Abstract: 【Objective】 In April 2023,chicks in a broiler farm in Taiyuan，Shanxi province showed depression,loss of appetite,rapid onset,short course of disease,and high mortality.This experiment was aimed at determining the pathogenic bacteria infection of dead chickens and studying its biological characteristics,in order to provide reference for the prevention and control of bacterial infection diseases in this area.【Method】 Pathological examination and observation were carried out on dead chickens.The heart,liver and spleen tissues of dead chickens were inoculated with BHI agar medium in sterile environment by plate streak method.The morphology of isolates were observed by Gram staining.The bacterial species were identified by biochemical characterization,16S rRNA gene and bacteria-specific primers PCR,and the phylogenetic tree was constructed by Mega 11.0 software.The sensitivity of the isolates to common antibiotics was detected by drug sensitivity test,and the pathogenicity of the isolates was analyzed.【Result】 Subcutaneous edema and exudate were found in the necropsy of the dead chicken.There were bleeding points in the heart,liver,spleen and pectoral muscle.12 isolates were obtained by isolation and purification，8 of them showed obvious hemolysis on blood agar medium,but did not grow on MacConkey agar medium and the Gram staining was purple.3 isolates showed purplish red colonies in MacConkey agar medium and Gram staining was red.1 strain was transparent white colony,Gram staining was red. Biochemical identification results showed that the biochemical characteristics of the isolates were consistent with Staphylococcus aureus,Escherichia coli and Proteus mirabilis,respectively.The plasma coagulase test of 8 isolates was positive and 4 isolates were negative.The results of 16S rRNA gene PCR amplification and sequencing showed that the isolates were more than 98% similar to Staphylococcus aureus,Escherichia coli and Proteus mirabilis in GenBank,and were named SA-1 to SA-8,E1 to E3，and P1,respectively. PCR identification of bacterial specific genes showed single bright bands at 280, 414 and 509 bp, which was consistent with expectations. The results of drug sensitivity test showed that isolates SA-1 to SA-8 were sensitive to imipenem, minocycline and ceftazidime, E1 to E3 were sensitive to flufenicol, imipenem and minocycline, and P1 was sensitive to imipenem, gentamicin, amikacin, ceftazidime and ampicillin. The results of pathogenicity test showed that all the chickens died within 20 h after single infection and mixed infection，and all the isolates had strong pathogenicity.【Conclusion】 In this study,8 strains of Staphylococcus aureus,3 strains of Escherichia coli and 1 strain of Proteus mirabilis were successfully isolated.All of the isolates showed multiple drug resistance and were highly pathogenic to chicks.The results provided important data support for the pathogenesis and prevention of mixed infection of Staphylococcus aureus,Escherichia coli and Proteus mirabilis.

Key words: chicken; Staphylococcus aureus; Escherichia coli; Proteus mirabilis; pathogenicity