B. Natural History Natural infections only occur in humans where it can be cultivated from respiratory tract and occassionally skin. Diphtheria begins in the upper respiratory tract where bacteria multiply in the throat in the superficial layers of mucous membranes. Elaboration of toxin causes necrosis of neighboring cells. Inflammation results in the classic pseudomembrane which can actually cause mechanical obstruction and suffocation. In the most serious form of disease, is characterized by irregular heartbeat, then coma and death.
Survivors are immune, but can carry and transmit the organisms.
Carriage rates among vaccinated individuals is much lower. Diphtheria toxin A. Structure Synthesized as a 560 amino acids polypeptide containing a 25-residue signal sequence for secretion. The mature protein has a molecular mass of 58,343-kDa. Its gene has been sequenced and its structure solved by X-ray crystallagraphy. The protein is proteolytically nicked to generate two chains, a 193 residue A chain and a 342 residue B chain.