Experimental Escherichia coli 06 infection in mice. 1. Effect of immunisation on resistance in relation to 06 antibody levels in mice of different strains.

The susceptibility to intraperitoneal infection with E. coli 06:k2 bacteria and the increase of resistance after immunization and immune serum injection was analysed in eight mouse strains (CBA, A/J, C3H, C3H/HeJ, C57 B1/6J congenitally athymic C57 B1 nu/nu and their heterozygous nu/+ litter mates as well as NMRI mice). A different susceptibility to the infection was found among the strains. This was not related to endotoxin resistance or thymus deficiency or to the ability of the animals to form antibodies as measured with the enzyme-linked immunosorbent assay, ELISA. Immunization of the animals with 5 x 10(7) E. coli 06:K13 bacteria resulted in a smaller increase in resistance in the less susceptible CBA mice than in the more susceptible A/J or C3H/HeJ mice. This pattern was further accentuated after repeated immunization. The development of resistance by immunization seemed independent of T-cells, since the nu/nu mice were as resistant as the nu/+ mice. The nu/nu mice, however, formed less antibodies after vaccination than did their nu/+ litter mates. The lowest antibody responses were noted in the NMRI mice, but this was accompanied with similar increase in resistance compared with the other strains forming 10-fold more 06 antibodies. Immunization with as little as 102-104 bacteria also resulted in a rise in resistance. This was, however, accompanied by a minute increase in antibody titer. Despite content of minute antibody levels administration of such immune serum gave protection of the recipients. It was concluded that very small amounts of antibodies would provide protection against E. coli infection varying from one mouse strain to another.