Listeria monocytogenes clones circulating in the natural environment of the Czech Republic and Slovakia

Listeria monocytogenes is not only a pathogen causing a serious food-borne disease in humans but can also occur as a saprophyte in the natural environment. This study aimed to evaluate the occurrence of L. monocytogenes obtained from the natural environment of the Czech Republic (hereinafter Czechia) and Slovakia in 2016-2018 and to compare the clonal relationship of strains circulating in the environment with the strains originating from the food chain and humans. Altogether, 217 samples of mud, surface water, vegetation and soil were collected in 61 locations. Samples were processed according to the modified EN ISO 11290-1 standard. The obtained L. monocytogenes isolates were characterised using serotyping, macrorestriction analysis, followed by pulsed-field gel electrophoresis, whole genome sequencing (WGS), and antimicrobial susceptibility testing. L. monocytogenes were detected in 8.8% of the examined samples and were isolated in 15 locations, mainly from the mud from the banks of the surface water sources. Altogether, 25 L. monocytogenes strains were obtained from 19 positive samples. Serotypes 1/2a, 4b, and 1/2b were detected among the strains. Twenty combined AscI/ApaI pulsotypes were obtained by macrorestriction analysis. Altogether, 12 sequence types (STs) were detected using Multi-locus sequence typing (MLST) by WGS, with ST451 being the most frequent. The core genome MLST analysis revealed a heterogeneous population of environmental strains. No phenotype resistance was detected by antimicrobial susceptibility testing. Screening of antimicrobial-resistance genes using the platform ResFinder revealed the genes fosX in 24 isolates and blaTEM-116 in one isolate. The occurrence of L. monocytogenes in various samples from natural environments within wide altitude range during different seasons of the year may highlight this bacterium's remarkable adaptability and exceptional tolerance to external factors. Serotype distribution of the strains circulating in the natural environment of Czechia and Slovakia seems to reflect distribution in the human population more than in the food chain.