Serum sensitivity of selected Brucella abortus isolates and their interaction with the bovine complement cascad

Brucella abortus strains have, historically, been considered to be sensitive to killing by normal bovine serum. Additionally, it has been assumed that the bovine complement system is responsible for killing these highly virulent organisms. Literature concerning the serum sensitivity of virulent B. abortus strains is paradoxical. Resistance to serum mediated killing is associated with virulence, while serum sensitivity is associated with avirulent organisms. This dissertation seeks to resolve this paradox by conclusively demonstrating the role of complement in serum mediated killing of B. abortus, the mechanism by which these organisms activate the bovine complement cascade, and by examining the interaction of these organisms with bovine serum. Additionally, a number of strains of B. abortus were examined for their interaction with bovine serum.;Examination of the interaction of B. abortus with the bovine complement system revealed that complement was responsible for the bactericidal effects of bovine serum. Additionally, evidence indicated the activation of the bovine complement cascade by these organisms resulted from an antibody independent interaction of bovine C1 with a structure on the cell surface. The ability of B. abortus outer membrane proteins to bind purified bovine C1q was later documented but did not appear to be related to serum resistance.;Virulent and avirulent laboratory isolates were demonstrated to be resistant to killing by bovine serum. Testing of thirteen B. abortus isolates recovered from infected cattle revealed that twelve were resistant to killing by normal bovine serum. These isolates included B. abortus strain 19 isolates, historically considered avirulent, and highly virulent carbon dioxide requiring B. abortus field isolates.;A comparison of serum resistant and sensitive isolates indicated that the presence of long chain O-antigen on the cell surface correlated with serum resistance. Isolates bearing no, or truncated, O-antigen were found to be serum sensitive. Serum resistant B. abortus isolates avoided killing by complement because they did not activate the bovine complement cascade. It is proposed that serum resistant B. abortus avoid antibody independent activation of the bovine complement cascade because long-chain O-antigen masks the surface molecule(s) capable of binding and activating bovine C1.