Identification and pathological observation of a pathogenic Plesiomonas shigelloides strain isolated from cultured tilapia(Oreochromis niloticus)

Abstract:

Abstract:［Objective］A mass mortality of tilapia broke out in an aquaculture farm in Panyu，Guangdong Province in May，2013．Affected fish showed blackening of body color，haemorrhageing on surface，scales shedding，fin rotting，and the presence of yellow liver，dark red spleen，enlarged gallbladder and ascitic fluid in the abdominal cavity．The purpose of this research was isolating and identifying the pathogen． ［Methods］We isolated a suspicious bacteria strain PYS1 from diseased fish with significant pathological signs．The homology of 16S rRNA gene sequence of strain PYS1 and its morphological，cultural，and physical and chemical characteristics were studied for its identification．Its pathogenicity was investigated by recursive infection experiment and histopathological study．Its effective medicines was screened by antibiotic sensitive test．［Results］The results showed that strain PYS1 was Plesiomonas shigelloides clustered with P．shigelloides strains isolated from other fishes in the molecular phylogenetic tree of 16S rRNA gene sequences．Strain PYS1 was multiple drug resistant and only sensitive to a small part of 31 tested antibiotics (e.g.，ceftriaxone，cefaclor，cefazolin，etc.)．The symptoms of tilapia (O. niloticus) artificially infected with strain PYS1 were similar with natural infected fish．The half lethal dose (LD50) of strain PYS1 to tilapia was 1.425×108 CFU per fish． Paraffin sections showed intestine，liver，spleen，kidney and heart tissue injury caused by the strain．［Conclusion］ Our study demonstrated that P. shigelloides was the pathogen of cultured tilapia in the aquaculture farm and first reported its bacterial pathogenicity on Nile tilapia.