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Listeria monocytogenes bacilli are a known cause of non-invasive gastrointestinal listeriosis with diarrhea, less frequently they cause invasive infections: meningitis and encephalitis, bacteremia and its complications: endocarditis, intraocular inflammation of the eye and others. The risk of human infection is mainly associated with consuming food contaminated with these bacteria. According to the European Food Safety Authority (EFSA), infections with L. monocytogenes etiology caused the highest hospitalizations and deaths among zoonotic diseases reported in 2022. A report by the European Center for Disease Prevention and Control (ECDC) reported 0.44 cases of listeriosis per 100,000 population in 2021, with the highest incidence rate for people over 64 years old at 1.7.

Listeriosis is one of the most serious foodborne diseases under surveillance, with an overall mortality rate in the EU currently being high at 18.1%. Therefore, this study aims to investigate Listeria monocytogenes strains isolated from clinical and food samples for susceptibility to antimicrobials, presence of virulence factors, and genetic diversity. Species were identified using the MALDI-TOF, resistance to 11 antibiotics was determined according to EUCAST guidelines, and multiplex PCR was used for serotyping and detecting virulence genes. Strains were genotyped using the PFGE method. Clinical strains showed full sensitivity to all tested antibiotics. In total, 33.3% of strains from food products were found to be resistant to ciprofloxacin and 4.2% to tetracycline. Most of the tested isolates (79.2%) belonged to serotype 1/2a-3a, and the rest (20.8%) belonged to serotype 4ab-4b,4d-4e. Five virulence genes (prfA, hlyA, plcB, inlA, and lmo2672) were detected in all strains studied. The llsX gene was the least common, in 37.5% of clinical strains and 18.75% of strains isolated from food products. Among the analyzed strains, 13 strains displayed unique PFGE profiles. The other 11 strains belong to 3 clusters of pulsotypes: cluster 1 (2 strains), cluster 2 (6 strains), and cluster 3 (2 strains). The percentage of hospitalizations and deaths of Polish patients with listeriosis indicates the seriousness of this disease, especially in an aging society, while the molecular testing of clinical strains has been rarely performed, which makes it difficult to determine the source of infection. [ABSTRACT FROM AUTHOR]

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