Antibiotic Resistance Profiles and Genetic Determinants of Listeria innocua Isolated from Food Sources in Poland

Background: Antimicrobial resistance (AMR) is a growing public health concern affecting both medicine and food safety. While Listeria monocytogenes is the primary pathogen of concern, Listeria innocua—commonly found in food and food-processing environments—may serve as a reservoir for resistance genes and a useful indicator of species for surveillance. This study aimed to assess the phenotypic antibiotic susceptibility and detect resistance-associated genes in L. innocua isolates from meat products and processing environments in Poland. Methods: A total of 51 L. innocua isolates were analyzed, originating from raw and processed meat products as well as meat-processing environments. Antimicrobial susceptibility was determined using the disc diffusion method against 18 antibiotics representing multiple classes. Phenotypic resistance was interpreted following CLSI guidelines (CLSI, 2020). Isolates exhibiting resistance or intermediate resistance were further screened for resistance-associated genes using PCR. Results: All isolates were fully susceptible to ampicillin, benzylpenicillin, chloramphenicol, gentamicin, rifampin, trimethoprim-sulfamethoxazole, and vancomycin. High susceptibility was observed for ciprofloxacin, erythromycin, meropenem, trimethoprim, and nitrofurantoin, with only sporadic intermediate responses. Moderate resistance levels were noted for streptomycin (10%) and tetracycline (12%). The lowest susceptibility was recorded for clindamycin and linezolid, with most isolates exhibiting intermediate or resistant phenotypes. Universal resistance to cefotaxime and oxacillin was found. Eighteen distinct resistance patterns were identified. PCR confirmed the presence of several resistance-associated genes, including mecA, lnuA, lnuB, cfr, optrA, and poxtA, consistent with observed phenotypes. Conclusions: This study provides the first detailed characterization of AMR in L. innocua from Polish meat and processing environments. The findings highlight its heterogeneous resistance profiles and potential role as a reservoir of clinically relevant resistance genes. Incorporating L. innocua into surveillance programs may strengthen early detection of emerging resistance and enhance food safety monitoring.