Abstract: [Objective] The aims of the present study were to isolate and identify Pasteurella multocida （Pm） from sheep, and to assess the biological characteristics of the isolate. [Method] On site dissection was performed on sheep with typical respiratory disease symptoms from a family farm in Togtoh County, Hohhot City, Inner Mongolia Autonomous Region. The pathological changes of the main respiratory organs and spleen were observed and the pathological sections of lung tissue were prepared. Lung tissue sample was aseptically collected for bacterial isolation and culture. After phenotypical identification, molecular biological identification and capsular serotype identification of the isolated strain were performed. PCR assay was used to detect the virulence genes. Disc diffusion method was used to test the sensitivity of the isolated strain to 11 antimicrobial agents. The pathogenicity of the isolated strain to mice was evaluated using intraperitoneal injection method. [Result] The diseased sheep had adhesions of the lungs to the pleura and pericardium, necrotic lesions in the lung tissues, enlarged lymph nodes in the hilum of the lung, and bleeding sites in the spleen. After isolation and purification, one bacterial strain was obtained and designated as 2021-TX-06. This strain appeared as a tiny, circular and white colony with slight protrusion on TSA medium containing 5% horse serum. It was a Gram negative bacterium that conformed to the biochemical characteristics of Pm and was capable of fermenting and utilizing mannitol, sorbitol, trehalose, xylose and glucose. After 16S rDNA PCR amplification and sequence alignment, the 2021-TX-06 strain was identified as Pm. Following capsular serotyping, it was identified as capsular serotype D, carrying 15 virulence genes such as ptfA, fimA, Fur, etc. It was sensitive to the antimicrobial agents including tetracycline, ceftriaxone, gentamicin, polymyxin B, and resistant to norfloxacin, enrofloxacin, and chloramphenicol. Seventy-two hours after the intraperitoneal injection with 5 × 10⁸ CFU/mL bacterial suspension of 2021-TX-06 strain, a 100% mortality rate was observed in the experimental group mice. A strain consistent with the challenging strain was recovered from the lungs of the dead mice. [Conclusion] A Pm prevalent strain belonging to capsular serotype D was isolated from the lung tissue of sheep with typical respiratory disease symptoms. This strain harbored multiple virulence genes, showed resistance to some antimicrobial agents, and had strong pathogenicity to mice.

Key words: sheep, Pasteurella multocida, isolation, identification, capsular serotype