Molecular basis for cytadsorption of Mycoplasma pneumoniae

Hemadsorbing (HA⁺) virulent M. pneumoniae and spontaneously derived nonhemadsorbing (HA^-) avirulent mutants were compared by biochemical and ultrastructural techniques in an attempt to understand the molecular basis for cytadsorption. Lactoperoxidase-catalyzed iodination of intact mycoplasmas indicated that both virulent and avirulent mycoplasmas displayed similar surface protein patterns. A specific external protein. P1 (molecular weight, 165,000), previously implicated as a major ligand mediating attachment, was readily detected in HA⁺ and HA^- mycoplasma strains. However, immunoferritin electron microscopy, with monospecific antibody against P1, revealed that differences in P1 topography existed among these strains. Only virulent mycoplasmas exhibited high concentrations of P1 at the terminal organelle. Avirulent mycoplasmas which possessed P1 showed no P1 clustering at the terminus. Both virulent M. pneumoniae and avirulent P1-containing mutants possessed numerous less dense P1 regions along the mycoplasma surface. Not surprisingly, an HA^- mutant lacking P1 exhibited only background immunoferritin labeling. Negative staining of intact mycoplasmas revealed a well-defined, naplike terminus (associated with P1 clusters) confined at the tip of virulent M. pneumoniae. Previous characterization of HA⁺ virulent and HA^- avirulent strains of M. pneumoniae by one- and two-dimensional polyacrylamide gel electrophoresis suggests that identified groups of mycoplasma proteins, lacking in specific HA^- mycoplasmas, regulate the physical arrangement of P1 and the ultrastructure of the terminus, thus influencing adherence to the respiratory epithelium and virulence.