Characterization of Listeria monocytogenes Isolates from Pork Production in Southern Sonora, Mexico: Serotyping, Antimicrobial Resistance, Chitosan Susceptibility, and Pathogenicity in a Chicken Embryo Model

Listeria monocytogenes is often found in pork intestines and can contaminate pork production, posing a risk to consumers. This study aimed to characterize 16 L. monocytogenes isolates from fresh and packaged pork loin, identify their serotypes, and assess antibiotic resistance. To evaluate chitosan susceptibility as a potential strategy to control L. monocytogenes in the pork industry and to determine its effectiveness in a eukaryotic model to demonstrate pathogenicity. Among the 16 isolates examined, 2 were identified as 1/2a, 12 as 1/2b, 2 as 4b, and 2 could not be assigned a serotype. Variations were observed in their pathogenicity factors. Some isolates were lacking in some virulence factors. In the antibiotic assays, all isolates demonstrated resistance to at least three antibiotics, and one of them exhibited resistance to as many as ten antimicrobial agents. To propose an alternative in the food industry as a decontamination agent, a low-molecular-weight chitosan was evaluated. It was shown that chitosan inhibits the growth of L. monocytogenes in a concentration of 0.25% in 45 min, resulting in a viable alternative against this pathogen, but in this work, one isolate exhibited resistance to chitosan (isolate Lm 1.2). Regarding infection in eukaryotic models, all isolates had the capacity to infect chicken embryos, except for isolate 1.2, which exhibited attenuated pathogenicity. These findings highlight the potential public health risk L. monocytogenes poses in pork and the need for continued research to develop effective control strategies.