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