Abstract: To investigate the germ-carrying condition, molecular evolution and transitional condition of stx in 5 Escherichia coli O157 strains which were separated and identified from Henan and Gansu province during 2009 to 2010, the stx gene of the isolated strains was amplified and sequenced, and phylogenetic analysis had been completed. The results revealed that all of the isolated strains had stx1 and stx2 gene: the homology of nucleotide and amino acid of five isolated strains were high with each other; the stx1 gene of the five isolated strains showed that a high degree of omology with those referenced Escherichia coli O157 strains deprived from goat and food and their genetic distance of the cladogram are near; the stx2 gene of the isolated strains showed that a high degree of homology with those referenced Escherichia coli O157 strains deprived from cattle and human,though the isolated strains were in one main branch according to phylogenic trees of stx2 gene, the genetic distance of the isolated strain 27 was the farthest with other strains and only the strain 27 was in one secondary branch; the isolated strain L37 and W were in one secondary branch with the referenced strains deprived from cattle; the isolated strains 12 and 50 were in one secondary branch with the referenced strains deprived from human. We could infer from these that stx1 of the five isolated strains might be Escherichia coli O157 strains deprived from food or goat; the stx2 of L37 and W might deprive from cattle, 12 and 50 might from human while 27 was not apparent. The research revealed that all the five isolated strains had stx1 and stx2 gene, but the origin of the two genes were different.

Key words: E.coli O157; stx1; stx2; detection; phylogenetic analysis