| Escherichia coli O157 are important enteropathogens causing outbreaks of haemorrhagic colitis and haemolytic uraemic syndrome of human and animals with high mortality. So it is noticed worldwidely. Although the mechanism of which factors make toxigenic E.coli O157 more virulent is unclear, the one of important virulent factors of the pathogens is Shiga toxin, which can cause the CPE of Vero cells, also called Vero toxin (VT). The VT gene is encoded on the chromosome of bacteriophage which contains VT gene (VT phage). The characteristics of VT phage virion, the conditions of horizontal gene transmition, the effect on the bacterial virulence of lysogenic conversion, all these will be related to the virulence of pathogen. So it will be contributed to prevention, diagnosis, and treatment of E.coli O157 infection to research all above.To detect the contamination of E.coli O157 in market, the beeves from the five supermarkets and three food markets were collected and detected by selective culture, biochemical characteristic evaluation, serologic agglutinative test and BINAX NOW?EH E.coli test kit. Animal experiments were made to determine the virulence of the isolates. Results showed that 3 strains of E.coli O157: H7 and 2 strains of E.coli O157: NM were identified from 120 samples, and the isolates of E.coli O157: H7 had virulence to mice. It revealed that E.coli O157: H7 contamination existed in the market beeves and had potential risk to human being.Occuring high virulent E.coli O157: H7 in market beef revealed that there was verocytotoxigenic bacteriophage (VT phage) in E.coli. Five strains of VT2 phage were isolated and purified by two layers agar assay, and identified by amplifying VT2 gene. These phages were from E. coli O157 strains, cow and chicken feces, and sewage, respectively. Phages were 0.5-2mm diameter and had regular hexagonal heads and short stubby tails. The litre of phages with lucidity plaques against host strain MC1061 was more than 109 PFU/mL.A lysogen (MC1061/SH W1 ) with VT2 gene was gained after transfecting MC1061 by VT2 phage SH W1 from sewage.The cytopathic effect on Vero cells were produced after inoculating the filtrates from the E. coli strains isolated VT2 phages and the above lysogen. All these showed that VT2 phages were common inenvironment and VT2 virulent factor could transfer horizontally through the lysogenic infection of VT2 phage. VT2 phage was related to the virulence of bacteria,A recombinant verocytotoxigenic bacteriophage (VT phage) contained Kan was used to detect the difference of adsorbability to soils and survival in soils of VT phage. The soils model was set up used clay, loam and sandy soils, respectively. The susceptibility of different strains to VT phage was also researched. The results showed that VT phage adsorbed strongly to clay and loam soils, but not to sandy soils. A small proportion of infective VT phage particles could survive for more than 30 days in all three soils type. Soils were important place for survival of VT phages. Of 15 E. coli strains tested, 12 were susceptible to lysogenic or lytic infection. 2 Shigella flexneri strains were all susceptible to infection by the phage, while 3 Salmonella strains were not. VT2 gene was detected in all above lysogens. All theses indicated VT phage had specific host rang. The virulence of sensitivity bacteria was enhanced because of obtaining VT2 gene.VT phage had the specific host rang might be related to the receptor on the surface of bacteria. A cloned gene with not clear function, named vpr was studied. The pJP5603 plasmid is a suicide plasmid and contains the Kan resistance cassette, which was digested with Xmal. Hin II and the pUC19phi RID plasmid contains the vpr gene, digested with Age , Hin II to obtain 778bp piece of vpr aim gene. After cohesive ligation. the recombinant plasmid was transformed into CC118 pir. Colonies were selected on Kan plates. A recombinant plasmid, pYYvpr, was confirmed by PCR to amplify vpr gene and digestion with restriction enzyme Acc I The recombinant suici...
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