Rozali Siti N M, Milani Elham A, Deed Rebecca C, Silva Filipa V M
Chemical and Materials Engineering Department, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand.
School of Biological Sciences, School of Chemical Sciences, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand.
Int J Food Microbiol. 2017 Dec 18;263:17-25. doi: 10.1016/j.ijfoodmicro.2017.10.008. Epub 2017 Oct 4.
Spores are the most resistant form of microbial cells, thus difficult to inactivate. The pathogenic or food spoilage effects of certain spore-forming microorganisms have been the primary basis of sterilization and pasteurization processes. Thermal sterilization is the most common method to inactivate spores present on medical equipment and foods. High pressure processing (HPP) is an emerging and commercial non-thermal food pasteurization technique. Although previous studies demonstrated the effectiveness of thermal and non-thermal spore inactivation, the in-depth mechanisms of spore inactivation are as yet unclear. Live and dead forms of two food spoilage bacteria, a mould and a yeast were examined using scanning electron microscopy before and after the inactivation treatment. Alicyclobacillus acidoterrestris and Geobacillus stearothermophilus bacteria are indicators of acidic foods pasteurization and sterilization processes, respectively. Neosartorya fischeri is a phyto-pathogenic mould attacking fruits. Saccharomyces cerevisiae is a yeast with various applications for winemaking, brewing, baking and the production of biofuel from crops (e.g. sugar cane). Spores of the four microbial species were thermally inactivated. Spores of S. cerevisiae were observed in the ascus and free form after thermal and HPP treatments. Different forms of damage and cell destruction were observed for each microbial spore. Thermal treatment inactivated bacterial spores of A. acidoterrestris and G. stearothermophilus by attacking the inner core of the spore. The heat first altered the membrane permeability allowing the release of intracellular components. Subsequently, hydration of spores, physicochemical modifications of proteins, flattening and formation of indentations occurred, with subsequent spore death. Regarding N. fischeri, thermal inactivation caused cell destruction and leakage of intracellular components. Both thermal and HPP treatments of S. cerevisiae free spores attacked the inner membrane, altering its permeability, and allowing in final stages the transfer of intracellular components to the outside. The spore destruction caused by thermal treatment was more severe than HPP, as HPP had less effect on the spore core. All injured spores have undergone irreversible volume and shape changes. While some of the leakage of spore contents is visible around the deformed but fully shaped spore, other spores exhibited large indentations and were completely deformed, apparently without any contents inside. This current study contributed to the understanding of spore inactivation by thermal and non-thermal processes.
芽孢是微生物细胞中抗性最强的形式，因此难以被灭活。某些产芽孢微生物的致病或导致食物变质的作用一直是杀菌和巴氏杀菌过程的主要依据。热杀菌是灭活医疗设备和食品上存在的芽孢的最常用方法。高压处理（HPP）是一种新兴的商业化非热食品巴氏杀菌技术。尽管先前的研究证明了热杀菌和非热芽孢灭活的有效性，但芽孢灭活的深入机制尚不清楚。在灭活处理前后，使用扫描电子显微镜检查了两种食物腐败细菌、一种霉菌和一种酵母的活态和死态形式。嗜酸嗜热栖热菌和嗜热栖热芽孢杆菌分别是酸性食品巴氏杀菌和杀菌过程的指示菌。费氏新萨托菌是一种侵害水果的植物病原霉菌。酿酒酵母是一种酵母，在酿酒、酿造、烘焙以及利用作物（如甘蔗）生产生物燃料等方面有多种应用。对这四种微生物的芽孢进行了热灭活处理。在热杀菌和高压处理后，观察到酿酒酵母的芽孢存在于子囊内和游离状态。每种微生物芽孢都观察到了不同形式的损伤和细胞破坏。嗜酸嗜热栖热菌和嗜热栖热芽孢杆菌的细菌芽孢通过攻击芽孢的内核而被热灭活。热量首先改变了膜的通透性，使细胞内成分释放出来。随后，芽孢发生水合作用、蛋白质的物理化学修饰、扁平化和凹陷形成，随后芽孢死亡。对于费氏新萨托菌，热灭活导致细胞破坏和细胞内成分泄漏。酿酒酵母游离芽孢经热杀菌和高压处理均攻击内膜，改变其通透性，并在最后阶段使细胞内成分转移到外部。热处理导致的芽孢破坏比高压处理更严重，因为高压处理对芽孢核心的影响较小。所有受损芽孢都经历了不可逆的体积和形状变化。虽然在变形但形状完整的芽孢周围可以看到一些芽孢内容物的泄漏，但其他芽孢呈现出大的凹陷并完全变形，内部显然没有任何内容物。这项研究有助于理解热杀菌和非热过程对芽孢的灭活作用。
