Analysis of bacterial cell wall proteins and human serum proteins bound to bacterial cell surfaces

A method was developed for the characterization of proteins non-covalently bound to the cell wall of Gram-positive cocci. The method employs radioactive labelling of cell wall proteins followed by solubilisation and analysis on polyacrylamide gel electrophoresis in sodium dodecyl sulphate. Using this experimental procedure, protein patterns obtained from group A, C and G streptococcal strains showed marked similarities within each group. Protein peaks were also found to be shared between group C and G strains. Two major peaks with molecular weights of about 30 000 and 70 000 characterized group A strains, whereas group C and G strains showed one consistent peak of about 45 000, thus reflecting the closer relationship between these two groups as compared to group A streptococci. By incubating bacteria with human serum proteins before labelling, solubilisation and electrophoretic analysis, it was also possible to study external proteins specifically bound to the bacterial surface. A group G streptococcus, strain G 148, showed protein peaks corresponding to its known specific binding of human albumin and immunoglobulin G, but also additional protein peaks. When Staphylococcus aureus, strain Cowan I, was pre-incubated with human serum in excess, protein peaks corresponding to heavy and light chains of immunoglobulins were seen. Three more protein peaks of serum origin were also detected, indicating binding of proteins other than Ig to S. aureus. Experiments with protein A-coated Sepharose beads resulted in the same protein pattern, suggesting that binding of these different polypeptides is indeed mediated by protein A.