Corynebacterium diphtheriae characteristics, morphology, culture

David Holt

Corynebacterium diphtheriae It is a Gram positive bacterium, but one that discolors easily, especially in old cultures. It is a straight, mallet-shaped, or slightly curved bacillus. It is resistant to extreme environmental conditions, including freezing and drying out. Some strains of this bacterium are pathogenic and capable of producing diphtheria.

C. diphtheriae It has four biotypes: gravis, intermedius, mitis and belfanti. Any of these biotypes can be toxigenic. Toxigenicity, or the ability to produce toxins, occurs only when the bacillus is infected (lysogenized) by a bacteriophage that carries the genetic information for toxin production. This information is carried by a gene known as the tox gene..

Corynebacterium diphteriae, transmission electron micrograph. Source:

Article index

  • 1 General characteristics
  • 2 Taxonomy
  • 3 Morphology
  • 4 Cultivation
  • 5 Clinical manifestations
  • 6 Pathogenesis
  • 7 Treatment
    • 7.1 Diphtheria antitoxin
    • 7.2 Complementary treatments
    • 7.3 Vaccination
  • 8 Disease reservoirs
  • 9 References

General characteristics

It is Gram positive, however in old cultures it can easily discolor. Often contains metachromatic granules (polymethaphosphate). These granules are stained blue-purple with the methylene blue dye..

Corynebacterium diphtheriae it is aerobic and facultative anaerobic, it does not produce spores. Its optimal development is achieved in a medium containing blood or serum at 35 to 37 ° C.

In tellurite-enriched agar plate cultures, colonies of C. diphtheriae have a black or gray coloration, after 24-48 h.


Corynebacterium diphtheriae it was discovered in 1884 by the German bacteriologists Edwin Klebs and Friedrich Löffler. It is also known as the Klebs-Löffler bacillus.

It is an Actinobacteria of the suborder Corynebacterineae. It belongs to the CMN group (bacteria of the families Corynebacteriaceae, Mycobacteriaceae and Nocardiaceae) that includes many species of medical and veterinary importance.

Four distinct biotypes or subspecies are recognized, mitis, intermedius, gravis and belfanti. These subspecies show slight differences in the morphology of their colony, their biochemical properties and their ability to metabolize certain nutrients..


Corynebacterium diphtheriae it is a bacillus in the shape of a straight club or with slightly curved ends. It does not present a scourge, so it is not mobile.

It contains arabinose, galactose, and mannose in its cell wall. It also has a toxic 6,6'-diester of corynemycolic and corynemylene acids..

Bacilli of the biotype gravis are generally short. Bacteria of the mitis biotype are long and pleomorphic. The intermedius biotype ranges from very long to short bacilli.


Corynebacteria, in general, are not very demanding in relation to culture media. Its isolation can be optimized using selective media.

The Loeffler medium, developed in 1887, is used to cultivate these bacteria and differentiate them from others. This medium consists of horse serum, meat infusion, dextrose and sodium chloride..

Loeffler's medium enriched with tellurite (tellurium dioxide) is used for the selective growth of C. diphtheriae. This medium inhibits the development of other species and by being reduced by C. diphtheriae leaves colonies gray-black.

Clinical manifestations

Diphtheria is, in most cases, transmitted by C. diphtheriae, even if C. ulcerans it can produce the same clinical manifestations. Diphtheria can affect almost any mucous membrane. The most common clinical forms include:

-Pharyngeal / Tonsillar: Is the most usual way. Symptoms include general malaise, sore throat, anorexia, and mild fever. May form a pseudomembrane in the region of the pharynx and tonsils.

-Laryngeal: can appear as an extension of the pharynx or individually. It produces fever, hoarseness, shortness of breath, high-pitched sounds when breathing, and a barking cough. Death may occur from airway obstruction.

-Nasal anterior: it is a rare clinical form. It manifests as a nosebleed. There may also be a purulent mucous discharge and a pseudomembrane develop in the nasal septum..

-Cutaneous- May present as a scaly rash on the skin or as well-defined ulcers. Depending on the location of the affected membrane and its extension, complications such as pneumonia, myocarditis, neuritis, airway obstruction, septic arthritis, osteomyelitis and even death can occur..


The disease is transmitted from a sick person to a healthy person through exhaled particles during respiration. It can also occur from contact with the secretion of skin lesions.

The acquisition of the diphtheria bacillus occurs in the nasopharynx. The pathogen produces a toxin that inhibits the synthesis of cellular proteins by the infected person.

This toxin is also responsible for the destruction of local tissue and the formation of a pseudomembrane. The toxin affects all cells in the body, but mainly the heart (myocarditis), nerves (neuritis) and kidneys (tubular necrosis).

Other effects of the toxin include thrombocytopenia, and proteinuria. Thrombocypenia is a decrease in the number of platelets in the blood. Proteinuria is the appearance of protein in the urine.

Within the first days of respiratory tract infection, the toxin causes a necrotic clot, or pseudomembrane, made up of fibrin, blood cells, dead cells of the respiratory tract epithelium, and bacteria..

The pseudomembrane can be local or extend widely, covering the pharynx and tracheobronchial tree. Membrane aspiration asphyxia is a common cause of death in both adults and children.


Diphtheria antitoxin

In case of suspected diphtheria, immediate administration of diphtheria antitoxin is necessary. This should be administered as soon as possible, even without waiting for confirmation of the diagnosis by laboratory tests..

The dose and route of administration will depend on the extent and duration of the disease..

Complementary treatments

In addition to diphtheria antitoxin, antimicrobial therapy is required to stop toxin production and eradicate C. diphtheriae.

This therapy may consist of Erythromycin (administered orally or parenterally), Penicillin G (intramuscularly or intravenously), or Procaine Penicillin G (intramuscularly), administered for two weeks.


Immunization with diphtheria toxoid will produce long-lasting but not necessarily permanent immunity. Because of this, an age-appropriate vaccine containing diphtheria toxoid should be administered during convalescence..

Disease reservoirs

Humans are considered to be the only reservoir of the disease. However, recent studies have isolated nontoxogenic strains of C. diphtheriae of domestic cats and cows.

A virulent strain of C. diphtheriae gravis biotype of horses. To date there is no evidence of zoonotic transmission of the disease, however, given these results, this possibility should be reevaluated..


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