Protein M and fibronectin-binding proteins are not sufficient to promote internalization of group a streptococci into HeLa cells

Infection by group A streptococci is often associated with highly invasive diseases resulting in sepsis and shock. Recently, this species has renewed interest in the mechanism by which some strains are better able to invade mucosal epithelia and to penetrate into the bloodstream than are others, In this work we have evaluated the invasive ability of eight group A Streptococcus pyogenes strains isolated either from patients with severe invasive diseases, or with pharyngitis, or from healthy carrier, Five out of the eight strains studied were efficiently internalized within HeLa cells and, of these, four produced the M6 protein. A recombinant S.gordonii strain constitutively expressing the M6 protein failed to invade HeLa cell monolayers, suggesting that the expression of the M6 protein is not sufficient to allow the non-invasive S.gordonii to be internalized within Hela cells, As fibronectin-binding proteins have been implicated as primary adhesins in host-parasite interactions, we assayed the adhesiveness and the invasiveness of five invasive GAS strains in competition experiments where HeLa cells were infected with bacteria in the presence of purified fibronectin. The results obtained indicated that fibronectin moderately inhibits bacterial adhesion, while it does not affect internalization. These results indicate that other factors, together with fibronectin-binding proteins, participate in the adhesion of streptococci, and that fibronectin-mediated adhesion does not seem to be important in the internalization process of streptococci within HeLa cells.