Which bacteria is responsible for causing diphtheria?

The bacterium responsible for causing diphtheria is Corynebacterium diphtheriae. It is a Gram-positive, rod-shaped bacterium whose disease-causing power depends mainly on whether it carries a bacteriophage-borne gene for diphtheria toxin.

Toxigenic strains usually colonize the nose, throat, or other mucous membranes and can produce a thick gray pseudomembrane. The toxin blocks protein synthesis by inactivating elongation factor 2, potentially damaging the heart, nerves, kidneys, and other organs. Diphtheria can also occur as a cutaneous infection.

The organism is sometimes called the Klebs–Löffler bacillus, after Edwin Klebs and Friedrich Löffler, who identified it in the 1880s. A frequent misconception is that every C. diphtheriae strain produces toxin; nontoxigenic strains generally do not cause classic toxin-mediated diphtheria. Vaccination uses diphtheria toxoid, which trains the immune system to neutralize the toxin.