Objective In order to identify the pathogenic bacteria causing the tilapia disease outbreak in the geese-fish polyculture system, the growth characteristics, pathogenicity and drug sensitivity of the pathogenic bacteria were analyzed, with an aim to provide scientific basis for further research and effective prevention and control of tilapia disease.

Method The pathogenic bacteria were isolated from the organs of diseased tilapia and identified by various methods such as morphological observation, physiological and biochemical identification, and 16S rRNA gene sequence analysis. The impacts of temperature, salinity and pH on the growth of pathogenic bacteria were analyzed, and a growth curve for the strain was plotted. The pathogenicity of the bacterial strain was assessed by intraperitoneal injection and oral feeding of mixed materials. Additionally, the susceptibility of the strain to 26 different antimicrobial agents belonging to 11 classes was determined by the drug sensitive slips method.

Result A dominant strain SAYJN was isolated and purified from the brain tissue of the diseased tilapia. Through morphological observation, physiological and biochemical identification, 16S rRNA gene sequence analysis and artificial regression infection test, the strain was identified as Streptococcus agalactiae. The growth retardation stage of SAYJN was 0-5 h, and its logarithmic stage was 5-12 h. The optimal conditions for growth were found at a temperature of 37 ℃, pH value of 7.0 and salinity of 5‰. The strain SAYJN with intraperitoneal injection and oral feeding of mixed materials could infect the healthy tilapia. In an artificial infection test by intrabitoneal injection, the strain SAYJN showed high pathogenicity to tilapia at a temperature of 29 (±1) ℃, with a lethal concentration of 2.4×10⁸ CFU/mL. The pathogenicity of the strain SAYJN was found to be closely related to bacterial concentration and water temperature. Further, the strain SAYJN displayed resistance to six drugs such as trimethopridine, gentamicin, neomycin and etc. It showed moderate sensitivity to four drugs such as enrofloxacin, ciprofloxacin and amikacin. Additionally, it was found to be sensitive to 16 drugs such as doxycycline, flufenicol, rifampicin and etc.

Conclusion The strain SAYJN of S. agalactiae is the primary pathogen responsible for the outbreak of tilapia disease in the goose-fish polyculture environment, and the pathogenicity of this pathogen is closely related to the bacterial concentration and water temperature.