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