Role of serotype-specific polysaccharide in the resistance of Streptococcus mutans to phagocytosis by human polymorphonuclear leukocytes

To clarify the role of cell surface components of Streptococcus mutans in r esistance to phagocytosis by human polymorphonuclear leukocytes (PMNs), sev eral isogenic mutants of S. mutans defective in cell surface components wer e studied with a luminol-enhanced chemiluminescence (CL) assay, a killing a ssay, and a transmission electron microscope, The CL responses of human PMN s to mutant Xc11 defective in a major cell surface antigen, PAc, and mutant Xc16 defective in two surface glucosyltransferases (GTF-I and GTF-SI) were the same as the response to the wild-type strain, Xc. In contrast, mutant Xc24R, which was defective in serotype c-specific polysaccharide, induced a markedly higher CL response than the other strains. The killing assay show ed that human PMNs killed more Xc24R than the parent strain and the other m utants. The transmission electron microscopic observation indicated that Xc 24R cells were more internalized by human PMNs than the parental strain Xc. These results may be reflected by the fact that strain Xc24R was more phag ocytosed than strain Xc, The CL response of human PMNs to a mutant defectiv e in polysaccharide serotype e or f was similar to the response to Xc24R. F urthermore, mutants defective in serotype-specific polysaccharide were mark edly more hydrophobic than the wild-type strains and the other mutants, sug gesting that the hydrophilic nature of polysaccharides may protect the bact erium from phagocytosis. We conclude that the serotype-specific polysacchar ide, but not the cell surface proteins on the cell surface of S. mutans, ma y play an important role in the resistance to phagocytosis.