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Web Review of Todar's Online Textbook of Bacteriology. "The Good, the Bad, and the Deadly".

Tag words: diphtheria, Corynebacterium diphtheriae, C. diphtheriae, diphtheria bacteria, pseudomembrane, diphtheria toxin, dtx, Beta phage, Theobald Smith, Freeman, Pappenheimer, diphtheria toxoid, DPT, DTP, DTaP.

Corynebacterium diphtheriae

Kingdom: Bacteria
Phylum: Actinobacteria
Order: Actinomycetales
Suborder: Croynebacterineae
Family: Corynebacteriaceae
Genus: Corynebacterium
Species: C. diphtheriae








Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

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Diphtheria (page 2)

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Human Disease

CDC describes diphtheria as an upper respiratory tract illness characterized by sore throat, low-grade fever, and an adherent membrane of the tonsil(s), pharynx, and/or nose. Diphtheria is a rapidly developing, acute, febrile infection which involves both local and systemic pathology. A local lesion develops in the upper respiratory tract and involves necrotic injury to epithelial cells. As a result of this injury, blood plasma leaks into the area and a fibrin network forms which is interlaced with with rapidly-growing C. diphtheriae cells. This membranous network, called a pseudomembrane, covers over the site of the local lesion leading to respiratory distress, even suffocation.


Figure 3. Diphtheria pseudomembrane. CDC.

The diphtheria bacilli do not tend to invade tissues below or away from the surface epithelial cells at the site of the local lesion. However, at this site they produce the toxin that is absorbed and disseminated through lymph channels and blood to the susceptible tissues of the body. Degenerative changes in these tissues, which include heart, muscle, peripheral nerves, adrenals, kidneys, liver and spleen, result in the systemic pathology of the disease.

Pathogenicity The pathogenicity of Corynebacterium diphtheriae includes two distinct phenomena:

1. Invasion of the local tissues of the throat, which requires colonization and subsequent bacterial proliferation. Little is known about the adherence mechanisms of C. diphtheriae, but the bacteria produce several types of pili. The diphtheria toxin, as well, may be involved in colonization of the throat.

2. Toxigenesis: bacterial production of the toxin. The diphtheria toxin causes the death eucaryotic 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 gravis strain has a generation time (in vitro) of 60 minutes; the intermedius strain has a generation time of about 100 minutes; and the mitis stain has a generation time of about 180 minutes. The faster growing strains typically produce a larger colony on most growth media. In the throat (in vivo), a faster growth rate may allow the organism to deplete the local iron supply more rapidly in the invaded tissues, thereby allowing earlier or greater production of the diphtheria toxin. 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.


Figure 4. Corynebacterium diphtheriae colonies on blood agar. CDC.




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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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