Molecular characteristics and comparative genomics analysis of a clinical Enterococcus casseliflavus with a resistance plasmid Min Yin,1,* Yi Jiang,1,* Changrui Qian,1 Fei Wu,1 Yuanyuan Ying,1 Chongyang Wu,1 Peizhen Li,1 Jun Ying,1 Kewei Li,1 Teng Xu,1 Qiyu Bao,1 Caixia Sun2 1Department of Microbiology and Immunology, School of Laboratory Medicine and Life Sciences/Institute of Biomedical Informatics, Wenzhou Medical University, Wenzhou, Zhejiang 325035, China; 2Nursing Department, The First Affiliated Hospital of Wenzhou Medical University, Wenzhou, Zhejiang 325000, China *These authors contributed equally to this work Purpose: The aim of this work was to investigate the molecular characterization of a clinical Enterococcus casseliflavus strain with a resistance plasmid. Materials and methods: En. casseliflavus EC369 was isolated from a patient in a hospital in southern China. The minimum inhibitory concentration was found by means of the agar dilution method to determine the antimicrobial susceptibilities of the strains. Whole-genome sequencing and comparative genomics analysis were performed to analyze the mechanism of antibiotic resistance and the horizontal gene transfer of the resistance gene-related mobile genetic elements. Results: En. casseliflavus EC369 showed resistance to erythromycin, kanamycin, and streptomycin, but was susceptible to vancomycin, ampicillin, and streptothricin and other antimicrobials. There were six resistance genes (aph3′, ant6, bla, sat4, and two ermBs) carried by a transposon identified on the plasmid pEC369 and a complete resistance gene cluster of vancomycin and a tet (M) gene encoded on the chromosome. This is the first complete plasmid sequence reported in clinically isolated En. casseliflavus. The plasmid with the greatest sequence identity with pEC369 was the plasmid of Enterococcus sp. FDAARGOS_375, followed by the plasmids of Enterococcus faecium strains F12085 and pRE25, whereas the sequence with the greatest identity to the resistance genes carrying a transposon of pEC369 was on the chromosome of Staphylococcus aureus strain GD1677. Conclusion: The resistance profiles of En. casseliflavus EC369 might contribute to the resistance genes encoded on the plasmid. The fact that the most similar sequence to the transposon carrying resistance genes of pEC369 was encoded in the chromosome of a S. aureus strain provides insights into the mechanism of dissemination of multidrug resistance between bacteria of different species or genera through horizontal gene transfer. Keywords: Enterococcus casseliflavus, antimicrobial resistance, transposon, molecular characteristics, comparative genomics analysis