Objective The causative agent was isolated from the diseased Micropterus salmoides tissue. The physiological properties of the pathogenic bacteria were identified, and their virulence genes and drug resistance genes were analyzed, with an aim to provide guidance for the scientific use of drugs in clinical practice.

Method Samples were collected from pathologic tissues of diseased M. salmoides from a fish breeding factory in Zhanjiang, Guangdong Province, China. The bacteria were isolated by dilution coating method, the 16S rRNA sequence comparison and biochemical tests were performed to identify the species, and the virulence genes, drug resistance genes and drug sensitivity characteristics were also identified.

Result Bacteria with round colony morphology, smooth milky white surface and opaque color were isolated from pathologic tissues of M. salmoides, and numbered as MSB-2, which was identified as Aeromonas in Gram-negative bacteria through phenotypic identification by Gram staining and physiological and biochemical reactions. By amplifying the 16S rRNA gene sequence and constructing a phylogenetic tree, the bacterium was further identified as A. veronii. The strain had virulence-related genes such as act, aer and aexT and resistance-related genes such as tetC, tetA and sull. The results of the drug sensitivity test showed that the bacterium was sensitive to carbenicillin, doxycycline, cefalexin, neomycin, gentamicin, cefradine, minocycline, cefazolin and etc. Artificial regression of healthy M. salmoides infected with A. veronii showing clinical signs similar to those of diseased fish, which were the same as those of the naturally occurring disease, such as congestion of the ventral and pectoral fin bases, redness and swelling of the anus, and obvious enlargement of the abdomen, etc. And a large amount of fluid in the abdominal cavity was found by autopsy.

Conclusion The above results indicate that A. veronii is the pathogenic bacterium causing this disease of M. salmoides, and the approved fishery drugs such as neomycin and doxycycline can be used to control the disease during the breeding process.