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Diphtheria (Greek διφθέρα (diphthera) "pair of leather scrolls") is an upper respiratory tract illness caused by Corynebacterium diphtheriae, a facultative anaerobic Gram-positive bacterium.[1][2] It is characterized by sore throat, low fever, and an adherent membrane (a pseudomembrane) on the tonsils, pharynx, and/or nasal cavity.[3] A milder form of diphtheria can be restricted to the skin. Uncommon consequences include myocarditis (about 20% of cases) [4] and peripheral neuropathy (about 10% of cases).[5]

Diphtheria is a contagious disease spread by direct physical contact or breathing the aerosolized secretions of infected individuals. Historically quite common, diphtheria has largely been eradicated in industrialized nations through widespread vaccination. In the United States for example, there were 52 reported cases of diphtheria between 1980 and 2000; between 2000 and 2007 there were only three cases[6] as the DPT (Diphtheria–PertussisTetanus) vaccine is recommended for all school-age children. Boosters of the vaccine are recommended for adults since the benefits of the vaccine decrease with age without constant re-exposure; they are particularly recommended for those traveling to areas where the disease has not been eradicated.



Diphtheria toxin consists of a single protein or polypeptide. The protein is broken down (Proteolysis) to yield two fragments (A and B), which are held together by a disulfide bond. Fragment A prevents the host cells from undergoing protein synthesis. Fragment B is a recognition subunit (the host cell basically allows it to enter). Fragment B gains access to the host cell by binding to the plasma membrane at specific sites called the EGF-like domain of Heparin-binding EGF-like growth factor (HB-EGF). The HB-EGF then takes in the fragments through receptor-mediated endocytosis. The low pH of the endosome (sac which transports the HB-EGF and bound fragments A and B) induces fragment B to begin producing pores and catalyses the release of a catalytic fragment A into the cytosol or intracellular fluid. The toxin catalyses the ADP-ribosylation of (and inactivates) the elongation factor eEF-2. This elongation factor is a protein that is essential to protein synthesis; by inactivating it, the translation portion of protein synthesis is inhibited. The toxin enters the host cell and is hydrolysed by a trypsin-like protease to produce a toxic fragment. The toxin then transfers an ADP-ribose from NAD+ to a diphthamide residue (a modified Histidine amino acid) found within the EF-2 protein. EF-2 is needed for the moving of tRNA from the A-site to the P-site of the ribosome during translation. The ADP-ribosylation is reversible when by giving high doses of nicotinamide (or vitamin B3), one of the reaction's products. Diphtheria toxin is produced by C. diphtheriae only when it is infected with a bacteriophage. The bacteriophage integrates a gene into the bacteria that causes the toxin to be produced. [7]

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