Mixed Infection in Common Carp () Caused by , , , and .

, , , and 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, , with symptoms of hemorrhaging along the body surface, abdominal distention, and flatulence in the intestine. According to the results of biochemical characteristics and sequencing analyses, the isolates were identified as , , , and . 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 of . , . , , , and the mixture group were 1.95 × 10, 4.74 × 10, 5.12 × 10, 1.53 × 10, and 5.41 × 10 CFU, respectively. The antibiotic resistance results showed that . , , and were most sensitive to ceftizoxime, florfenicol, enrofloxacin, and norfloxacin. . was most sensitive to ceftizoxime and gentamicin.