Diphtheria

Interestingly, diphtheria toxin is produced by strains of Corynebacterium diptheriae that have themselves been infected with a special type of virus called a bacteriophage. The particular bacteriophage that infects C. diptheriae carries with it the gene that produces the diphtheria toxin. Strains of C. diptheriae without the bacteriophage do not produce the toxin.

The diphtheria toxin consists of two subunits, A and B. The B subunit binds to the plasma membrane of a cell. Once it is bound to the membrane, it pulls the A subunit into the cell. The A subunit is the active segment of the toxin, producing most of the effects. Once inside the cell, the A subunit disrupts protein synthesis; once this mechanism is disrupted, the cell cannot survive for long. Diphtheria toxin thus kills cells. Cells in the throat and respiratory tract are killed first; if the toxin spreads in the bloodstream to other organs—such as the heart, kidney, and brain—severe and even fatal damage can result.