EXPRESSION OF 2 DIFFERENT ANTIPHAGOCYTIC M-PROTEINS BY STREPTOCOCCUS-PYOGENES OF THE OF+ LINEAGE

All clinical isolates of Streptococcus pyogenes (group A streptococcus ) share the ability to resist phagocytosis and grow in human blood. In many strains, this property is due to the expression of a single anti phagocytic M protein, while other strains express more than one M-like molecule, of which the role in phagocytosis resistance is unclear, In particular, all S. pyogenes strains of the OF+ lineage, representing approximately halt: of all isolates, express two M-like proteins, Mrp and Emm, which are immunologically unrelated. These two proteins bind different ligands that have been implicated in phagocytosis resistance : Mrp binds fibrinogen and Emm binds the complement inhibitor C4BP, Us ing a clinical isolate of the common serotype 22, we created mutants a ffected in the mrp and emm genes and characterized them in phagocytosi s experiments and by electron microscopy, A double mutant mrp(-)emm(-) showed strongly decreased resistance to phagocytosis, while mrp(-) an d emm(-) single mutants grew well in blood. However, optimal growth re quired the expression of both Mrp and Emm. Experiments in which coagul ation was inhibited using the specific thrombin inhibitor, hirudin, ra ther than heparin, indicated that Emm is more important than Mrp for r esistance to phagocytosis. Tuftlike surface structures typical for S. pyogenes were still present in the mrp(-)emm(-)double mutant, but not in a mutant affected in the regulatory gene mga, indicating that the p resence of these surface structures is not directly correlated to phag ocytosis resistance. Our data imply that OF+ strains of S. pyogenes ex press two antiphagocytic M proteins with different ligand-binding prop erties.