Edwardsiella piscicida, a pathogenic bacterium newly detected in spotted sea bass Lateolabrax maculatus in China

In this study, Edwardsiella piscicida strain 18BJ136 was isolated from diseased spotted sea bass Lateolabrax maculatus, in China, and its pathogenicity in this newly recognized host was investigated. Physiological and biochemical tests showed that 18BJ136 did not produce H2S but utilized d-maltose, and was positive for succinate alkalinization, coumarate, and the ellman reaction. Its growth characteristic showed that it proliferated in a wide range of salinities, but was sensitive to acidity and alkalinity. A PCR analysis showed that 18BJ136 carried at least nine virulence genes: gadB, katB, citC, usp13, esrB, sodB, luxS, fimA, and mukF. A spot test showed that the strain had a variety of extracellular enzyme activities, including hemolysin and protease. Artificial infection experiments indicated that 18BJ136 was strongly virulent in the spotted sea bass, with a median lethal dose (LD50) of 1.23 × 103 cfu/g. Similar to the naturally infected fish, the main clinical symptoms of the artificially infected fish were festering scores of skin, scale shedding, and the swelling of multiple internal organs. The 18BJ136 caused the following histopathological lesions in the spotted sea bass: different degrees of inflammation in each tissue; multiple necrotic foci in the liver, spleen, and head kidney; and severe hemorrhage in the spleen and head kidney. The expression of IL-8, TNF-α, TLR-23, and HSP70 was significantly upregulated after infection (P < 0.05), indicating that the pathogen activated the host's innate immune response. These results extended our understanding of E. piscicida infection in the spotted sea bass.