Abstract: The objective of this paper is to study the genetic evolutionary relationship of E.coli from yaks and identify their genetic relationship with human and other species of animals. A total of 60 anal swabs from healthy adult yaks in Ganzi andAba prefecture were collected and inoculated in MaConkey agar. Suspected colonies were further identified by morphological observation, cultural character, biochemical assays and 16S rRNA PCR detection, and 49 strains of E.coli were finally obtained. The 16S rRNA of 9 E.coli strains was cloned and sequenced, and the phylogenetic analysis showed that the homology was high up to 98.8% to 99.9% among yak-origin strains, while the homology with other strains in GenBank was 94.0% to 100%. The results in this paper indicate that the yak-origin E.coli strains have close genetic distance with strains from cattle, and have closer distance with the strains from human, pig, duck and chicken, and have farther distance with the caprine strains, in which two strains are gathered to one cluster with human enterohemorrhagic E.coli (EHEC) O₁₅₇∶H₇ and Shigella, and these two strains may be the potential pathogenic bacteria for human being.

Key words: E.coli; yak; 16S rRNA; phylogenetic analysis