Co-infections of Aeromonas dhakensis and Chryseobacterium indologenes in largemouth bass (Micropterus salmoides)

As one of the most important economic fishes, largemouth bass (Micropterus salmoides) is being widely farmed around the world. With the improper expansion and high-density culture, largemouth bass is susceptible to an increasing number of pathogens. Particularly, co-infection of multiple pathogens is ubiquitous, which has become a main obstacle to the development of largemouth bass industry. In the study, a co-infected case caused by Aeromonas dhakensis (A. dhakensis) and Chryseobacterium indologenes (C. indologenes) in largemouth bass was firstly reported. Diseased fish with clinical signs such as exophthalmia, hemorrhage on gill cover, rot or ulceration on gill, skin and fin were collected for further diagnostics. Tissue lesions of liver, spleen and kidney were observed, such as necrosis, hemorrhage, vacuolization and disintegration. Two bacteria strains were isolated from liver, one appeared as a white or yellowish colony, and the other was characterized by golden yellow color. Based on the conjoint analysis of 16S rRNA and gyrB gene sequences, the isolated strains were identified as A. dhakensis and C. indologenes, respectively. Antibiotic sensitivity testing indicated that both A. dhakensis and C. indologenes were sensitive to flumequine, enrofloxacin, ceftazidime and minocycline, and resistant to penicillin, sulfamethoxine sodium, ampicillin and cefamezin. Largemouth bass showed similar clinical signs and tissue lesions to naturally diseased fish following co-infection with the isolated strains, indicating that the isolated strains fulfill Koch's postulates. Besides, a slightly increased pathogenicity was observed following co-infection compared with single infection. The data firstly reveal a co-infected case caused by A. dhakensis and C. indologenes in largemouth bass, not only enriching the relevant information of largemouth bass diseases, but also promoting the diagnosis, prevention and control of A. dhakensis and C. indologenes co-infection.