Resistance to phagocytosis and epithelial cell binding of Group A streptococci

Resistance to phagocytosis and epithelial cell binding of Group A streptococci is undoubtedly mediated by surface to surface interactions between host and bacterial cells. Previous ultrastructural studies have demonstrated that the M antigen is associated with surface projections which have been referred to as "fimbriae" (1). The observations were based on ultrastructural studies comparing M-protein-rich and M-protein-negative variants of Group A streptococci, or the regeneration of both fimbriae and M antigen after their complete removal by enzymatic digestion. It could not be determined, therefore, whether or not the fimbriae were composed of M protein alone. Consequently, differences in biological behavior of the streptococcal variants which lack fimbriae cannot be attributed solely to lack of M protein. It has been suggested that the M protein which is known to be antiphagocytic also mediates binding of Group A streptococci to oral epithelial cells (2). This hypothesis was based on observations employing M-positive and M-negative variants, the limitations of which have been indicated above. In previous studies (3-5), however, we presented evidence to suggest that lipoteichoic acid (LTA) j which binds spontaneously to mammalian cell membranes via ester-linked fatty acids may play a central role in the binding of these organisms to mucosal surfaces. We undertook the present investigation to study the surface ultrastructure in relation to immunochemical and biological activities of Group A streptococci after selective removal of M protein. The behavior of the M-protein-denuded fimbriae of streptococci was compared to that of chemically or enzymatically defimbriated organisms in phagocytosis and epithelial-binding experiments. Our data indicate that in addition to M protein, LTA can be located on the fimbriae of Group A streptococci and that the role of fimbriae in epithelial cell binding is due to their content of LTA rather than M protein.