Surface receptors for serum albumin in group C and G streptococci show three different types of albumin specificity.

A total of 100 bacterial strains were tested for binding uptake of radiolabeled albumin preparations from 15 mammalian species. Three types of surface structures with specific binding sites for albumin were defined. A previously described receptor for albumin was separated into type a in Streptococcus equisimilis strains and in human group G streptococcal strains and type b in bovine group C streptococci. A new type of albumin receptor, type c, was found in Streptococcus dysgalactiae strains, the only receptor type so far with high affinity for bovine serum albumin. Type of albumin receptor correlated with bacterial species. The three receptor types showed high binding capacities; 2 X 10(8) bacterial organisms bound from 5 to 16 micrograms of albumin. All types of albumin receptors were stable to heat treatment at 80 degrees C for 5 min, but susceptible to both pepsin and trypsin treatment. Bacteria-bound albumin preparations were eluted at various concentrations of KSCN, reflecting differences in affinity. Up to 500 micrograms of human fibrinogen or polyclonal human immunoglobulin G had no inhibitory effect on the uptake of albumin, indicating a separate molecular localization of receptors for these proteins.