Phenotypic and molecular typing of Listeria monocytogenes isolated from the processing environment and products of a sandwich-producing plant

Listeria monocytogenes as a food-borne pathogen has significant public health and economic impacts. The present study investigated the diversity of L. monocytogenes in a Swiss sandwich-producing plant over a 12-months period. L. monocytogenes were detected by culture after enrichment in 70 (3.5%) of 2028 environmental swabs and 16 (7.4%) of 217 samples from ingredients and sandwiches. Of the 86 L. monocytogenes strains, 93% belonged to serotype 1/2a and genetic lineage II. Rep PCR and PFGE analysis yielded each six profiles. Sixty-seven (77.9%) strains belonged to only one genotype, which was repeatedly found on/in slicers, conveyor belts, tables, a bread-feeding machine, spattles, air blow-guns, salmon, and egg sandwiches. Strains of this genotype persisted for more than 9 months in the processing environment, in particular on slicers and conveyor belts, which probably contributed to the contamination of sandwiches. After revision of the cleaning and disinfection procedures, L. monocytogenes were no longer found on slicers, conveyor belts, or in products. Besides, these results emphasize the importance of environmental monitoring schemes to identify potential contamination sources and as an early warning system for food business operators.