In vitro studies on the mechanism of adherence and pathogenicity of mycoplasmas

Most pathogenic mycoplasmas adhere to the cytoplasmic membrane of host cells in an avid, almost irreversible way. This interaction seems hydrophobic in nature and may be induced in the area of contact when the surface proteins are cleared away by electrostatic forces. In several mycoplasmas (e.g., Mycoplasma pneumoniae, M. gallisepticum and M. genitalium) the initial steps of adherence are reversible interactions occurring between mycoplasma adhesins that recognize specific sialoglycoconjugates on the host cell membrane. In M. pneumoniae the major adhesin (P1) is an integral membrane protein of about 165 kDa. About 10% of the P1 molecules are linked to the cytoskeleton elements. Part of the proteins in these cytoskeletal elements are phosphorylated, which may explain the anchorage and possible mobilization of P1 to the tip structure, as was indicated by immunohistochemical electron microscopy. With the close contact caused by attachment of the mycoplasmas to the host cells, their pathogenicity can be expressed. Studies with M. pneumoniae on a variety of human cells in culture indicated that superoxide anions are generated during the infection. They drastically inhibit the catalase activity of the host cells. Addition of exogenous superoxide dismutase or increasing its level endogenously minimize the inhibition of catalase. With much of the catalase inhibited, oxidation of cell components occurs, among which are membrane lipids as indicated by elevated malonyldialdehyde levels in infected cells. These may lead to membrane leakage and to the cytopathology of mycoplasma infection.