The association of staphylococci with the mucus gel that overlays the mucosa of the respiratory tract may lead to clearance of cocci or, in certain conditions such as cystic fibrosis (CF), to colonization. In the present study, a quantitative radioassay was used to study the effect of Ca2+, which is elevated in CF sputa, on the adhesion of 3H-labelled Staphylococcus aureus to submaxillary gland mucin immobilized in MaxiSorp 96-well, break-apart modules. Ca2+ significantly enhanced the adhesion of S. aureus (five strains) and Staphylococcus epidermidis (four strains). The reaction was specific because adhesion was not enhanced in the presence of Mg2+, Ca2+ + EGTA (a Ca2+ chelator) or protamine and was not attributable to hydrophobicity of the test strains. Staphylococcal adhesion was significantly (P ≤ 0.005) blocked in the presence of highly sialated and sulphated reagents, which suggests that Ca2+ binds to the sialic acid and sulphate residues of immobilized mucin. The Ca2+-binding sites on the surface of S. aureus were trypsin-sensitive; in addition, 125I-labelled solubilized S. aureus surface proteins reacted with immobilized mucin in a direct binding assay, and the reaction was significantly enhanced by Ca2+. Autoradiography demonstrated that 45Ca bound directly to two polypeptides (M(r) 170000 and 150000) of solubilized staphylococcal surface proteins separated by SDS-PAGE, and that 125I-labelled mucin bound directly to three staphylococcal polypeptides (M(r) 40000, 35000, and 29000). These results suggest that S. aureus adhesion to mucin is mediated by at least two mechanisms: via Ca2+-binding surface proteins in the presence of Ca2+ and via mucin-binding surface proteins unrelated to Ca2+.
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