In order to cause disease, Mycoplasma pneumoniae must adhere to host cells and produce cytotoxic molecules.
Despite its small genome, M. pneumoniae achieves cytadherence through the synthesis of a complex cellular structure, the
terminal organelle or attachment organelle, which initiates binding to host cells through carbohydrate moieties associated
with protein and lipid molecules on their surfaces. The construction of this organelle is remarkable, making use of proteins
that are generally dissimilar in composition to those found in organisms other than the closest relatives of M. pneumoniae.
The attachment organelle also plays an essential role in the crucial but poorly understood process of gliding motility.
Cytadherence is also carried out by other proteins involved in binding to ligands found on host cell surfaces. Having
accomplished cytadherence, M. pneumoniae damages host cells though various means. One is production of hydrogen
peroxide, which occurs via metabolism of glycerol. Another is the production of CARDS toxin, an ADPribosyltransferase
that induces vacuolation and, when administered on its own, mimics many of the clinical symptoms of
M. pneumoniae-associated disease. A nuclease has been implicated in promoting apoptosis of host cells. Finally, certain
M. pneumoniae lipoproteins elicit host immunologic responses that contribute significantly to cytokine production and
concomitant inflammation. These various virulence factors make M. pneumoniae difficult to clear, resulting in not just
acute but also chronic disease.