Diversity and antimicrobial resistance of Listeria spp. and L. monocytogenes clones from produce handling and processing facilities in the Pacific Northwest

Abstract

The widespread nature of Listeria monocytogenes and its presence in soil and agricultural environments, make it a challenging pathogen to control in the produce industry. The objective of this study was to assess diversity of Listeria spp. isolates (n = 113) previously recovered from five produce handling and processing facilities in the Pacific Northwest, using molecular and antibiogram typing. Antimicrobial resistance (AMR) profiles were obtained for all isolates. L. monocytogenes isolates were also characterized using molecular serogrouping (n = 75) and multi-locus sequence typing (MLST; n = 48). Most individual facilities contained a single serogroup of L. monocytogenes; two facilities were positive for serogroup 1 only and two facilities were positive for serogroup 4 only. The facility with the highest prevalence of Listeria spp. was positive for both serogroups. MLST identified 10 unique sequence types (STs) within 10 clonal complexes (CCs), with hypervirulent clones of CC2 and CC4 being overrepresented. All Listeria spp. isolates were sensitive to ampicillin, erythromycin, gentamicin, imipenem, co-trimoxazole, tetracycline and vancomycin and resistant to cefoxitin and nalidixic acid. A high proportion (66%) of Listeria spp. isolates was resistant to clindamycin, whereas resistance to penicillin, ciprofloxacin, rifampin, and novobiocin was less common. Three L. monocytogenes isolates and one L. innocua isolate were multi-drug resistant (MDR; resistant to ≥3 antibiotics). While the majority of Listeria spp. remain sensitive to clinically relevant antibiotics, the presence of MDR in strains recovered from produce handling and processing environments is concerning and prompts further studies to understand the pressures driving the AMR changes in these microorganisms.