Abstract
Spotted sea bass (Lateolabrax maculatus) is a high-economic-value aquacultural fish widely distributed in the coastal and estuarine areas of East Asia. In August 2020, a sudden outbreak of disease accompanied by significant mortality was documented in L. maculatus reared in marine cage cultures located in Nanhuang island, Yantai, China. Two coinfected bacterial strains, namely, NH-LM1 and NH-LM2, were isolated from the diseased L. maculatus for the first time. Through phylogenetic tree analysis, biochemical characterization, and genomic investigation, the isolated bacterial strains were identified as Vibrio harveyi and Photobacterium damselae subsp. piscicida, respectively. The genomic analysis revealed that V. harveyi possesses two circular chromosomes and six plasmids, while P. damselae subsp. piscicida possesses two circular chromosomes and two plasmids. Furthermore, pathogenic genes analysis identified 587 and 484 genes in V. harveyi and P. damselae subsp. piscicida, respectively. Additionally, drug-sensitivity testing demonstrated both V. harveyi and P. damselae subsp. piscicida exhibited sensitivity to chloramphenicol, ciprofloxacin, ofloxacin, orfloxacin, minocycline, doxycycline, tetracycline, and ceftriaxone. Moreover, antibiotic resistance genes were detected in the plasmids of both strains. Extracellular product (ECP) analysis demonstrated that both V. harveyi and P. damselae subsp. piscicida can produce hemolysin and amylase, while V. harveyi additionally can produce caseinase and esterase. Furthermore, infected fish displayed severe histopathological alterations, including infiltration of lymphocytes, cellular degeneration and necrosis, and loose aggregation of cells. Artificial infection assays determined that the LD(50) of P. damselae subsp. piscicida was 3 × 10(5) CFU/g, while the LD(50) of V. harveyi was too low to be accurately evaluated. Furthermore, the dual infection of V. harveyi and P. damselae subsp. piscicida elicits a more rapid and pronounced mortality rate compared to single challenge, thereby potentially exacerbating the severity of the disease through synergistic effects. Ultimately, our findings offer compelling evidence for the occurrence of coinfections involving V. harveyi and P. damselae subsp. piscicida in L. maculatus, thereby contributing to the advancement of diagnostic and preventative measures for the associated disease.