Identification, Characterization, and Pathogenic Investigation of Aeromonas veronii, Aeromonas hydrophila, Plesiomonas shigelloides, and Citrobacter freundii from a Case of Natural Mixed Infection in Common Carp (Cyprinus carpio)

Aeromonas veronii, Aeromonas hydrophila, Plesiomonas shigelloides, and Citrobacter freundii were opportunistic pathogen widely distributed in water environment and fish population, causing fish diseases under stressful conditions. In this study, bacteria were isolated from diseased common carp Cyprinus carpio with symptoms of hemorrhage on body surface, abdominal distention, and flatulence in the intestine. According to the results of biochemical characteristics, 16S rRNA sequencing analyses, the isolates were identified as A. veronii, A. hydrophila, P. shigelloides, and C. freundii. The experimentally infected fish showed identical symptoms as observed in the naturally infected common carp. The LD50 of C. freundii, P. shigelloides, A. veronii, A. hydrophila, and mixture group were 1.95×104, 4.74×104, 5.12×104, 1.53×105, and 5.41×104 respectively. Antibiotic resistance results showed that P. shigelloides, A. veronii, and A. hydrophila were sensitive to streptomycin, enrofloxacin, florfenicol, gentamicin, kanamycin, neomycin, norfloxacin, co-trimoxazole, ceftizoxime, and resistant to ampicillin. C. freundii was sensitive to streptomycin, gentamicin, and ceftizoxime, moderately susceptible to kanamycin, neomycin, norfloxacin, and ampicillin, and resistant to enrofloxacin, florfenicol, tetracycline, and co-trimoxazole.