Deletion of wboA enhances activation of the lectin pathway of complement in Brucella abortus and Brucella melitensis

Brucella spp, are gram-negative intracellular pathogens that survive and mu ltiply within phagocytic cells of their hosts. Smooth organisms present O p olysaccharides (OPS) on their surface. These OPS help the bacteria avoid th e bactericidal action of serum. The wboA gene, coding for the enzyme glycos yltransferase, is essential for the synthesis of O chain in Brucella. In th is study, the sensitivity to serum of smooth, virulent Brucella melitensis 16M and B. abortus 2308, rough wboA mutants VTRM1, RAI, and WRR51 derived f rom these two Brucella species, and the B. abortus vaccine strain RB51 was assayed using normal nonimmune human serum (NHS). The deposition of complem ent components and mannose-binding lectin (MBL) on the bacterial surface wa s detected by flow cytometry. Rough B. abortus mutants were more sensitive to the bactericidal action of NHS than were rough B, melitensis mutants. Co mplement components were deposited on smooth strains at a slower rate compa red to rough strains. Deposition of iC3b and C5b-9 and bacterial killing oc curred when bacteria were treated with Clq-depleted, but not with C2-deplet ed serum or NHS in the presence of Mg-EGTA. These results indicate that (i) OPS-deficient strains derived from B. melitensis 16M are more resistant to the bactericidal action of NHS than OPS-deficient strains derived from B. abortus 2308, (ii) both the classical and the MEL-mediated pathways are inv olved in complement deposition and complement-mediated killing of Brucella, and (iii) the alternative pathway is not activated by smooth or rough bruc ellae.