Transferable tetracycline resistance in Listeria monocytogenes from food in Italy

This study investigated tetracycline resistance mechanisms in Listeria monocytogenes isolates from food in Italy. Two L. monocytogenes strains (266 and 286) and one L. innocua strain (52P) isolated from poultry and fresh meat were found to be tetracycline-resistant. Using conjugation experiments, the researchers demonstrated that tetracycline resistance could be transferred from all three donor strains to L. ivanovii, and from two strains to Enterococcus faecalis. Molecular analysis using PCR, Southern blotting, and pulsed-field gel electrophoresis revealed that the tetracycline resistance gene was chromosomally located in all strains and identified as tetM. Notably, no plasmids were detected in any donors or transconjugants, indicating chromosomal rather than plasmid-mediated resistance. This is the first evidence that foodborne L. monocytogenes can serve as a source of transferable tetracycline resistance genes capable of spreading to other bacterial species, representing a potential public health concern.

Key findings

• Tetracycline resistance in foodborne L. monocytogenes is mediated by the tetM gene located on the chromosome, not plasmids
• The tetM resistance gene can be conjugatively transferred from L. monocytogenes to both L. ivanovii and Enterococcus faecalis
• This is the first demonstration that foodborne L. monocytogenes can transfer tetracycline resistance genes to other microorganisms
• Resistance frequencies ranged from 1.9×10⁻⁶ to 6×10⁻⁶ for L. ivanovii and 2.4×10⁻⁷ to 3×10⁻⁷ for E. faecalis