Opsonization of Four Bacteroides Species: Role of the Classical Complement Pathway and Immunoglobulin

Abstract

Opsonization of 2 Bacterioides spp. may be mediated exclusively by the alternative complement pathway and may require immunoglobulins [Ig]. The nature of the opsonic factors in non-immune human serum for 4 Bacteroides spp. was investigated by measuring uptake of [3H]thymidine-labeled bacteria by human polymorphonuclear leukocytes. Normal human serum, C2[complement component 2]-deficient serum, Ig-deficient serum and serum chelated with ethylene glycol-bis(.beta.-aminoethyl ether)-N,N-tetraacetic acid (EGTA), MgEGTA and EDTA were used as opsonic sources. Heat inactivation of each of these sera significantly reduced its opsonic activity for all 4 Bacteroides spp. Serum C apparently was essential for effective opsonization. All strains were opsonized in the absence of the classical C pathway; kinetics studied revealed that opsonization proceeded at a significantly faster rate when the classical C pathway was intact. Although 2 strains were opsonized in Ig-deficient sera, opsonization was less efficient and apparently occurred via the alternative C pathway. All strains were well opsonized by the classical C pathway in 10% serum which had been effectively chelated with EGTA or EDTA. Cell wall cations of Bacteroides spp. may participate in the activation of C in chelated serum, resulting in effective opsonization. Bacteroides, when incubated with an Escherichia coli strain in normal serum, could compete for opsonins and reduce phagocytosis of E. coli. Competition for opsonins among bacterial species may contribute to the synergistic role these organisms share in mixed floral infections.