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From an O18ac:K1:H7 ColV+ strain of Escherichia coli (designated MW) that had caused meningitis in a human baby, mutant forms were isolated that lacked different combinations of its O, K and H antigens and its ColV plasmid. These characters were also transmitted by conjugation to E. coli K12 and the virulence, immunogenicity and other properties of the different forms of both strains were studied. All the forms of the MW strain that lacked either the O18 or K1 antigens or the ColV plasmid, but not the H7 antigen, were much less virulent for chickens and mice than the parent form of MW. Another form derived by N-methyl-N'-nitro-N-nitrosoguanidine (NTG) treatment of the parent strain and that possessed all these four characters was also much less virulent. Some of the forms of the K12 strain to which the characters had been transferred were slightly more virulent than the K12 parent, but the virulence of all of them, including one possessing the O18 and K1 antigens and the ColV plasmid, did not approach that of the MW parent. Pathogenesis studies in chickens and colostrum-deprived calves revealed that the loss of virulence exhibited by the forms of the MW strain that lacked O18, K1 and ColV and by the NTG-derived form was associated with decreased ability to invade the body. This was also the reason for the ow virulence of the forms of the K12 strain that had acquired these characters. Possession of both the O18 and K1 antigens was largely responsible for the ability of the different forms of the MW strain to survive in fresh chicken serum; organisms of K12 that possessed the K1 antigen survived as long as those of the parent form of the MW strain. A substantial degree of immunity against lethal infection with the parent form of the MW strain was produced in chickens and mice by all the forms of the MW and K12 strains that possessed the O18 antigen. The K1 and H7 antigens and the ColV plasmid produced no detectable immunity.

The Association of the O18, K1 and H7 Antigens and the CoIV Plasmid of a Strain of Escherichia coli with its Virulence and Immunogenicity