Corynebacterium diphtheriae bacteria, illustration. C. diphtheriae is a gram-positive rod-shaped bacterium that is transmitted via the respiratory route and causes the disease diphtheria. Diphtheria is characterised by formation of fibrin-containing membranes on the tonsils and on the surface of the respiratory tract that can lead to respiratory obstruction and difficulties breathing. C. diphtheriae also produces a toxin that affects the heart, nervous system and the kidneys.