The chemical basis of the virulence of Brucella abortus. X. A surface virulence factor which facilitates intracellular growth of Brucella abortus in bovine phagocytes.

In a phagocytosis system previously described (Smith and FitzGeorge, 1964) streptomycin penetrated the phagocytes of bovine “buffy coat” to a small extent in untreated phagocytes but to a large extent in phagocytes pretreated with killed or their cell walls. In the same test sytem, but using only the bactericidins of normal bovine serum to prevent extracellular growth, the intracellular growth of an attenuated strain 45/0, which is normally destroyed by phagocytes, was promoted if the phagocytes were pretreated with killed organisms of the virulent strain 544, and to an increased extent if these had been grown in a medium supplemented with bovine allantoic fluid. Washing the “544” organisms from the supplemented medium with water or ether/water reduced their capacity to promote the intracellular growth of “45/0” organisms. When the washed “544” or “45/0” organisms were used to absorb a bovine antiserum against live “544” organisms or a rabbit antiserum against the ether/water wash, the absorbed sera agglutinated “544” organisms from the supplemented medium to a higher titre than those from the unsupplemented one. Furthermore agglutination of “544” organisms from the supplemented medium by the absorbed serum was inhibited by the prior addition of the ether/water wash but not by a surface, immunizing antigen purified by Ellwood, Keppie and Smith (1967). A second surface antigen which inhibits the intracellular bactericidins of bovine phagocytes appears to be produced in the supplemented medium and to be removed by washing with ether/water.