Mycoplasma pneumoniae - Pathogenesis, Virulence factors, Host immunity, Transmission

Virulence factors of Mycoplasma pneumoniae

Mycoplasma pneumoniae has several virulence factors.

P1 is an adhesion protein. P1 Ag is a membrane-associated protein that helps in the adhesion of mycoplasmas to epithelial cells (isolated glycoprotein receptors present at the base of cilia on the epithelial surfaces). This receptor is also present on the surface of erythrocytes. antibody cross-reaction can cause RBC to agglutinate.

Hydrogen peroxide and Superoxide radicals are important virulence factors of Mycoplasma pneumoniae.

Pathogenesis of Mycoplasma pneumoniae

Mycoplasma pneumoniae colonizes mucosal surfaces of respiratory and genital tracts. They predominantly reside extracellularly attached to ciliated and non-ciliated epithelial cells. Mycoplasma pneumoniae has now been identified intracellularly.

Intracellular invasion contributes to hist immune evasion, the persistent nature of the infection, and difficulty in isolation/cultivation of Mycoplasmas.

After attachment, Mycoplasma pneumoniae causes direct damage to cilia and then ciliated epithelial cells. Loss of cells interferes with the normal functioning of URT. This results in LRT becoming infected with microbes and mechanically irritated.

This mechanical irritation is seen in patients with a respiratory infection and causes persistent cough. Mycoplasma pneumoniae acts as a superantigen. This causes the migration of inflammatory cells to the site of infection and produces cytokines (TNF- α, IL-1, IL-6). The cytokines help in clearing the bacteria.

The bacteria do not usually invade blood to produce systemic infections. But in cases of immunosuppression or following instrumentation they penetrate the sub-mucosa and invade the bloodstream and cause infection in different organs of the body.

Host immunity of Mycoplasma pneumoniae

Mycoplasma pneumoniae infection does not induce any protective host immunity and is susceptible to reinfection. Antibody against P1 Ag is found in 50% of patients. Ab against P1 Ag is an autoantibody and it cross-reacts with Antigen I of RBS. It is not protective.

Transmission of Mycoplasma pneumoniae

Patients with active infection are more likely to transmit Mycoplasma pneumoniae infection by close contact through nasal secretions (inhalation of aerosolized droplets).