Corynebacterium diphtheriae

Life History

corynebacteriumCorynebacterium 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.


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.


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. The A portion contains an enzyme that cleaves nicotinamide from NAD, resulting in ADP-ribose. The ADP-ribose is then attached to elongation factor-2, which is necessary for polypeptide chain growth during translation. By inactivating EF-2, diphtheria toxin prevents protein synthesis by the host cell, which leads to host cell death.


diphtheriaC. diphtheriae is the causative agent of diphtheria. The incubation period is 2-7 days and medical treatment is necessary. Diphtheria is a respiratory disease with a range of symptoms and localizations. Nasal diphtheria is characterized by watery or blood discharge and a small, visible membrane in the nasal passage. It rarely causes complications by itself; however, it spreads the disease rapidly through aerosol droplets. Pharyngeal diphtheria is the most common form and is mainly characterized by a gray or greenish-gray membrane on the throat. Other symptoms include: mild sore throat, fever (101- 102∞F), rapid pulse, and general body weakness. Laryngeal diphtheria is most likely to cause serious complications including myocarditis (inflammation of heart muscle), damage to nervous system by toxin, and swelling of the neck (“bull neck”), which causes airway obstruction and leads to respiratory compromise. Symptoms include fever (103- 104∞F), extreme weakness, severe cough, difficulty breathing, and loss of voice. Cutaneous diphtheria constitutes about 33% of all cases and is a result of poor hygiene. Infected skin lesions are slow to heal and may be numb and/or insensitive; it is rarely fatal.


It is important to diagnose and begin antitoxin treatment as soon as diphtheria is suspected. Because the other symptoms are similar to mononucleosis, sinus infections, and strep throat, the most important symptom is the characteristic diphtheria membrane. Lab tests include a culture of the infected area followed by an identifying gram stain and growth on Loeffler’s medium, a medium selective for C. diphtheriae. Because a rapid diagnosis is key, these are methods applied, but treatment is started before the results are obtained. Consequently, new quicker methods of identifying C. diphtheriae are being developed. One such test is an immunochromatographic strip (ICS) test. This test employs equine polyclonal antibody and colloidal gold-labeled monoclonal antibody specific for fragment A of diphtheria toxin to detect the toxins presence. The study by Engler et al. gathered data from former USSR countries where diphtheria is an epidemic. 850 throat swabs were examined by the ICS test and conventional culture; the ICS test proved to be 98% sensitive, 99% specific, and showed a 99% concordance to the conventional test. In addition to being reliable, this method only requires 16 hours for results and is cost effective because it doesn’t require specialized equipment. 


The most important step is the prompt administration of antitoxin. The chance of serious complications increases as the time between onset and administration of antitoxin increases. The antitoxin is made from horse serum and neutralizes circulating exotoxin. About 10% of the population has a sensitivity to animal serum; thus, a test must be performed be for the antitoxin is given. If there is sensitivity, the individual is desensitized with diluted antitoxin. Usually a dosage of 20,000- 100,000 units is given intravenously, depending on the severity of the case. Antibiotics that kill the bacteria, prevent their spread, and protect against pneumonia follow the antitoxin administration. Penicillin, ampicillin, and erythromycin are commonly used. Hospitalization and monitoring for heart problems, airway blockage, and nervous system problems are required for severe cases. Finally, patients should be immunized to prevent re-infection.


The most effective way of prevention is immunization. Children receive a series of DPT (diphtheria-tetanus-pertussis) shots. The prep shot is given at 2-6 months of age, while two booster shots are given at 18 months and before entry into school. Adults are immunized with Td toxoid (tetanus-diphtheria). To maintain protection, immunization every ten years is recommended. Isolation of patients, 1- 7 days, is also important to prevent the spread, since its main route is person-to-person contact. Finally, reporting cases to public health authorities to track potential epidemics, identify strains, and check for antibiotic resistance is a method of indirect prevention. 

Epidemiological Data 

Since the introduction of the diphtheria vaccine in the 1920’s the incidence in the U.S. has decreased. Before the vaccine, the incidence rate was 100-200 cases per 100,000 and children were the highest risk group. Since 1980, the incidence rate in the U.S. has been 0.001 cases per 100,000. Conversely, in the former countries of the USSR, there has been an epidemic, more than 150,000 cases since 1990. “Incidence ranges from 0.5 to 1 per 100,000 population in Armenia, Estonia, Lithuania, and Uzbekistan, to 27- 32 cases per 100,000 in Russia and Tajikistan.”(who website) “Case fatality rates range from 2 –3% in Russia and Ukraine, to 6- 10% in Armenia, Kazakhstan, and Moldova, and to 17 –23% in Azerbaijan, Georgia, and Turkmenistan.” (who website) Interestingly, adults have become the high-risk group in the Eastern European epidemic.


K. H. Engler,1* A. Efstratiou,1 D. Norn,2 R. S. Kozlov,3 I. Selga,4 T. G. Glushkevich,5 M. Tam,2 V. G. Melnikov,6 I. K. Mazurova,6 V. E. Kim,7 G. Y. Tseneva,8 L. P. Titov,9 and R. C. George1,Immunochromatographic Strip Test for Rapid Detection of Diphtheria Toxin: Description and Multicenter Evaluation in Areas of Low and High Prevalence of DiphtheriaJournal of Clinical Microbiology, January 2002, p. 80-83, Vol. 40, No. 1

"Diphtheria." Rebecca J. Frey, PhD. The Gale Encyclopedia of Medicine. Second Edition. Jacqueline L. Longe, Editor. 5 vols. Farmington Hills, MI: Gale Group,2001.

Diana Marra Oram, Ana Avdalovic, and Randall K. Holmes*, Construction and Characterization of Transposon Insertion Mutations in Corynebacterium diphtheriaeThat Affect Expression of the Diphtheria Toxin Repressor (DtxR)Journal of Bacteriology, October 2002, p. 5723-5732, Vol. 184, No. 20

Jerome J Perry, James T. Staley, Stephen Lory.Microbial Life. Sinauer Associates. Sunderland, MA. 2010