Listeria monocytogenes is the causative bacteria of listeriosis, a food-borne disease that can cause severe illness in pregnant women, newborns, the elderly, and others with weakened immune systems. Although relatively rare, it has a high mortality rate (20-30%) in these vulnerable groups. L. monocytogenes is stress resistant, can grow at refrigeration temperatures, and can form biofilms, making it difficult to control or eliminate in food production environments. A key point of control for L. monocytogenes is in the food production area where cleaning can prevent the spread from equipment to food product. Knowing where L. monocytogenes is located and how it is getting into food product is valuable for directing control efforts.
This study aimed to use whole genome sequencing (WGS) for tracing L. monocytogenes within a food production plant. In total, 253 L. monocytogenes isolates from a single commercial cooked chicken processing plant were sequenced. Isolates were primarily collected between April 2022 and September 2023. The isolates were obtained from both environmental swab and food product samples. Isolates prior to 2022 were included as background isolates.
Whole genome SNP analysis was performed and a SNP cutoff of 2 was used to assign isolates into SNP subgroups to infer transmission events. One subgroup, containing 188 isolates, was found to be the sole cause of food contamination in 2022-2023 samples and was present in swab samples from food-contact surfaces in the production line during this time. Further by examining the isolation locations of the isolates of this subgroup, the likely reservoir was identified.
This study demonstrates the utility of WGS for control of L. monocytogenes. With the increasing affordability of the next generation sequencing technologies, WGS based surveillance in food production facilities is becoming increasingly viable. The use of WGS can help reduce the risk of listeriosis.