Identification of as a novel pathogen of the largemouth bass () and histopathological analysis of diseased fish.

The largemouth bass () is an economically important aquaculture species in China, and its production has increased rapidly in recent years. Although is known to infect several fish species, its role in infecting is relatively unknown. Here, we isolated a gram-negative bacterial strain (termed XX2021) from farmed largemouth bass. Based on the results of 16S rRNA sequencing and phylogenetic analyses, the isolate was identified as The virulence of XX2021 was dependent on water temperature, such as the LD values were 4.21×10, 7.26×10, and 2.47×10 CFU/g fish weight at 10°C, 18°C, and 25°C, respectively. Four virulent genes-including , and -were screened through a PCR assay. The results of an antibiotic resistance test showed that XX2021 was sensitive to kanamycin, cefotaxime, doxycycline, sulfamethoxazole, florfenicol, tetracycline, and gentamicin; showed intermediate susceptibility to streptomycin, ampicillin, and norfloxacin; and was resistant to nalidixic acid and penicillin. XX2021-infected fish showed clinical symptoms typical of infection. In addition, we re-isolated XX2021 from infected fish and confirmed its identity using 16S rRNA sequencing. Histopathological changes were observed in the intestine, head kidney, spleen, and liver of diseased fish. This study presents the first report of the pathogenic effects of in farmed largemouth bass. Our findings may help develop effective disease control strategies for aquaculture fish and prevent disease outbreaks under low water temperatures.