Isolation, identification, and comparative genomic analysis of Vibrio harveyi as a causative agent for a skin ulcer disease of endangered fish Chinese bahaba (Bahaba taipingensis)

Chinese bahaba, a fish species native to China and protected under national law, has faced significant challenges in artificial breeding efforts. In August 2024, a farming facility in Guangdong Province experienced a mass die-off of the species, with affected fish displaying a range of symptoms including skin ulcers, ascites, gastroenteritis, and swollen internal organs. Upon further investigation, researchers discovered that the cause of the outbreak was the bacterium Vibrio harveyi.

Whole-genome analysis of the isolated strain, designated HCY1, confirmed its identity as Vibrio harveyi. When this bacterium was introduced to healthy fish in a controlled environment, it resulted in 100% mortality and the development of symptoms resembling those observed in the naturally infected fish population. Antibiotic susceptibility testing revealed that HCY1 was susceptible to certain medications, such as florfenicol and trimethoprim-sulfamethoxazole, but showed resistance to a range of antibiotics, including beta-lactams and tetracyclines.

Further genomic examination of HCY1 highlighted the presence of genes responsible for antibiotic resistance, particularly those conferring immunity to beta-lactam and tetracycline antibiotics. The bacterium also possessed numerous virulence factors, including complete type III and type VI secretion systems, as well as genes involved in immune evasion. Additionally, HCY1's plasmids contained multiple genes that enhanced its virulence, such as mcpA, pal, and esiB.

This investigation marks the first detailed examination of a disease outbreak in cultured Chinese bahaba, offering critical insights for managing the health of this endangered species and preventing further population declines.