Abstract
Listeria monocytogenes, the bacterial foodborne pathogen responsible for the severe disease listeriosis, frequently exhibits heavy metal resistance. Concurrent resistance to cadmium and arsenic in L. monocytogenes is strongly associated with the 35-kb chromosomal island LGI2. LGI2 has been encountered repeatedly among L. monocytogenes serotype 4b hypervirulent clones but, surprisingly, not among non-pathogenic Listeria spp. Here we describe a novel LGI2 variant, LGI2-3, in two L. welshimeri strains from an urban aquatic environment. Whole genome sequence analysis revealed that the genomes were closely related except for one prophage region and confirmed a chromosomally integrated LGI2-3. It harbored a cystathionine beta-lyase gene previously only encountered in LGI2-1 of L. monocytogenes clonal complex 1 but was otherwise most closely related to LGI2. LGI2-3 harbored a novel cadAC cassette (cadA7C7) that, like LGI2′s cadA4C4, was associated with lower-level tolerance to cadmium (MIC 50 μg/mL) than other cadAC cassettes (MIC ≥ 140 μg/mL). CadA sequence analysis identified two amino acids that may be important for mediating different levels of cadmium tolerance. Our findings clearly demonstrated the potential for LGI2-like islands to be harbored by non-pathogenic Listeria spp. and generate intriguing hypotheses on the genetic diversity mediated by this island and its transfer among Listeria spp.
Highlights
Listeria monocytogenes is the only human pathogen in the genus Listeria and is responsible for the severe foodborne disease listeriosis, which is characterized by symptoms such as stillbirths, meningitis and septicemia, and has high hospitalization and case fatality rates [1,2,3,4]
One colony from each sample was chosen for Whole Genome Sequencing (WGS), and analysis of the whole genome sequence data indicated that both strains were L. welshimeri
We obtained clear evidence of the presence of a Listeria genomic island 2 (LGI2)-like chromosomal island, LGI2-3, in two L. welshimeri strains recently isolated from an urban aquatic ecosystem in North
Summary
Listeria monocytogenes is the only human pathogen in the genus Listeria and is responsible for the severe foodborne disease listeriosis, which is characterized by symptoms such as stillbirths, meningitis and septicemia, and has high hospitalization and case fatality rates [1,2,3,4]. The 13 known serotypes of L. monocytogenes are classified into four lineages (I to IV); the lineage II serotype 1/2a and the lineage I serotypes 1/2b and 4b contribute to the majority of human cases of listeriosis [5,6,7]. L. monocytogenes is notorious for its capacity to persistently colonize food-processing environments and equipment [6,10,11]. Persistence in food-processing environments is mediated by multiple mechanisms and adaptive traits of L. monocytogenes, including the capacity to grow at low temperatures, biofilm formation, and resistance to disinfectants and phages [6,10,11,12]. Resistance to heavy metals, especially cadmium and arsenic, has long been recognized as a major adaptation of L. monocytogenes [13,14]. Resistance to cadmium has been extensively investigated, leading to the identification of a large array of cadAC cadmium
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