Phenotypic and genotypic characterization of listeria monocytogenes strains isolated from meat industries

Listeria monocytogenes is a major foodborne pathogen of concern for the food industry due to its ability to persist in processing environments and contaminate ready-to-eat foods. This study aimed to characterize phenotypically and genotypically L. monocytogenes isolates recovered from meat-processing environments in Greece, with emphasis on antimicrobial susceptibility, resistance determinants, plasmid content, and sequence types. Eighteen environmental isolates were analyzed using broth microdilution MIC testing and Oxford Nanopore whole-genome sequencing followed by in silico screening for antimicrobial resistance genes and plasmids. All isolates were susceptible to ampicillin, erythromycin, and trimethoprim-sulfamethoxazole according to EUCAST criteria. For agents categorized by EUCAST as having insufficient evidence ("IE") for clinical categorization in Listeria (moxifloxacin and linezolid), MICs were reported descriptively. Elevated MICs were observed for clindamycin (≥ 8 mg/L) in three isolates and for tetracycline (≥ 16 mg/L) in two isolates, while all isolates showed high fosfomycin MICs (≥ 128 mg/L) and intermediate fusidic acid MICs (4-8 mg/L). Genomic screening identified two acquired resistance genes, vga (G) and tet (M), and detected plasmids in a subset of isolates, including plasmid types previously associated with stress tolerance and persistence in food environments. MLST revealed four sequence types (ST87, ST155, ST6, and ST504), including lineages linked to food-processing persistence and clinically relevant clonal complexes. Overall, the findings indicate limited clinically relevant resistance among the examined isolates, while highlighting the contribution of mobile genetic elements and diverse sequence types to environmental fitness and potential public-health risk in meat-processing settings.