Abstract
Environmental adaptation of Listeria monocytogenes is a complex process involving various mechanisms that can contribute to their survival in the environment, further spreading throughout the food chain and the development of listeriosis. The aim of this study was to analyze whole-genome sequencing data in a set of 270 strains of L. monocytogenes derived from human listeriosis cases and food and environmental sources in order to compare the prevalence and type of genetic determinants encoding cadmium, arsenic, and benzalkonium chloride resistance. Most of the detected genes of cadmium (27.8%), arsenic (15.6%), and benzalkonium chloride (7.0%) resistance were located on mobile genetic elements, even in phylogenetically distant lineages I and II, which indicates the possibility of their horizontal spread. Although no differences were found in the prevalence of these genes between human and food strains, they have been detected sporadically in strains from the environment. Regarding cadmium resistance genes, cadA1C1_Tn5422 predominated, especially in clonal complexes (CCs) 121, 8, and 3 strains. At the same time, qacH_Tn6188-encoding benzalkonium chloride resistance was most frequently detected in the genome of CC121 strains. Genes encoding arsenic resistance were detected mainly in strains CC2 (located on the chromosomal island LGI2) and CC9 (carried on Tn554). The results indicated a relationship between the spread of genes encoding resistance to cadmium, arsenic, and benzalkonium chloride in certain serotypes and CCs and showed the need for a more extensive study of L. monocytogenes strains to better understand their ability to adapt to the food production environment.
Highlights
Listeria monocytogenes is a facultative intracellular pathogen and at the same time ubiquitous saprophytic bacterium, which can be isolated from the soil, water, and vegetation (Gray et al, 2006)
Distribution of Cadmium, Arsenic, and Benzalkonium Chloride Resistance Genes in L. monocytogenes From Various Sources In L. monocytogenes strains isolated from food, in sequences obtained by whole-genome sequencing, resistance genes to cadmium (36.8%), arsenic (23.6%), and benzalkonium chloride (9.4%) were detected more frequently in comparison with strains isolated from human cases of listeriosis (Table 1)
Out of the L. monocytogenes strains obtained from the environment, cadmium and arsenic resistance genes were detected in a strain isolated from decaying vegetation, whereas cadmium resistance genes and simultaneously benzalkonium chloride resistance genes were found in a strain isolated from pond water
Summary
Listeria monocytogenes is a facultative intracellular pathogen and at the same time ubiquitous saprophytic bacterium, which can be isolated from the soil, water, and vegetation (Gray et al, 2006). L. monocytogenes is known for its ability to persist in the food-processing environment. Its isolates have been classified into a number of clonal complexes (CCs) with specific characteristics and frequency of occurrence. The classification of L. monocytogenes into hypervirulent and hypovirulent clones is used. The hypervirulent clones include strains of CC1, CC2, CC4, and CC6, which are mainly associated with human and ruminant listeriosis cases, and the hypovirulent clones, typically containing truncated internalin A, include strains of CC121 and CC9, which are mainly associated with the occurrence and persistence in food-processing environments (Maury et al, 2019)
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