Interactions between human serum proteins and oral streptococci reveal occurrence of receptors for aggregated beta 2-microglobulin

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Abstract
Strains (31) of oral streptococci representing Streptococcus mutans, S. sanguis, S. mitior [S. mitis], S. salivarius and S. milleri were tested for possible binding of human immunoglobulin [Ig] G, Ig G1, Ig G2, Ig G3, Ig G4, Ig A1, Ig A2, Ig M1 Ig and Ig M2 and haptoglobin, hemoglobin, fibrinogen and aggregated .beta.2-microglobulin. Radiolabeled .beta.2-microglobulin in aggregated form showed affinity for 20 of the 31 strains tested. Binding activity for the protein was found in strains belonging to all 5 species. The bacterial receptor was resistant to trypsin. Monomeric, unlabeled .beta.2-microglobulin did not interfere with the binding of the aggregated form. Of the other proteins tested, only the Ig A1 protein showed positive binding, and that was only with a single strain of S. milleri. .beta.2-Microglobulin is present on all nucleated cell membranes in vivo. The reaction between aggregated .beta.2-microglobulin and oral streptococci is a new type of human-bacterium interaction which should be considered in studies of bacterial adherence.