The role of various antigenic fractions of Bacteroides nodosus in eliciting protection against foot-rot in vaccinated sheep.

Sheep were vaccinated against foot-rot with various antigenic extracts of Bacteroides nodosus and challenged to assess their resistance. The protective immunogen was shown to be a heat-labile, high molecular weight surface antigen which was removed by ultrafiltration and was either identical to or closely associated with the fimbriae of the organism. Resistance appeared to be associated with the presence of a single high molecular precipitinogen distinct from the 0 antigen. Agglutinating antibodies to the surface antigen in immunised sheep were predominantly IgG1 whereas the 0 antigen induced predominantly IgM indirect haemagglutinating antibodies. Reactions at vaccination sites were less marked to the surface antigen than to the 0 antigen.