Milk Quality Improvement Program, Department of Food Science, Cornell University, Ithaca, New York 14853, USA.
(ORCID: https://orcid.org/0000-0002-7672-0982 [J.W.S.]).
J Food Prot. 2020 Dec 1;83(12):2074-2079. doi: 10.4315/JFP-20-195.
Thermophilic spore-forming bacteria are found ubiquitously in natural environments and, therefore, are present in a number of agricultural food products. Spores produced by these bacteria can survive harsh environmental conditions encountered during food processing and have been implicated in food spoilage. During research efforts to develop a standardized method for enumerating spores in dairy powders, the dairy powder-associated thermophilic sporeformer Anoxybacillus flavithermus was discovered growing in uninoculated control plates of tryptic soy agar (TSA) supplemented with 1% (w/v) starch, after incubation at thermophilic (55°C) growth temperatures. This article reports the investigation into the source of this thermophilic sporeformer in TSA medium components and characterization of the bacterial isolates collected. Aqueous solutions of tryptic soy broth powder from four suppliers and four agar-agar powders (two manufacturing lots from one supplier [agar A_1 and agar A_2] and two from separate suppliers [agar B and agar C]) were subjected to two different autoclave cycle times (121°C for 15 min or 121°C for 30 min) and then prepared as TSA. After incubation at 55°C for 48 h, bacterial growth was observed only in media prepared from both lots of agar A agar-agar powder, and only when they were subjected to a 15-min autoclave cycle, implicating these powders as a source of the sporeformer contamination. Genetic characterization of 49 isolates obtained indicated the presence of five unique rpoB allelic types of the thermophilic sporeformer Geobacillus spp. in agar-agar powder from agar A. These results not only highlight the importance of microbiological controls but also alert researchers to the potential for survival of thermophilic sporeformers such as Anoxybacillus and Geobacillus in microbiological media used for detection and enumeration of these same thermophilic sporeformers in products such as dairy powders.
嗜热芽孢形成细菌广泛存在于自然环境中，因此存在于许多农业食品中。这些细菌产生的孢子可以在食品加工过程中遇到的恶劣环境条件下存活，并与食品变质有关。在开发乳制品粉末中孢子计数的标准化方法的研究工作中，发现了一种与乳制品相关的嗜热芽孢形成菌 Anoxybacillus flavithermus 在未接种的对照平板中生长，这些平板是在补充有 1%（w/v）淀粉的胰蛋白酶大豆琼脂（TSA）中，在嗜热（55°C）生长温度下培养的。本文报道了对 TSA 培养基成分中这种嗜热芽孢形成菌来源的调查，并对收集到的细菌分离物进行了特征描述。来自四个供应商的胰蛋白酶大豆肉汤粉的水溶液和四个琼脂粉（一个供应商的两个批次[琼脂 A_1 和琼脂 A_2]和来自两个独立供应商的两个批次[琼脂 B 和琼脂 C]）分别经过两种不同的高压釜循环时间（121°C 15 分钟或 121°C 30 分钟）处理，然后制备成 TSA。在 55°C 孵育 48 小时后，仅在由琼脂 A 琼脂粉的两个批次制备的培养基中观察到细菌生长，并且仅当它们经受 15 分钟高压釜循环时才观察到细菌生长，这表明这些粉末是芽孢形成污染的来源。对从琼脂 A 琼脂粉中获得的 49 个分离物的遗传特征分析表明，在琼脂粉中存在 5 种独特的嗜热芽孢形成菌 Geobacillus spp. 的 rpoB 等位基因类型。这些结果不仅强调了微生物控制的重要性，还提醒研究人员注意在用于检测和计数乳制品等产品中相同嗜热芽孢形成菌的微生物培养基中，嗜热芽孢形成菌如 Anoxybacillus 和 Geobacillus 存活的可能性。
