Corynebacterium Diphtheriae :: Medical Biology Medicinal Essays

Corynebacterium Diphtheriae Life History: Corynebacterium diphtheriae is the microbial causative agent of diphtheria; however, it is the exotoxin produced by pathogenic strains of the bacteria that cause the symptoms of the disease. Non-pathogenic strains are normal inhabitants of the oral cavity and lack the bacteriophage DNA to produce exotoxin. Pathogenic C. diphtheriae can localize in different parts of the body, including the nasal cavity, the naso-pharynx, the larynx, and the skin. Depending on the localization, the symptoms range from slight to severe (death). The bacteria are spread by direct person-to-person contact; concurrently, humans are the reservoirs for C. diphtheriae. Since the introduction of a vaccine in the 1920’s, diphtheria has not been prevalent in developed countries. Conversely, it is endemic in developing countries, especially in the former countries of the USSR. Characteristics: C. diphtheriae are non-motile, aerobic, gram-positive bacilli (rods). Viewed with a microscope, the bacteria are club-shaped and group in patterns that resemble Chinese characters. Little is known about the details of the adhesion mechanism of C. diphtheriae, but the bacteria are known to produce a neuraminidase. This enzyme is able to split N-acetylneuuraminic acid, which is found on the surface of animal cells. This may facilitate bacterial entry into the host cell. There are four biotypes of C. diphtheriae, gravis, mitis, belfanti, and intermedius. Only intermedius can be differentiated by colony morphology; small, gray, or transluscent lipophilic colonies compared to larger, white or opaque colonies of the other biotypes. In addition, the biotype intermedius is rare in clinical infections, while the belfanti biotype rarely carries the tox gene. Virulence: C. diphtheriae are capable of producing exotoxin. The diphtheria toxin gene, tox, is located on a bacteriophage genome; thus, bacteriophage infection is necessary for virulence. tox is negatively regulated by diphtheria toxin repressor (DxtR), located on the bacterial chromosome, in response to iron concentration within the cell. When iron concentration is low, DxtR does not repress the tox gene because it requires iron to be functional. Thus, when iron concentration is low, the tox gene is transcribed and toxin is produced. Diphtheria toxin is made up of a single polypeptide chain with two portions, A and B. The toxin attaches to a cell membrane receptor of the host cell and is engulfed by receptor-mediated endocytosis. Once inside the vesicle, the disulfide bond between the A and B portion is cleaved and the A portion enters the cytosol, where is can resist degradation for a long period of time.