Deletion ofwboAEnhances Activation of the Lectin Pathway of Complement inBrucella abortusandBrucella melitensis

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Abstract
Brucellaspp. are gram-negative intracellular pathogens that survive and multiply within phagocytic cells of their hosts. Smooth organisms present O polysaccharides (OPS) on their surface. These OPS help the bacteria avoid the bactericidal action of serum. ThewboAgene, coding for the enzyme glycosyltransferase, is essential for the synthesis of O chain inBrucella. In this study, the sensitivity to serum of smooth, virulentBrucella melitensis16M andB. abortus2308, roughwboAmutants VTRM1, RA1, and WRR51 derived from these twoBrucellaspecies, and theB. abortusvaccine strain RB51 was assayed using normal nonimmune human serum (NHS). The deposition of complement components and mannose-binding lectin (MBL) on the bacterial surface was detected by flow cytometry. RoughB. abortusmutants were more sensitive to the bactericidal action of NHS than were roughB. melitensismutants. Complement components were deposited on smooth strains at a slower rate compared to rough strains. Deposition of iC3b and C5b-9 and bacterial killing occurred when bacteria were treated with C1q-depleted, but not with C2-depleted serum or NHS in the presence of Mg-EGTA. These results indicate that (i) OPS-deficient strains derived fromB. melitensis16M are more resistant to the bactericidal action of NHS than OPS-deficient strains derived fromB. abortus2308, (ii) both the classical and the MBL-mediated pathways are involved in complement deposition and complement-mediated killing ofBrucella, and (iii) the alternative pathway is not activated by smooth or rough brucellae.