Demonstration of cross-reactions between pneumococci and alpha-streptococci using gold-labelled mono- and polyclonal antibodies and electron microscopy.

Cross-reactions between alpha-streptococci and the pneumococcal C-polysaccharide (PnC) were investigated using electron microscopy and immunogold labelling of bacterial cells. Monoclonal antibodies against two different determinants of the PnC molecule were used, one directed against the chain sugar of the repeating unit 2-acetamido-4-amino-2,4,6-trideoxygalactose (Sug) and the other against the phosphorylcholine residue. Two different immunogold techniques were tested, either by direct labelling of the monoclonal antibody or by using an antimouse immunogold conjugate to demonstrate binding of the monoclonal antibodies. Antibodies against the "Sug" determinant reacted only with pneumococci, whereas antibodies against the phosphorylcholine determinant bound to cross-reacting streptococci as well as to pneumococci. These results indicate that the cross-reacting antigens of the alpha-streptococci contain phosphorylcholine residues, but that they are not identical to the C-polysaccharide molecule.