Genetic Diversity of Listeria monocytogenes Strains Recovered from the Food Sector in British Columbia, Canada

Introduction: Listeria monocytogenes (Lm) is an environmentally ubiquitous pathogen and a frequent contaminant of ready-to-eat foods. Studies examining the prevalence of Listeria spp. in British Columbia (BC), Canada, found problems with Lm contamination in fish, and to a lesser degree in dairy and meat sectors. However, little is known about strain relatedness, and potential virulence risk of Lm in the BC food continuum.

Purpose: To assess diversity and virulence potential of Lm isolates recovered from BC’s food chain.

Methods: Culture methods were used to recover Lm from food and environmental samples. Isolates were serotyped and subjected to pulsed-field gel electrophoresis (PFGE). Based on PFGE, premature stop codons (PMSC) within inlA were screened by DNA sequencing in 56 unique isolates.

Results: In total, 111 Lm isolates were recovered from three dairy, seven fish and five meat facilities. Isolates serotyped as 1/2a (42%), 4b (37%), 1/2c (12%), 1/2b (5%), and 3a (4%). PMSCs in inlA were observed in 36% of isolates, including eight 1/2b, six 1/2a, four 3a, and one 1/2b and 4b Lm serotype. Conversely, no PMSCs were seen in 64% of isolates (20 4b, 15 1/2a and one 1/2b), though 23% possessed a previously unreported three-codon deletion in positions 738-740. PFGE revealed 36 unique pulsotypes; closely related patterns were observed in dairy and meat, fish and meat, but not dairy and fish facilities. More than one Lm serotype and pulsotype were seen in 54% and 69% of the facilities, respectively. 

Significance: Genetically diverse Lm strains were observed across different food facilities. Most strains belonged to listeriosis-causing serotypes encoding a full-length InlA protein required for disease. Accordingly, Lm recovered along the BC food continuum, and specifically fish, may pose high risk to the consuming public.