Demonstration of an outer membrane protein with antiphagocytic activity from Pasteurella multocida of avian origin.

A strain of Pasteurella multocida of avian origin was found to inhibit phagocytosis of Candida albicans by mononuclear phagocytes in vitro. Whole-cell lysates of P. multocida showed this effect, as did a 50-kilodalton (kDa) protein eluted from sodium dodecyl sulfate-polyacrylamide gels obtained by electrophoresis of whole-cell lysates. Heat, digestion with trypsin, and antibody specific for this 50-kDa protein neutralized the antiphagocytic effects of P. multocida, of the whole-cell lysates, and of the 50-kDa protein itself. Evidence that this protein was in the outer membrane of the bacterial cell included the findings that (i) treatment of encapsulated or unencapsulated P. multocida with trypsin reduced the antiphagocytic effect; (ii) whole-cell lysates prepared from trypsinized, unencapsulated P. multocida had reduced antiphagocytic activity; and (iii) antibody to outer membrane proteins neutralized the antiphagocytic effect. Turkeys given antibodies specific for the 50-kDa outer membrane protein were protected against lethal challenge with P. multocida.