Diphtheria : Introduction, Pathogenesis, Lab Diagnosis and Treatment

Diphtheria introduction, pathogenesis ,lab diagnosis and treatment

Introduction of Diphtheria

From the Greek word, diphtheria (leather), diphtheria is named for the tough pseudomembrane that forms in the patient’s throat.
Caused by Aerobic Gram +ve bacilli, Corynebacterium diphtheriae.
Exotoxin production only if infected by virus phage infected carrying tox gene
Diphtheria is an acute, toxin-mediated disease caused by toxigenic Corynebacterium diphtheriae. 

It’s a very contagious and potentially life-threatening bacterial disease.
An infection of local tissue of upper respiratory tract with the production of toxin which causes systemic effects on Heart and Peripheral tissues.
It’s a localized infectious disease, which usually attacks the throat and nose mucous membrane.

Etiology of Diphtheria

C. diphtheriae is an aerobic gram-positive bacillus.
Pleomorphic, club-end

There are four biotypes
mitis and
The most severe clinical type of this disease is associated with the gravis biotype, but any strain may produce toxins.
The major virulence determinant is an exotoxin, diphtheria toxin.

After binding to the host cells, the active subunit will interrupt the protein synthesis of the target host cell and results in cell death.
Toxoids made from diphtheria toxins can be used as vaccine.

Pathogenesis  of Diptheria

Entry of the bacilli multiply locally in the throat and elaborate a powerful exotoxin that produces local and systemic symptoms.
Local lesions 
Exotoxin causes necrosis of the epithelial cells and liberates serous and fibrinous material which forms a grayish-white pseudo membrane.
The membrane bleeds on being dislodged.
The surrounding tissue is inflamed and edematous.
Local manifestation
Depend on the site of the lesion.
Nasal diphtheria 
Unilateral or bilateral serosanguineous ( blood and serous fluid ) discharge from the nose
Excoriation of the upper lip
Toxemia is minimal.
Faucial diphtheria 
Redness and swelling over fauces
Exudates on the tonsils coalesce to form grayish-white pseudomembrane
Regional lymph nodes are inflamed.
Sore throat and

Nasopharyngeal diphtheria
Cutaneous diphtheria
Systemic complications
Note:  Diagnosis must be clinical.

Clinical features

Malaise, sore throat, fever
Adherent grey pseudomembrane
Nasal ulcers
Obstruction of the larynx and lower airways
Difficulty in swallowing
Lead to myocarditis

Peripheral neuritis
Paralysis of limbs
May involve any mucous membrane
Classified based on site of infection
Anterior nasal
Pharyngeal and tonsillar


Faucial diphtheria is very common,
Malignant or Hyper toxic toxemia
Marked adenitis
circulatory failure
Paralytic sequela
Septic ulceration

cellulitis, gangrene, epistaxis, and bleeding tendency.
Asphyxia – causing mechanical obstruction

May need tracheotomy

Circulatory failure

Post diphtheria paralysis
Non toxigenic clinical manifestations
Bacteria can produce


Cerebral abscess


Laboratory Diagnosis of Diphtheria

Bacteriological study -direct smear examination and culture
Virulence test
Animal inoculation -subcutaneously and intracutaneously
In vitro test
Tissue culture test
Collection of Specimens
Throat swabs from the peritonsillar region if possible collect pseudomembrane
Smear examinations
Gram’s stain, Albert’s stain  or Neisser’s stain   or Ponder’s stain, or  Loeffler’s polychrome methylene blue staining
Immunofluorescent methods
Cultures on Loeffler’s serum slope
Tellurite Blood agar
Blood agar
Isolation of C. diphtheriae
Serum slope: Growth in 6 – 8 hours
Stain with Neisser’s stain Albert’s stain
Bacilli have metachromatic granules
Tellurite blood agar takes two days for the manifestation of colonies.

Virulence tests

In vivo and In vitro
In vivo in Animals
Subcutaneous tests
Inject broth from culture into two guinea pigs, 0.8 ml
One animal is given 500 units of antitoxin
Other no Vaccine.
Animal not given antitoxin will die.
The loss of animals restricts its testing.
Intracutaneous method
One animal is given 500 units of antitoxin before the toxin

Other 50 units antitoxin after toxin

So the animals can be saved
In Vitro Testing
Elek was modified in 1949 as Elek’s gel precipitation testing
Filter paper impregnated with Diphtheria antitoxin 1000 Units /ml
Tested on the horse serum agar
Positive or Negative-test strains tested for Immunodiffusion
Line of precipitation – test positive
Other methods of testing in tissue cultures

Toxigenicity Tests

In vitro Elek test
In vivo Animal inoculation
rabbit skin test-necrosis
guinea pig challenge test- lethal


Antibiotic is not useful in acute infections.
Antitoxin is also necessary.
Anti toxin obtained from horse serum
Mild 20,000 to 40,000 U
Moderate 40,000 to 60,000 U
Severe 80,000 to 1,00,000 U
Commonly used antibiotics-
Penicillin parentally
Oral Erythromycin

Further Readings on Diphtheria

  • Topley and Wilson’s microbiology and microbial infection Topley and Wilson’s microbiology and microbial infection – Bacteriology-2-10th Edn.
  • Manual of Clinical Microbiology   -Patrick R. Murray -8th Edn.
  • Bailey and Scott’s  Diagnostic Microbiology -13th Edn.
  •  Mackie & Mc Cartney  Practical Medical Microbiology- 14th  Edn.
  • Diagnostic Microbiology -Connie R. Mahon & George Manuselis
  • Cowan and Steel’s, manual for the identification of medical bacteria
  • Koneman Color Atlas and Text-Book of Diagnostic Microbiology-6th  Edn.
  • Jawetz Melnick and Adelberg’s Medical Microbiology- 25th Edn.
  • Lippincott’s –Illustrated- review-Microbiology-3rd Edn.
  •  Mandell’s Infectious Disease-7th Edn.
  • Bergey’s Manual of Systemic Bacteriology – 2nd  Edn.
  • Medical Microbiology-The Practice of Medical Microbiology Vol-2-12th Edn. –Robert Cruickshank
  • District Laboratory Practice in  Tropical Countries  –  Part-2-   Monica Cheesebrough-   2nd Edn Update
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