Serum sensitivity of field isolates and laboratory strains of Brucella abortus

Objective To evaluate the ability of bovine complement to kill a variety of field isolates and laboratory strains of Brucella abortus. Design The experimental approach was to determine the sensitivity of B abortus isolates to killing by bovine serum, and to document the role of complement in brucellacidal activity. Sample population Six laboratory isolates and 12 field isolates of B abortus were tested. Procedure The ability of B abortus to survive exposure to undiluted bovine serum for 2 hours at 37 C was assessed. The role of complement in killing was determined by examining the ability of heat (56 C for 60 minutes) and cobra venom factor to obliterate the activity in serum, and by detecting binding of the ninth component of bovine complement to serum-sensitive target cells. Results Isolates of B abortus that were resistant to the bactericidal activity of normal bovine serum were revealed. These included field isolates and laboratory strains. Furthermore, the study confirmed earlier reports that bovine serum-mediated killing of B abortus is caused by the complement cascade. Conclusions Some isolates of B abortus , like other gram-negative bacteria, were resistant to complement-mediated killing. Resistance was associated with smooth colony morphology. Isolates lacking detectable O antigen were serum sensitive.