Phenotypic profiling of biofilm formation and antibiotic susceptibility in poultry-derived Listeria monocytogenes isolates

Listeria monocytogenes is a critical foodborne pathogen, with poultry products serving as a potential reservoir. Its ability to form biofilms may aid in its persistence on processing equipment and food-contact surfaces, while antimicrobial resistance complicates efforts to control and treat infections. This study analyzed 93 L. monocytogenes isolates from poultry to assess their biofilm-forming capacity and antibiotic resistance. Biofilms were tested on polystyrene microtiter plates at 12 °C and 30 °C in a nutrient-rich medium (Brain Heart Infusion, BHI). Susceptibility to eight antibiotics relevant to clinical and food safety was tested by disk diffusion and evaluated using EUCAST breakpoints, where available. Most isolates produced detectable, typically weak biofilms at both temperatures, with some shifting to moderate biofilm formation after extended incubation at 12 °C; no strong biofilm producers were observed. This underscores the significant role of incubation time and temperature in surface colonization. Overall, the isolates remained mostly susceptible to ampicillin, penicillin G, vancomycin, tetracycline, and chloramphenicol, whereas some subpopulations exhibited resistance or low susceptibility to trimethoprim–sulfamethoxazole (TMP-SMX) and, notably, erythromycin and streptomycin. No consistent link was found between biofilm formation and antibiotic susceptibility, suggesting these traits are largely independent among these isolates. These results demonstrate that poultry-derived L. monocytogenes can form weak to moderate biofilms under the tested monoculture conditions while generally remaining susceptible to first-line antibiotics. Nevertheless, the emergence of resistance to macrolides and aminoglycosides, together with the potential for increased colonization in complex multispecies biofilms in real food processing environments, underscores the need for ongoing, integrated surveillance across animal food systems.