Listeria Monocytogenes Isolated from Ready-To-at Food Products in Tehran: Prevalence and Antimicrobial Resistance

L. monocytogenes is a significant foodborne pathogen that is associated with a range of clinical illnesses, from self-limited gastroenteritis to invasive infection, which can lead to hospitalization of immunocompromised individuals. In the present study, the incidence of L. monocytogenes in ready-to-eat (RTE) food samples from Tehran, Iran, was therefore measured. A total of 110 samples were collected from various ready-to-eat (RTE) foods in different zones of Tehran from April to September of 2022. The samples were obtained from various types of food, including Caesar salad, Olivier salad, burger, schnitzel, sushi, and sausage. The identification of isolates was facilitated by the detection of hlyA and prfA genes through a polymerase chain reaction (PCR) approach. The antimicrobial resistance profile of the isolates was assessed through the use of a disc diffusion assay and the PCR amplification of resistance genes. Among the 110 samples, 14 (12.7%) were identified as Listeria spp., and 6 (5.5%) were confirmed as L. monocytogenes by molecular methods. The prevalence of Listeria spp. was observed to be highest in schnitzel and burgers, with 30% of schnitzel samples and 25% of burger samples being positive. Among the 14 isolates, 6 samples (42%) were identified as L. monocytogenes. The highest rate of L. monocytogenes was observed in burgers, accounting for 20% of the total burger samples. In contrast, no L. monocytogenes was identified in Caesar salad, sausage, and sushi samples. The L. monocytogenes isolates demonstrated resistance to oxacillin, streptomycin, cotrimoxazole, clindamycin, and cefoxitin, and were susceptible to chloramphenicol. Furthermore, the isolates demonstrated intermediate susceptibility to fosfomycin and ampicillin. Furthermore, the isolates demonstrating resistance to erythromycin contained genes associated with resistance to the macrolide class of antibiotics, including ermA and ermB. However, the presence of cfxA and mecA genes was detected in a single isolate resistant to cefoxitin and oxacillin. The prevalence of these findings underscores significant concerns regarding the potential for listeriosis to pose a threat to consumers of ready-to-eat (RTE) food products.