Outer
membrane proteins (OMP) of P. multocida (serotype
B:2) field isolates (n = 6) and a
vaccine strain (P-52) were extracted by a
sarkosyl method and characterized using SDS-PAGE and immunoblotting. About 20
polypeptide bands were observed in the profile of the
vaccine strain with MW ranging from 16 to 90 kDa and, based on band thickness and intensity of staining, three
polypeptides of MW 31, 33 and 37 kDa were considered to be the major OMPs. The profiles of the field isolates showed minor differences when compared with that of the
vaccine strain. The OMP of 33 kDa was only expressed by the
vaccine strain. Four field isolates expressed an OMP of 39 kDa, which did not appear in the profiles of the remaining two field isolates and the P-52 strain. Similarly, an OMP of 25 kDa was exclusively seen in the profile of a single isolate. By immunoblotting studies, using anti-P. multocida (P-52) whole-cell hyperimmune serum raised in rabbits as well as buffalo
immune sera, it became evident that the
polypeptide of 37 kDa was the most antigenic OMP in the profiles of all the isolates, including the P-52 strain. Other
polypeptides were either weakly antigenic or visible in the profile of only a few of the isolates. The study thus identified the major OMP of P. multocida (
B:2) and suggested that this highly antigenic 37 kDa OMP has potential for further protective and immunodiagnostic studies.