Phenotypic and genotypic characterization of Listeria monocytogenes in clinical ruminant cases in Korea

Listeria monocytogenes, a foodborne human and veterinary pathogen, is associated with high mortality rates in ruminants. However, no studies have investigated the antimicrobial resistance of L. monocytogenes isolates from clinical ruminant cases. This study aimed to determine the phenotypic and genotypic characteristics of L. monocytogenes isolates from clinical cases of Korean ruminants. We collected 24 L. monocytogenes isolates from aborted bovine fetuses and goats presenting with listeriosis-related symptoms. The isolates were subjected to PCR serogrouping, conventional serotyping, virulence gene detection, and antimicrobial susceptibility testing. Furthermore, pulsed-field gel electrophoresis and multilocus sequence typing were used to classify and compare genetic diversity among the isolates, including human L. monocytogenes isolates. The most prevalent L. monocytogenes serotypes were 4b (Ⅳb), 1/2a (Ⅱa; Ⅱc), and 1/2b (Ⅱb). All isolates harbored the virulence genes; however, llsX-encoding listeriolysin were identified only in serotypes 4b and 1/2b. All isolates, including two found in humans, formed three genetically diverse pulsed-field gel electrophoresis clusters according to serotype, lineage, and sequence type. The most prevalent sequence type was ST1, followed by ST365 and ST91. The isolates from ruminants with listeriosis were resistant to oxacillin and ceftriaxone and showed diverse lineage, serotype (serogroup), and sequence type characteristics. Considering that the atypical sequence types exhibited clinical manifestations and histopathological lesions, further study is needed to elucidate the pathogenicity of genetically diverse ruminant L. monocytogenes isolates. Furthermore, continuous monitoring of antimicrobial resistance is required to prevent the emergence of L. monocytogenes strains resistant to common antimicrobials.