Clinical Strains of Listeria monocytogenes From the 2011 Cantaloupe Outbreak Show Evidence of Biofilm Formation on Cantaloupe Food-Conditioning Film

In 2011 an outbreak of listeriosis associated with cantaloupes was recorded as the deadliest foodborne disease outbreak in the United States in 90 years1. Interestingly, this was also the first recorded incident associated with cantaloupe, and one of comparatively few outbreaks of listeriosis associated with produce. It is known that Listeria monocytogenes can be isolated from food processing plants, and biofilm formation is thought to be a factor in how particular strains persist in these plants. Previous work from our lab2 has demonstrated that Listeria monocytogenes outbreak strains on food-conditioning films can grow to high cell densities to form biofilms composed of web-like extracellular polymeric substances. Therefore, we sought to identify biological factors that might facilitate Listeria monocytogenes biofilm formation on cantaloupe food-conditioning films. Four clinical strains associated with the cantaloupe outbreak were obtained from the Center for Disease Control and Prevention (CDC). Three of the four strains were also found in the cantaloupe processing plant. Molecular subtyping of the four outbreak strains revealed that they shared key molecular markers with isolates seen in previous outbreaks (Lomonaco et al., unpublished data). One outbreak strain represented a novel outbreak strain and the other three strains were identified as representing two novel epidemic clones. All strains were analyzed using an epifluorescence microscopy biofilm assay. Cantaloupe food-conditioning films enhanced biofilm formation, similar to that seen before with conditioning films made from ready-to-eat meat and poultry products. In particular, cantaloupe rind appeared to enhance attachment, growth and biofilm formation. Interestingly, there was some variability in the growth patterns of different strains, e.g. a strain containing the comK prophage region produced more extracellular polymeric substance. The two epidemic clones included serotype 1/2a and 1/2b strains that had also been previously isolated together from a poultry processing plant, and thus might represent a mix-serotype biofilm.

2011年，一场与哈密瓜相关的李斯特菌病暴发被记录为美国90年来致死性最强的食源性疾病暴发¹。值得注意的是，这也是首次有公开记录的与哈密瓜相关的李斯特菌病暴发，同时也是为数不多的与农产品相关的李斯特菌病暴发之一。已知单核细胞增生李斯特菌（Listeria monocytogenes）可从食品加工厂中分离得到，而生物被膜（biofilm）的形成被认为是特定菌株在这类工厂中长期定植的关键影响因素。本实验室此前的研究²已证实，食品调理膜表面的单核细胞增生李斯特菌暴发菌株可增殖至较高细胞密度，形成由网状胞外聚合物组成的生物被膜。因此，本研究旨在探究可能促进单核细胞增生李斯特菌在哈密瓜食品调理膜上形成生物被膜的生物学因子。研究团队从美国疾病控制与预防中心（Center for Disease Control and Prevention, CDC）获取了4株与此次哈密瓜暴发相关的临床菌株，其中3株同时也在哈密瓜加工厂中被分离得到。对这4株暴发菌株的分子分型结果显示，它们与此前暴发事件的分离株共享关键分子标记（Lomonaco等，未发表数据）。其中1株暴发菌株属于新型暴发菌株，其余3株则被鉴定为2个新型流行克隆株。所有菌株均通过落射荧光显微镜（epifluorescence microscopy）生物被膜检测法完成了分析。哈密瓜食品调理膜可促进生物被膜的形成，这与此前即食肉类及禽类产品调理膜的相关研究结果一致。具体而言，哈密瓜果皮似乎可促进细菌的附着、增殖及生物被膜形成。值得关注的是，不同菌株的生长模式存在一定差异，例如携带comK原噬菌体（comK prophage）区域的菌株可产生更多的胞外聚合物。这两个流行克隆株分别包含血清型1/2a和1/2b菌株，此前这两种血清型菌株也曾同时从禽类加工厂中分离得到，因此二者可能构成混合血清型生物被膜。