Molecular Characterisation and Antimicrobial Resistance Patterns of Listeria monocytogenes From Dairy Sources

Listeria monocytogenes (L. monocytogenes) is a major food pathogen that causes severe infections, especially in the susceptible population. Dairy products and milk are some of the possible vehicles through which it can be transmitted both as a threat to the health of people and as an economic issue. The study conducted was to explore the prevalence, virulence characteristics, and antimicrobial resistance patterns of L. monocytogenes along the supply chain of milk in Yazd Province, Iran.

Collection of 370 samples (raw milk, pasteurised milk, dairy equipment, transport containers and hands and gowns of workers) was done between 2022 and 2023. Identification and isolation of L. monocytogenes was done using standard biochemical, bacteriological and molecular tests. The assessment of antimicrobial susceptibility was done using the Kirby-Bauer disk diffusion, and the multi-antibiotic resistance (MAR) index was determined. The presence of important virulence genes and resistance determinants was identified by PCR.

Overall, 10 isolates (2.7%) were confirmed as L. monocytogenes. Raw milk had the highest contamination rate (5.29%), while pasteurised milk and worker samples were negative. High-level resistance was observed against nalidixic acid, penicillin G, ceftazidime (100%), cefepime and ceftriaxone (90%), whereas first-line clinical drugs such as gentamicin, amikacin, levofloxacin, vancomycin, trimethoprim-sulfamethoxazole and erythromycin showed full susceptibility. Virulence genes prfA, inlA and iap were universally present, with high prevalence of resistance genes including lnuA/B, ermA/B and tetA/M.

The results suggest that raw milk may be a significant reservoir of multi-drug-resistant L. monocytogenes in this region and the studied supply chain. Therefore, hygiene measures and continuous monitoring are key to preventing contamination and protecting public health.