This study evaluated the effects of NaCl on heat resistance, antibiotic susceptibility, and Caco-2 cell invasion of Salmonella. Salmonella typhimurium NCCP10812 and Salmonella enteritidis NCCP12243 were exposed to 0, 2, and 4% NaCl and to sequential increase of NaCl concentrations from 0 to 4% NaCl for 24 h at 35°C. The strains were then investigated for heat resistance (60°C), antibiotic susceptibility to eight antibiotics, and Caco-2 cell invasion efficiency. S. typhimurium NCCP10812 showed increased thermal resistance (P < 0.05) after exposure to single NaCl concentrations. A sequential increase of NaCl concentration decreased (P < 0.05) the antibiotic sensitivities of S. typhimurium NCCP10812 to chloramphenicol, gentamicin, and oxytetracycline. NaCl exposure also increased (P < 0.05) Caco-2 cell invasion efficiency of S. enteritidis NCCP12243. These results indicate that NaCl in food may cause increased thermal resistance, cell invasion efficiency, and antibiotic resistance of Salmonella.
This study developed models to predict the growth probabilities and kinetic behavior of Salmonella enterica strains on cutting boards. Polyethylene coupons (3 by 5 cm) were rubbed with pork belly, and pork purge was then sprayed on the coupon surface, followed by inoculation of a five-strain Salmonella mixture onto the surface of the coupons. These coupons were stored at 13 to 35°C for 12 h, and total bacterial and Salmonella cell counts were enumerated on tryptic soy agar and xylose lysine deoxycholate (XLD) agar, respectively, every 2 h, which produced 56 combinations. The combinations that had growth of ≥0.5 log CFU/cm(2) of Salmonella bacteria recovered on XLD agar were given the value 1 (growth), and the combinations that had growth of <0.5 log CFU/cm(2) were assigned the value 0 (no growth). These growth response data from XLD agar were analyzed by logistic regression for producing growth/no growth interfaces of Salmonella bacteria. In addition, a linear model was fitted to the Salmonella cell counts to calculate the growth rate (log CFU per square centimeter per hour) and initial cell count (log CFU per square centimeter), following secondary modeling with the square root model. All of the models developed were validated with observed data, which were not used for model development. Growth of total bacteria and Salmonella cells was observed at 28, 30, 33, and 35°C, but there was no growth detected below 20°C within the time frame investigated. Moreover, various indices indicated that the performance of the developed models was acceptable. The results suggest that the models developed in this study may be useful in predicting the growth/no growth interface and kinetic behavior of Salmonella bacteria on polyethylene cutting boards.
Sophorae radix extract (SRE) has been registered as an environment-friendly organic material that is widely used in the cultivation of crops in Korea. Matrine, the active ingredient in SRE, was reported as a toxic substance in the nervous system in mice. However, no information is available on its toxic effects in other organisms. Therefore, antimutagenicity and two kinds of genotoxicity tests (bacterial reverse mutation and chromosome aberration test) of two samples of SRE were investigated in this study. Antimutagenicity test was experimented by using bacterial reverse mutation test. In the reverse mutation test, Salmonella Typhimurim TA98, TA1535 and TA1537 were used to evaluate the mutagenic potential of SRE. Bacterial reverse mutation test was also performed on positive and negative control groups in the presence of the metabolic activation system (with S-9 mix) and metabolic non-activation system (without S-9 mix). In the chromosome aberration test, Chinese hamster lung cells were exposed to SRE for 6 or 24 hours without S-9 mix, or for 6 hours with S-9 mix. Negative and positive control groups were experimented for chromosome aberration test. As a result, the number of mutated colonies induced by 4-NQO were reduced by SRE treatment in all strains, indicating that SRE may have antimutagenic effects. Reverse mutation was not shown at all concentrations of SRE, regardless of application of the metabolic activation system. In the chromosomal aberration test, one of the SRE sample gave a suspicious positive result at 250 µg/ml in the presence of S-9 mix. For the more adequate evaluation of the genotoxic potential of SRE samples, other in vivo genotoxicity study is needed.
This study evaluated the effects of NaCl on heat resistance and Caco-2 cell invasion of Listeria monocytogenes in broth media and sausage. A 10-strain mixture of L. monocytogenes was inoculated in tryptic soy broth containing 0.6% yeast extract (TSBYE), and sausage formulated with 0, 2, 4, and 6% NaCl. The medium was stored at 7, 15, 20, and 25 o C for 3-16 d, and medium samples were withdrawn at the appropriate time and challenged to 55, 60, and 63 o C to evaluate the thermal resistance of the pathogen. Sausage samples were stored at 7 and 25 o C, and they were exposed to 63 o C to evaluate thermal resistance. NaCl-habituated L. monocytogenes strains NCCP10811 and NCCP10943 were examined for 12 antibiotics and Caco-2 cell invasion assay (only L. monocytogenes NCCP10943). Bacterial populations of L. monocytogenes generally increased (p<0.05) during the heat challenge as NaCl concentrations increased in both TSBYE and sausage samples. The antibiotic resistance of L. monocytogenes was not observed (p≥0.05) when it was exposed to a single concentration of NaCl in TSBYE, but the pathogen obtained resistance to some antibiotics when exposed to a sequential increase of NaCl concentration. Invasion efficiency of L. monocytogenes NCCP10943 was not increased (p≥0.05) with NaCl concentration increase. These results indicate that NaCl may increase the resistance of L. monocytogenes to heat and to some antibiotics, but may not increase Caco-2 cell invasion of L. monocytogenes.
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