. L. monocytogenes was isolated and identified according to ISO11290-1. Listeria spp. was found in 288 of 439 samples (65.6%). L. monocytogenes was isolated from 116 samples (25.6%).The prevalence of L. monocytogenes from plant B is 94.6% in total prevalence.2. Phenotype and genotype of L. monocytogenes isolates from different plantsSerotypes, antibiotic sensitivity test, major virulence genes-prfA, plcA, hlyA, actA, iap and molecular-typing had been done by using multiplex PCR, Kirby-Bauer method, PCR and RAPD, respectively. L. monocytogenes isolated from three abattoirs were divided into 2 serotypes: 1/2a for plant C and 1/2c for plant A and B, and the serotypes from different plants are significantly different. Antibiotic sensitivity test was done for 55 from all L. monotytogenes isolated. All 55 isolated strains were susceptible to penicillin G, norfloxacin and sinomin compositea. Of 55 isolates, 1.82% was resistant to cephalosporins thiophene, 25.5% of isolates to polymyxin B, 36.4% to cefotaxime and 38.2% to enoxacin. Besides, there were different levels of multi-drug resistance. There was loss of virulence-associated genes in different degrees. In general, 3 of 55 L. monocytogenes isolates were lack of virulence genes. In fact, Gene hly, iap and prfA were detected in all isolates. 2 isolates hadn't Gene actA. 1 isolate hadn't Gene plcA. 55 strains of L. monocytogenes could be divided into 34 different RAPD types.3. Thermal inactivation model of L. monocytogenes in ground beef ?In the study of microbial sterilization, the predictive microbiology model is used more and more widely, which can reduce or replace routine experiments to insure food safety. In order to set up suitable kinetic models for the evaluation of heat inactivation rate of L. monocytogenes in ground beef, 3 strains of L. monocytogenes were inoculated and the samples were heated at 55℃, 57.5℃, 60℃, 63℃, 66℃or 70℃respectively. The survival curves were fitted with modified Gompertz model and linear model after the bacterial counts were decreased from 109CFU?g-1 to 103CFU?g-1. With the rise of heating temperature, inactivation rates of L. monocytogenes increased gradually, and D value decreased, with Z value 6.25℃. In addition, survival curves fitted with modified Gompertz model was better than linear model. We established a secondary-level model to predict the effect of temperature on the first-level model parameters, which was lnM=27.516-0.364T and lnμ=-27.358+0.366T. The two models established were validated by survival count of L. monocytogenes at 59℃and 64℃. The Bf and Af of the modified Gompertz model were acceptable. The RMSE value of the modified Gompertz model was significantly lower than that of linear model, which indicate that modified Gompertz model can predict the influence of heat treatment on L. monocytogenes better than that of linear model. The models established in this study should be suitable for describing the survival of L. monocytogenes in ground beef at different temperatures (55-70℃).
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