Diphtheria overview and Definition

The causative agent for the diphtheria is the Corynebacterium diphtheriae. It is bacterial infection that affects the mucous membrane of the throat and the nose. It can be fatal if prompt treatment is not done.

Structure of Diphtheria

The cause of diphtheria is bacterial species termed as Corynebacterium diphtheriae a gram positive bacillus that usually produces exotoxins. They shows the palisading arrangements when viewed under the microscope. There are 4 types of strains of C. Diphtheriae, they include the following biotypes:





The intermedius strain produces more amount of exotoxins when compared to other strains.

The organisms easily invade the tissue lining the throat and during this invasion they produce exotoxins that destroy the tissue and lead to the development of the pseudomembrane.

Non toxin producing strains and other Corynebacterium species such as C. ulcerans can still cause infection.

Non toxin produced strains and other Corynebacterium species such as C.ulcerns  can still cause infection, but the infection is less severe and  sometimes only remains in the skin and leads to the formation of the skin rashes.


Routes of Transmission

Diphtheria spreads from person to person through the cough or sneeze from the infected persons. And also might spread through the contacting the inanimate objects.


The Corynebacterium secreate exotoxins such as polypeptide chain A and chain B.

Corynebacterium diphtheriae may produce a neuraminidase that cleaves sialic acid from the cell surface into its pyruvate and N-acetylneuraminic acid components. Cord factor (6,6′-di-O-mycoloyl-α,α'-D-trehalose) is a surface component of C diphtheriae, but its role in colonization of the human host is unclear.

A thick, gray-green fibrin membrane, the pseudomembrane, often forms over the site(s) of infection as a result of the combined effects of bacterial growth, toxin production, necrosis of underlying tissue, and the host immune response. Recognition that the systemic organ damage was due to the action of diphtheria toxin led to the development of both an effective antitoxin-based therapy for acute infection and a highly successful toxoid vaccine.

This toxin interferes the protein synthesis of the host cells.

These above features result in tissue necrosis and also pseudomembrane formation resulting in clinical manifestation of the disease.

Clinical signs & symptoms

Early features of the infection, before the pseudomembrane appears, include:

  • swollen glands on the neck
  • low fever, malaise and weakness
  • Swelling of soft tissue in the neck, giving a ‘bull neck’ appearance
  • nasal discharge
  • fast heart rate

Children with a diphtheria infection in a cavity behind the nose and mouth are more likely to have the following early features:

  • nausea and vomiting
  • chills headache and fever

After a person is first infected with the bacteria, there is an average incubation period of 5 days before early signs and symptoms appear.

After the initial symptoms have appeared, within 12 to 24 hours, a pseudomembrane will begin to form if the bacteria are toxic, leading to:

  • difficulty swallowing
  • a sore throat
  • possible obstruction that causes breathing difficulties

If the membrane extends to the larynx, hoarseness and a barking cough are more likely, as is the danger of complete obstruction of the airway. The membrane may also extend further down the respiratory system toward the lungs.

Differential Diagnosis

For microbiologic and epidemiologic surveillance, it is crucial that the laboratory receive the following information for each specimen from suspected cases, contacts, and carriers: name, age, and sex of patient; hospital to which the patient was admitted; name of physician caring for the patient; laboratory details (source of specimen, date collected); clinical details (symptoms, onset date, treatment regime); epidemiologic information (case, contact, or carrier); and immunization and travel histories.

Screening tests for presumptive identification

Unless a clinical diagnosis of diphtheria is suspected, laboratory diagnosis may be difficult because C. diphtheriae is not easily identified on blood agar. In addition, swabs with specimens from asymptomatic carriers or contacts may contain only small numbers of organisms, which may be obscured by the overgrowth of normal throat flora. To obtain the better results all the swabs are investigated using the tellurite medium.

Clinical specimens should ideally be cultured onto blood agar and selective tellurite media; Hoyle's tellurite medium is recommended . Tellurite medium enhances the growth of the diphtheria bacteria and reduces the growth of normal flora.

Significance of biotyping:

Pathogenic strains can be easily identified by the use of simple tests such as API CORYNE and  Rosco diagnostics tests are readily available. They shows pyrazinamidase negative, and cystinase positive. They will also ferment glucose, maltose, and starch. They also shows catalase and nitrate positive, urease negative.

Elk Gel Precipitation test:

A strip of filter paper is impregnated with diphtherial antitoxin is buried just beneath the surface of the special agar plate before the agar hardens. Strains should be tested are streaked with known positive and known negative  toxigenic strains on the agar’s surface in a line across the plate and at right angles to the antitoxin paper strip. After incubation for 24 hours at 37 degree Celsius plates are examined with transmitted light for the presence of fine precipitin lines at a 45 degree angle to the streaks.

Polymerase chain reaction:

This uses the amplification of the DNA of the bacteria which is in investigation.


Most people with diphtheria shows good prognosis. The fatality rate in complicated cases shows 10-15 %.


Vaccinate earlier

Try to avoid contacts with the infected peoples

If the infection is detected isolated yourself from the surroundings.