Agglutination of Streptococcus mutans by low-molecular-weight salivary components: Effect of β2-microglobulin

Abstract

Radiolabeled monomeric human β2-microglobulin (β2m) was tested for binding to Streptococcus mutans strains in buffers containing 1 mM calcium (Ca2+). Binding was seen to strains with a previously established binding capacity of aggregated β2m. Monomeric β2m agglutinated β2m-binding strains when Ca2+ was present. At Ca2+ concentrations of 1.4 mM, 0.032 μg of monomeric β2m per ml caused bacterial agglutination. Parotid saliva was gel filtered on a Sephadex G-75 column, and low-molecular-weight fractions containing β2m could agglutinate S. mutans cells. Five of six strains that could bind β2m were agglutinated by these fractions, but only one of five nonbinding strains was. All strains tested were agglutinated by void volume fractions. A new method for the measurement of turbidity in bacterial agglutination inhibition experiments with parotid saliva was used. Suspensions containing parotid saliva, bacteria, and control serum were directly compared in a spectrophotometer with test suspensions containing goat anti-human β2m, bacteria, and saliva. Thus, the spectrophotometer directly read the difference in agglutination of the two suspensions, and the result was presented as one curve by the recorder. Agglutination of five β2m-binding strains of S. mutans was inhibited or decreased by the addition of goat anti-human β2m as compared with control serum. The agglutination of two β2m-nonbinding strains and one with variable binding was not inhibited. Thus, salivary β2m may contribute to agglutination of S. mutans cells in parotid saliva.