Identification and biological characterization of the pathogen responsible for acute haemorrhagic disease in largemouth bass

To investigate the cause of mass mortality in largemouth bass ( Micropterus salmoides ) at an aquaculture farm in Hubei due to acute hemorrhagic disease, a dominant bacterium named LY02 was isolated from the brain and major internal organs of diseased fishdise. A re-infection experiment confirmed that LY02 was the pathogenic bacterium responsible for the mass mortality. Histopathological studies revealed that the infection caused varying degrees of necrosis and congestion in the brain and major internal organs of the largemouth bass. Polymerase chain reaction (PCR) detection showed that the LY02 isolate carried the toxin genes simA , pdi , sagA , and cfi . Whole-genome analysis identified LY02 as Streptococcus iniae , carrying 330 toxin factor-related genes and 164 resistance factor-related genes. LY02 was found to be highly sensitive to ceftizoxime, cefradine, cefotaxime, azithromycin, erythromycin, enoxacin, doxycycline, and netilmicin; moderately sensitive to norfloxacin and enrofloxacin; and resistant to gentamicin, streptomycin, kanamycin, tobramycin, and neomycin. This study is the first to isolate and identify S. iniae from infected largemouth bass, providing a theoretical basis for the diagnosis and prevention of acute hemorrhagic disease in this species. • It was found for the first time that Streptococcus iniae caused the disease of Largemouth bass. • LY02 carries the virulence genes simA, pdi, sagA, and cfi. • LY02 showed severe drug resistance.