Serum resistance and aerobactin iron uptake in avian Escherichia coli mediated by conjugative 100-megadalton plasmid.

A total of 115 strains of Escherichia coli isolated from chickens with colisepticemia in Japan were examined for chicken lethality and virulence factors. It was found that serum resistance and aerobactin-mediated iron uptake are the most prevalent characteristics in these strains. Among them, S-20, a representative virulent strain of serotype O2, was further studied. S-20 harbored a conjugative 100-megadalton (Mdal) plasmid, designated pKI100. Curing and reintroduction experiments showed that pKI100 encodes both serum resistance and aerobactin-mediated iron uptake, and the diminished virulence of the pKI100-cured strain was fully restored by the reintroduction of the plasmid. These results demonstrated that pKI100 is the virulence plasmid of the S-20 strain, and that serum resistance and aerobactin-mediated iron uptake are the virulence factors in E. coli strains which cause avian colibacillosis.