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Microbiology · Gram-positive rods

Corynebacterium diphtheriae !

  • Bacterium
  • Gram-positive
  • Droplet
  • Contact
  • Exam priority: medium

A bacterium whose toxin can form a grey membrane in the throat and damage the heart and nerves; vaccination prevents it.

Updated 2026-11-01

Also called: Klebs-Löffler bacillus

At a glance

Gram
Gram-positive
Shape
Club-shaped rod in «Chinese letter» arrangements
Oxygen
Aerobe or facultative anaerobe
Family
Corynebacteriaceae
Stain
Gram positive; metachromatic granules

Morphology

  • Club-shaped Gram-positive rods in V shapes or palisades
  • Metachromatic granules (methylene blue)
  • No spores

Transmission

  • Respiratory droplets from patients or carriers
  • Contact with skin lesions
  • Objects contaminated with secretions

Isolation precautions

What the care team adds to standard precautions.

Droplet
Surgical mask within about a metre; single room.
Contact
Gown and gloves; dedicated equipment. For C. difficile, soap-and-water hand washing.

Virulence factors

  • Diphtheria toxin, encoded by a bacteriophage (beta)
  • ADP-ribosylates elongation factor 2 and halts protein synthesis
  • Local necrosis (pseudomembrane) and damage to heart muscle and nerves

Diseases it causes

Clinical picture

  • Sore throat, low fever and an adherent grey pseudomembrane that bleeds when lifted
  • Bull neck from nodes and oedema
  • Myocarditis and peripheral neuropathy from the toxin

Diagnosis

  • Culture on Löffler medium or tellurite agar (Tinsdale)
  • Elek test or PCR to confirm the strain makes toxin
  • In Colombia: SIVIGILA event 230; field investigation starts within 48 hours

Treatment

Diphtheria antitoxin as early as possible to neutralise circulating toxin, plus an antibiotic (a macrolide such as erythromycin or azithromycin, or penicillin) to stop the bacterium and transmission. Contacts receive antibiotic prophylaxis, and everyone is vaccinated after the acute phase.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Vaccination with diphtheria toxoid and boosters
  • Droplet precautions (contact for the skin form)
  • Prophylaxis and vaccination of close contacts

Vaccine: Diphtheria toxoid is in Colombia's PAI: pentavalent vaccine in infants, a DTP booster at five years, Td in adolescents and adults and Tdap in pregnancy.

Test yourself

Think of the answer, then tap the question to see it.

What is the mechanism of diphtheria toxin?

It ADP-ribosylates elongation factor 2 (EF-2) and halts the cell's protein synthesis.

Which test confirms that a C. diphtheriae strain makes toxin?

The Elek test (immunodiffusion) or PCR for the toxin gene.

Why is antitoxin given without waiting for confirmation?

It only neutralises free toxin, not toxin already inside cells; delay increases heart and nerve damage.

Which isolation precautions does pharyngeal diphtheria need?

Standard plus droplet precautions; the skin form needs contact precautions.

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← Back to the group: Gram-positive rods

Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Diphtheria (WHO, 2025); About Diphtheria (CDC, 2024); Difteria (OPS/OMS, 2026); Protocolo de vigilancia en salud pública difteria (Instituto Nacional de Salud (INS), Colombia, 2024).

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Representative model, for educational purposes only.