Characterization of a Novel Conjugative Plasmid in Strain MS-18-199.

is a pathogenic bacterium responsible for significant losses in important wild and cultured fish species. strain MS-18-199 recovered from a diseased hybrid catfish from East Mississippi and showed resistance to florfenicol, chloramphenicol, oxytetracycline, doxycycline, erythromycin, tetracycline, azitromycin, spectinomycin, sulfonamide, and bacitracin. To explore the mechanisms of resistance in strain MS-18-199, genomic DNA was extracted and subjected to whole genome sequencing (WGS) using a combination of long (Oxford Nanopore) and short (Illumina) reads. The genome of strain MS-18-199 revealed a novel plasmid named pEPMS-18199. The 117,448 bp plasmid contains several antimicrobial resistance (AMR) elements/genes, including florfenicol efflux pump (), tetracycline efflux pump (), tetracycline repressor protein (), sulfonamide resistance (), aminoglycoside O-phosphotransferase -Id (), and aminoglycoside O-phosphotransferase -Ib (). Two genes, and , that encode protein components related to transport/resistance to arsenic were also found in pEPMS-18199. In addition, pEPMS-18199 carried twelve conjugative transfer genes (), eight transposases and insertion elements, two plasmid stability proteins, two replication proteins, and three partitioning proteins ( system). Results from mobilization and stability experiments revealed that pEPMS-18199 is highly stable in the host cell and could be transferred to and by conjugation. To our knowledge, this is the first detection of a multidrug resistance (MDR) conjugative plasmid in in the United States. Careful tracking of this plasmid in the aquaculture system is warranted. Knowledge regarding the molecular mechanisms of AMR in aquaculture is important for antimicrobial stewardship.