Abstract
Aeromonas veronii, Aeromonas hydrophila, Plesiomonas shigelloides, and Citrobacter freundii are opportunistic pathogens that are widely distributed in aquatic environments and fish populations, causing fish diseases under stressful conditions. In this study, bacteria were isolated from a group of diseased common carp, Cyprinus carpio, with symptoms of hemorrhaging along the body surface, abdominal distention, and flatulence in the intestine. According to the results of biochemical characteristics and 16S rRNA sequencing analyses, the isolates were identified as A. veronii, A. hydrophila, P. shigelloides, and C. freundii. The virulence genes from these four representative bacterial strains were amplified. The experimentally co-infected fish showed similar symptoms to those observed in the naturally infected common carp. The LD(50) of C. freundii, P. shigelloides, A. veronii, A. hydrophila, and the mixture group were 1.95 × 10(4), 4.74 × 10(4), 5.12 × 10(4), 1.53 × 10(5), and 5.41 × 10(4) CFU, respectively. The antibiotic resistance results showed that P. shigelloides, A. veronii, and A. hydrophila were most sensitive to ceftizoxime, florfenicol, enrofloxacin, and norfloxacin. C. freundii was most sensitive to ceftizoxime and gentamicin.