A 78-kilodalton (kDa) outer membrane protein (OMP) of Haemophilus somnus was one of the two antigens most consistently and most intensely immunoreactive in Western immunoblots of whole cells of H. somnus reacted with convalescent-phase serum obtained from cattle with experimental H. somnus pneumonia. This antigen was isolated by gel filtration chromatography of sodium dodecyl sulfate-solubilized OMP. Reactions of Western blots with bovine monospecific antiserum prepared against the 78-kDa antigen indicated that this 78-kDa OMP was present in each of 22 isolates of H. somnus obtained from cattle with pneumonia, thromboembolic meningoencephalitis, and abortion as well as from vaginal or preputial carriers. The 78-kDa OMP was also present in each isolate obtained weekly throughout the course of experimental H. somnus pneumonia in a calf. Monospecific antiserum to the 78-kDa OMP also reacted with proteins from closely related bacterial species in the family Pasteurellaceae but not with bacteria of 13 other genera. The 78-kDa OMP of H. somnus is of interest because it is surface accessible, highly conserved, immunogenic, cross-reactive with other members of the family Pasteurellaceae, and reactive with convalescent-phase serum which is passively protective against H. somnus pneumonia
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