SUMMARY: A comparative electron microscopic study was made of virulent ovine strains, benign ovine strains, bovine strains and culture variants of using negative staining, thin section and freeze-fracture etch techniques. The plasma membrane, peptidoglycan layer and outer membrane structures were similar in all the organisms, but there were marked differences in the presence of pili, diffuse polar material and additional layer. The variations in these surface structures were examined in relation to the virulence and immunoprotection of towards foot-rot in sheep. Only organisms with abundant pili caused virulent foot-rot; diffuse polar material and perhaps the additional layer may also be associated with virulence, but conclusive evidence was lacking. It appeared that pili and one or more unknown cell components, possibly diffuse polar material but not the additional layer, were necessary for immunoprotection.


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