Binding of horse-spleen ferritin to group A streptococci

Receptors for horse-spleen ferritin were found on group A streptococci. Both electron microscopic and chemical investigation of Streptococcus cells treated with ferritin showed that M + variants of group A streptococci were able to bind substantially more ferritin than M - variants of the same serotypes. Ferritin receptors were located on the tops of filamentous protrusions of Streptococcus cell walls and only on the outer surface of isolated cell walls. Trypsin treatment destroyed the ferritin-binding capacity of streptococci completely, while mild pepsin treatment left the ferritin receptors undisturbed, or uncovered additional ones. The ferritin receptors were not identical with receptors for the Fc-portion of swine IgG. The finding of ferritin receptors on bacteria necessitates careful interpretation of results obtained by immunoferritin localization techniques.