The diphtheria toxin causes the death eukaryotic cells and tissues by inhibition protein synthesis in the cells. Although the toxin is responsible for the lethal symptoms of the disease, the virulence of C. diphtheriae cannot be attributed to toxigenicity alone, since a distinct invasive phase apparently precedes toxigenesis. However, it has not been ruled out that the diphtheria toxin plays an essential role in the colonization process due to short-range effects at the colonization site. Three strains of Corynebacterium diphtheriae are recognized, gravis , intermedius and mitis . They are listed here by falling order of the severity of the disease that they produce in humans.
All strains produce the identical toxin and are capable of colonizing the throat.
The differences in virulence between the three strains can be explained by their differing abilities to produce the toxin in rate and quantity, and by their differing growth rates. The faster growing strains typically produce a larger colony on most growth media. Also, if the kinetics of toxin production follow the kinetics of bacterial growth, the faster growing variety would achieve an effective level of toxin before the slow growing varieties. The gene for toxin production occurs on the chromosome of the prophage, but a bacterial repressor protein controls the expression of this gene. The repressor is activated by iron , and it is in this way that iron influences toxin production.