IJFM：沙门氏菌对恶劣环境的抵抗力比预想更强

弗吉尼亚理工大学的科学家们提供了新的研究证据，表明细菌表面附着的生物膜能形成一种保护性外膜，这对人类病原体沙门氏菌的存活起到保护作用。

根据疾病预防与控制中心的数据，每年每6位美国人中就有1位因食用了受污染的食物而生病，其中因沙门氏菌感染所致的超过一百万人。找出沙门氏菌对抗生素治疗耐药的原因有助于控制疫情。

Fralin生命科学研究所的研究人员发现，生物膜除了保护沙门氏菌免受热处理和消毒液如漂白剂的杀灭外，还使细菌在极端干燥的情况中存活，甚至使细菌在正常的消化活动中存活。这个研究已经在《食品微生物学国际性杂志》（International Journal of Food Microbiology）的在线刊发表，即将在4月刊中发行。

“生物膜作为一种潜在的污染源是食品加工厂一个日益严重的问题，”农业和生命科学学院的食品科学和技术部的助理教授莫尼卡.庞德说，“我们已经发现，具有生物膜的沙门氏菌在干燥的食物中存活状况比之前预想的更严重，因此也而更容易导致疾病。”在过去5年中，与坚果、谷物、香料、奶粉和宠物食品等相关的沙门氏菌感染爆发引起了超过900例的相关感染。因为这些干产品的干燥特性阻止了细菌的生长，在以前，这些食物都被认为是安全的。“大多数人都认为，沙门氏菌在生肉中繁殖，而不知道它可以在水果、蔬菜或者干产品这些常常不需要煮熟的食物中生长。”庞德说。

在潮湿的环境中，沙门氏菌大量生长和繁殖。如果放到一个干燥的环境中，他们就停止繁殖，但通过开启基因产生的生物膜可以在不利的环境中保护它们。为检测沙门氏菌生物膜的顺应力，研究人员将其晾干并将存放到干燥的奶粉中长达30天。研究人员还在一个模拟消化系统的不同点检测生物膜。大量沙门氏菌在这种长期的存放中生存了下来。有生物膜的沙门氏菌比自由浮动的细菌对同一处理条件有着更强的顺应力。细菌对干旱的应激反应也使它更容易引起疾病。生物膜使得沙门氏菌在严酷的酸性胃液环境中得以生存，增加沙门氏菌到达肠道的机会，并在此引起的感染，导致了食物中毒的症状。

该研究可以帮助美国食品和药物管理局修改规定，通过强调更好的卫生条件和研究新的策略以减少生物膜的形成，从而减少沙门氏菌感染再次暴发的可能。

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Biofilms promote survival and virulence of Salmonella enterica sv. Tennessee during prolonged dry storage and after passage through an in vitro digestion system.
Abstract
Salmonella enterica serotypes have been linked to outbreaks associated with low water activity foods. While the biofilm-forming abilities of Salmonella improve its survival during thermal processing and sanitation it is unclear whether biofilms enhance survival to desiccation and gastric stresses. The purpose of this study was to quantify the effect of physiological state (planktonic versus biofilm) and prior exposure to desiccation and storage in dry milk powder on Salmonella survival and gene expression after passage through an in vitro digestion model. Planktonic cells of Salmonella enterica serotype Tennessee were deposited onto membranes while biofilms were formed on glass beads. The cells were subsequently dried at room temperature and stored in dried milk powder (aw=0.3) for up to 30days. Salmonella survival was quantified by serial dilution onto Brilliant Green Agar before desiccation, after desiccation, after 1-day storage and after 30-day storage. At each sampling period both physiological states were tested for survival through a simulated gastrointestinal system. RNA was extracted at the identical time points and Quantitative Real-Time PCR was used to determine relative expression for genes associated with stress response (rpoS, otsB), virulence (hilA, invA, sipC) and a housekeeping gene 16S rRNA. The physiological state and length of storage affected the survival and gene expression of Salmonella within the desiccated milk powder environment and after passage through an in vitro digestion system (p<0.05). Larger numbers of S. Tennessee were recovered by plate counts for biofilms compared to planktonic, however, the numbers of Salmonella genomes detected by qPCR were not significantly different suggesting entry of the planktonic cells of S. Tennessee into a viable but non-culturable state. The increased expression of stress response genes rpoS and otsB correlated with survival, indicating cross-protection to low water activity and acid stress. Increased expression of virulence-associated genes was seen in cells exposed to dry storage for short periods, however the largest amount of expression occurred in biofilm cells stored for 30days at aw 0.3, suggesting increased virulence potential.