Agglutination of Streptococcus mutans by low-molecular-weight salivary components: effect of beta 2-microglobulin

Radiolabeled monomeric human beta 2-microglobulin (beta 2m) 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 beta 2m. Monomeric beta 2m agglutinated beta 2m-binding strains when Ca2+ was present. At Ca2+ concentrations of 1.4 mM, 0.032 micrograms of monomeric beta 2m per ml caused bacterial agglutination. Parotid saliva was gel filtered on a Sephadex G-75 column, and low-molecular-weight fractions containing beta 2m could agglutinate S. mutans cells. Five of six strains that could bind beta 2m 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 beta 2m, 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 beta 2m-binding strains of S. mutans was inhibited or decreased by the addition of goat anti-human beta 2m as compared with control serum. The agglutination of two beta 2m-nonbinding strains and one with variable binding was not inhibited. Thus, salivary beta 2m may contribute to agglutination of S. mutans cells in parotid saliva.