Laboratory of Food Microbiology, Department of Microbial and Molecular Systems (M(2)S), Faculty of Bioscience Engineering, KU Leuven, Leuven, Belgium.
Research Unit Food Microbiology and Food Preservation (FMFP-UGent), Department of Food Technology, Safety and Health, Part of Food2Know, Faculty Bioscience Engineering, Ghent University, Ghent, Belgium.
Int J Food Microbiol. 2021 Mar 2;341:109072. doi: 10.1016/j.ijfoodmicro.2021.109072. Epub 2021 Jan 20.
Proper elimination of bacterial endospores in foods and food processing environment is challenging because of their extreme resistance to various stresses. Often, sporicidal treatments prove insufficient to eradicate the contaminating endospore population as a whole, and might therefore serve as a selection pressure for enhanced endospore resistance. In the sporeforming Bacillus cereus group, Bacillus weihenstephanensis is an important food spoilage organism and potential cereulide producing pathogen, due to its psychrotolerant growth ability at 7 °C. Although the endospores of B. weihenstephanensis are generally less heat resistant compared to their mesophilic or thermotolerant relatives, our data now show that non-emetic B. weihenstephanensis strain LMG 18989 can readily and reproducibly evolve to acquire much enhanced endospore heat resistance. In fact, one of the B. weihenstephanensis mutants from directed evolution by wet heat in this study yielded endospores displaying a > 4-fold increase in D-value at 91 °C compared to the parental strain. Moreover, these mutant endospores retained their superior heat resistance even when sporulation was performed at 10 °C. Interestingly, increased endospore heat resistance did not negatively affect the vegetative growth capacities of the evolved mutants at lower (7 °C) and upper (37 °C) growth temperature boundaries, indicating that the correlation between cardinal growth temperatures and endospore heat resistance which is observed among bacterial sporeformers is not necessarily causal.
在食品和食品加工环境中，由于细菌内孢子具有极端的抗逆性，因此很难彻底消除它们。通常，杀菌处理不足以彻底消灭污染的内孢子群体，因此可能成为增强内孢子抗性的选择压力。在产芽孢的蜡状芽孢杆菌群中，由于其在 7°C 时的耐冷生长能力，解淀粉芽孢杆菌是一种重要的食品腐败菌和潜在的呕吐毒素产生病原体。尽管与中温或耐热亲缘体相比，解淀粉芽孢杆菌的内孢子通常耐热性较差，但我们的数据现在表明，非致吐解淀粉芽孢杆菌菌株 LMG 18989 可以轻易且可重复地进化以获得更高的耐热性。事实上，本研究中通过湿热定向进化获得的一个解淀粉芽孢杆菌突变体的内孢子在 91°C 时的 D 值比亲本菌株增加了 4 倍以上。此外，即使在 10°C 下进行孢子形成，这些突变体的内孢子仍保持其优异的耐热性。有趣的是，内孢子耐热性的增加并没有对进化突变体在较低（7°C）和较高（37°C）生长温度边界下的营养生长能力产生负面影响，这表明在细菌产芽孢体中观察到的主要生长温度与内孢子耐热性之间的相关性不一定是因果关系。
