Corynebacterium diphtheriae
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Direct answer
Corynebacterium diphtheriae is a non-motile, non-spore-forming Gram-positive bacillus that causes diphtheria through an A-B exotoxin encoded by a lysogenic corynebacteriophage. The toxin inactivates elongation factor 2 by ADP-ribosylation, halting protein synthesis and producing a grey adherent pseudomembrane in the pharynx. Toxin production is confirmed by the Elek test or tox gene PCR, but treatment starts on clinical suspicion.
What you must remember
- Morphology: club-shaped Gram-positive bacilli in V or Chinese-letter arrangements, with metachromatic granules seen on Albert or Neisser staining.
- Culture: grows rapidly on Loeffler serum slope and on cysteine tellurite agar, where colonies turn black because the organism reduces tellurite.
- Toxin gene: the tox gene is carried by a lysogenic corynebacteriophage, so non-lysogenised strains are non-toxigenic.
- Elek test: an immunodiffusion test in which toxin from the isolate and antitoxin meet in agar to form a precipitin line.
- Pharyngeal diphtheria: a grey-white adherent pseudomembrane on the tonsils and pharynx that bleeds if forcibly removed, sometimes with a bull neck in severe cases.
- Complications: myocarditis, usually in the second or third week, peripheral neuropathy, and airway obstruction from laryngeal disease.
- Management and prevention: diphtheria antitoxin plus an antibiotic such as erythromycin or penicillin, and DPT vaccination in the Universal Immunisation Programme with Td boosters later in life.
Toxigenic and non-toxigenic strains compared
A throat swab grows Gram-positive bacilli, but the clinical question is whether the strain produces toxin. The comparison below separates the toxigenic strain that causes diphtheria from the non-toxigenic strain that does not.
| Feature | Toxigenic strain | Non-toxigenic strain |
|---|---|---|
| Elek test | Positive, precipitin line | Negative |
| tox gene PCR | Positive | Negative |
| Clinical picture | Classic diphtheria | Usually no systemic toxicity |
| Antitoxin | Given on clinical suspicion | Not indicated |
The Elek test and tox gene PCR are read together, because a discordant result calls for repeat testing. Antitoxin is given to a patient with the classic membrane even before the Elek result returns, while non-toxigenic strains need only the appropriate antibiotic and follow-up.
Culture on cysteine tellurite agar and microscopic morphology remain the first steps, and the team must still treat the patient clinically rather than waiting for the laboratory. Swabs are taken from under the membrane and from the nose as well, since nasal carriage can persist after the throat has cleared.
Where the exam tries to catch you
Diphtheria questions often hinge on the toxin mechanism, the test used to prove toxigenicity and the timing of antitoxin.
- Trap: dismissing a grey throat membrane as tonsillitis because the isolate is only a Gram-positive rod Correct: diagnosis is clinical, and antitoxin should start on suspicion while toxigenicity is confirmed later.
- Trap: assuming every Corynebacterium diphtheriae isolate causes diphtheria Correct: non-toxigenic strains lack the tox gene, and the Elek test distinguishes them from toxigenic strains.
- Trap: believing vaccination gives lifelong protection Correct: antitoxin levels fall over time, which is why booster doses are needed in later life.
Frequently asked questions
Which test confirms toxin production by Corynebacterium diphtheriae?
The Elek test confirms toxin production by Corynebacterium diphtheriae, showing a precipitin line where toxin and antitoxin meet in agar. PCR for the tox gene is an alternative method used in many laboratories.
Which medium is used to isolate Corynebacterium diphtheriae?
Cysteine tellurite agar is used to isolate Corynebacterium diphtheriae, and colonies appear black because the organism reduces tellurite. Loeffler serum slope gives rapid growth, and the bacilli show metachromatic granules on Albert staining.
Why is diphtheria antitoxin given before laboratory confirmation?
Diphtheria antitoxin is given before laboratory confirmation because it neutralises only unbound toxin, and damage from toxin is rapid. Treatment is started on clinical suspicion, since waiting for culture or the Elek test can allow fatal toxin injury to occur.
What complications of diphtheria need monitoring?
Myocarditis and peripheral neuropathy are the main complications of diphtheria needing monitoring, along with airway obstruction from laryngeal disease. Myocarditis usually appears in the second or third week, so cardiac monitoring is needed during this period.
How does immunisation protect against diphtheria in India?
Immunisation against diphtheria in India uses the DPT vaccine, which contains diphtheria toxoid, within the Universal Immunisation Programme. Td boosters later in life help maintain protective immunity against Corynebacterium diphtheriae toxin.