Phenotypic Profiling of Biofilm Formation and Antibiotic Susceptibility in Poultry-Derived Listeria monocytogenes Isolates

This study examined the biofilm-forming capacity and antibiotic resistance of 93 L. monocytogenes isolates from poultry to better understand how this pathogen persists and how it can be more effectively controlled throughout the poultry production process. Biofilms were evaluated on polystyrene microtiter plates at 12 and 30 °C in a nutrient-rich laboratory medium (Brain Heart Infusion, BHI). Susceptibility to eight clinically and food-relevant antibiotics was tested using disk diffusion and interpreted according to European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints when available. Most isolates produced detectable but generally weak biofilms at both temperatures, with a subset shifting to moderate biofilm formation after prolonged incubation at 12 °C; no strong biofilm producers were identified. This highlights the significant influence of incubation time and temperature on surface colonization. Overall, the isolates remained largely susceptible to ampicillin, penicillin G, vancomycin, tetracycline, and chloramphenicol, although resistant or low-susceptibility subpopulations were noted for trimethoprim–sulphamethoxazole (TMP-SMX) and, particularly, erythromycin and streptomycin. No consistent correlation was found between biofilm-forming ability and antibiotic susceptibility, indicating that these phenotypic traits are largely independent in this collection. These findings reveal that poultry-derived L. monocytogenes isolates can form weak to moderate biofilms under the tested monoculture conditions while generally maintaining susceptibility to first-line antibiotics. However, the development of macrolide- and aminoglycoside-resistant subpopulations, along with the potential for increased colonization within complex multispecies biofilms in real processing environments, emphasizes the importance of ongoing integrated surveillance across animal food systems.