Variable Sensitivity of Oral Isolates of Eikenella Corrodens to Serum Bactericidal Activity: Role of Antibody

Abstract

Eikenella corrodens is a facultatively anaerobic Gram-negative bacterium which is among the predominant cultivable microflora of periodontal lesions characterized by loss of attachment level. In the present study, we examined the potential role of complement-mediated killing in host defense against this periodontopathic organism. Seven clinical isolates obtained from human subgingival plaque and one reference strain of E. corrodens were characterized with respect to (a) susceptibility to the bactericidal properties of pooled human serum and (b) the role of the classical and/or alternative pathway(s) of complement in effecting killing of sensitive strains. Six strains, including the reference strain, were found to be variably serum-sensitive, exhibiting 1-12.5% survival after two hr of incubation in the presence of 20% pooled human serum. The remaining two isolates were serum-resistant. Both serum-resistant and serum-sensitive strains consumed complement via the classical pathway in normal but not in hypogammaglobulinemic serum, thus ruling out an antibody-independent mechanism of classical pathway activation. Four of six serum-sensitive strains exhibited little or no loss of viability following incubation with serum depleted of the classical pathway component Clq. One strain which was resistant to killing by normal human serum was, nevertheless, highly susceptible to complement-mediated killing in the presence of rabbit immune serum. Two additional serum-sensitive strains were killed, albeit to a lesser extent, in Clq-depleted serum, indicative of a role of the alternative pathway in killing of some serum-sensitive strains. These results indicate a potential role for complement-mediated killing in host defense against Gram-negative periodontal bacteria such as E. corrodens. However, the ultimate contribution of this immune defense mechanism may be defined, at least in part, by the presence of a humoral response to key bacterial membrane constituents.