Purpose: The main goal of the present study was the molecular characterization of L. monocytogenes strains isolated from a frozen-vegetable processing plant to determine contamination pathways of the pathogen.
Methods: Six samplings were conducted during one year in a frozen-vegetable processing plant. A total of 574 samples (contact-surfaces, non-contact surfaces and product) were collected and L. monocytogenes was detected in 49 samples (8.54%). From positive samples, 61 strains of L. monocytogenes were isolated and subtyped using pulse field gel electrophoresis (PFGE) technique following the CDC standardized PulseNet protocol using ApaI as the restriction enzyme. PFGE patterns were analyzed and a dendrogram UPGMA (unweighted pair group method with averages) was constructed using Dice coefficient method with a 1.0–1.5% tolerance window using PAST program.
Results: L. monocytogenes isolated strains were differentiated in 11 pulsotypes and were identified with letters from A to K. Pulsotype B was found in 39 strains (63.93%) recovered from all samplings. The dendrogram showed that pulsotype C (1.64%), D (4.92%), and E (3.28%) were genetically related with pulsotype B. The persistent pulsotypes distribution inside the processing plant suggests a cross-contamination between product and contact surfaces (conveyor belts, dispenser) located at the packing area. Additionally, these pulsotypes were found on non-contact surfaces (strainer, floor, forklift skates, waste collector).
Significance: L. monocytogenes pulsotype B strains and related pulsotypes are established inside the frozen-vegetable processing plant. Strains characterization by PFGE technique allowed the identification of` the main sources and contamination pathways of the pathogen. This information should assist processors to implement effective L. monocytogenes control methods.