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strains revealed that they shared key molecular markers with isolates seen in previous outbreaks (Lomonaco et al., unpublished data).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.