Koutsoumanis K P, Sofos J N
Center for Red Meat Safety, Department of Animal Sciences, Colorado State University, Fort Collins, Colorado 80523, USA.
Lett Appl Microbiol. 2004;38(4):321-6. doi: 10.1111/j.1472-765x.2004.01491.x.
The aim of the study was to evaluate the effect of habituation at different pH conditions on the acid resistance of Listeria monocytogenes, Escherichia coli O157:H7 and Salmonella enterica serotype Typhimurium, and to identify potential differences between the adaptive responses of the three pathogens.
Stationary phase cells of L. monocytogenes, E. coli O157:H7 and S. Typhimurium, grown in glucose-free media, were exposed to pH 3.5 broth directly or after habituation for 90 min at various pH conditions from 4.0 to 6.0. Survivors at pH 3.5 were determined by plating on tryptic soy agar and incubating at 30 degrees C for 48 h. The kinetics (death rate) of the pathogens at pH 3.5 was calculated by fitting the data to an exponential model.
Habituation to acidic environments provided protection of the pathogens against lethal acid conditions. This acid protection, however, was found to be pH dependent. For example, for E. coli O157:H7 an increased acid resistance was observed after habituation at a pH range from 4.0 to 5.5, while the maximum acid tolerance was induced at pH 5.0. Furthermore, the effect of low pH habituation was different among pathogens. For L. monocytogenes, E. coli O157:H7 and S. Typhimurium, the pH range within which habituation resulted to increased acid resistance was 5.0-6.0, 4.0-5.5 and 4.0-5.0, respectively, while the maximum acid tolerance was induced after habituation at pH 5.5, 5.0 and 4.5, respectively.
Acid stress conditions are common within current food processing technologies. The information on adaptive responses of L. monocytogenes, E. coli O157:H7 and S. Typhimurium after habituation to different pH environments provided in the present study, could lead to a more realistic evaluation of food safety concerns and to a better selection of processes in order to avoid adaptation phenomena and to minimize the potential for food safety risks.
本研究旨在评估在不同pH条件下适应环境对单核细胞增生李斯特菌、大肠杆菌O157:H7和鼠伤寒沙门氏菌耐酸性的影响，并确定这三种病原体适应性反应之间的潜在差异。
在无糖培养基中生长的单核细胞增生李斯特菌、大肠杆菌O157:H7和鼠伤寒沙门氏菌的稳定期细胞，直接暴露于pH 3.5肉汤中，或在4.0至6.0的各种pH条件下适应90分钟后再暴露于其中。通过接种在胰蛋白胨大豆琼脂上并在30℃下培养48小时来确定pH 3.5时的存活菌数。通过将数据拟合到指数模型来计算病原体在pH 3.5时的动力学（死亡率）。
适应酸性环境可保护病原体免受致死性酸性条件的影响。然而，这种酸保护作用发现是依赖于pH的。例如，对于大肠杆菌O157:H7，在pH范围为4.0至5.5的环境中适应后观察到耐酸性增加，而在pH 5.0时诱导出最大耐酸性。此外，低pH适应的效果在病原体之间有所不同。对于单核细胞增生李斯特菌、大肠杆菌O157:H7和鼠伤寒沙门氏菌，适应后导致耐酸性增加的pH范围分别为5.0 - 6.0、4.0 - 5.5和4.0 - 5.0，而最大耐酸性分别在pH 5.5、5.0和4.5适应后诱导产生。
酸性应激条件在当前食品加工技术中很常见。本研究提供的关于单核细胞增生李斯特菌、大肠杆菌O157:H7和鼠伤寒沙门氏菌在适应不同pH环境后的适应性反应信息，可能会导致对食品安全问题进行更现实的评估，并更好地选择加工工艺，以避免适应现象并将食品安全风险降至最低。
