Isolation, identification and pathogenicity of Proteus vulgaris from moribund common carp (Cyprinus carpio) farmed in China

Abstract

Common carp (Cyprinus carpio) is one of the most important aquaculture species in the world. In recent years, several outbreaks of unknown etiology has caused huge economic losses in common carp culture. To determine the death cause of common carp in Chongqing, China, we isolated potential pathogens from the infected common carp. The isolated bacteria (CQC01) recovered from moribund common carp was identified as Proteus vulgaris based on biochemical characterization and 16S rDNA sequence analysis. The pathogenicity of CQC01 was determined by artificial infection. Common carp experimentally infected with CQC01 showed the same symptoms as the naturally infected fish, including swelling on the anus, congestion on the eyelids, fins and abdominal muscle. At 6 h post infection (hpi), malondialdehyde (MDA), catalase (CAT), alanine aminotransferase (ALT), alkaline phosphatase (AKP), and lysozyme (LYZ) level in serum of fish infected with high concentration of CQC01 (109 CFU/mL) were significantly increased, but superoxide dismutase (SOD) activity was significantly decreased; whereas, liver tissue of infected fish showed immunological and oxidative stress. The histopathology of the challenged fish showed pathological lesions on hepatopancreas, kidney, spleen and muscle, suggesting CQC01 infection caused great damage to fish. Antimicrobial susceptibility test showed that the strain CQC01 was not resistant to most of antibiotics. Narrow-spectrum antibiotics were recommended for the control of strain CQC01. This study highlights the incrimination of P. vulgaris in the disease outbreak of cultured common carp for the first time. These findings provide guidance for the prevention and control of Proteus related diseases.