[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Gram-positive rods](https://durgical.com/en/microbiology/groups/gram-positive-rods/) › Corynebacterium diphtheriae

[Microbiology](https://durgical.com/en/microbiology/) · [Gram-positive rods](https://durgical.com/en/microbiology/groups/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

## Key points

- Diphtheria toxin ADP-ribosylates EF-2.
- The toxin is encoded by a bacteriophage (lysogenic conversion).
- Grey pseudomembrane that bleeds when lifted.
- Tellurite medium and the Elek test.
- Antitoxin + antibiotic; toxoid vaccine.

## 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

- Respiratory and cutaneous diphtheria [Diphtheria →](https://durgical.com/en/conditions/diphtheria/)

## 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:

- [Erythromycin](https://durgical.com/en/pharmacology/erythromycin/)
- [Azithromycin](https://durgical.com/en/pharmacology/azithromycin/)
- [Benzylpenicillin (penicillin G)](https://durgical.com/en/pharmacology/benzylpenicillin/)

## 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.

## In Colombia

Diphtheria is SIVIGILA event 230. Colombia has confirmed isolated cases in recent years, some imported, against the background of regional outbreaks (Venezuela, Haiti); a single confirmed case is handled as an outbreak.

## Memory aid

Diphtheria is a double F: it freezes elongation Factor 2 and leaves a False membrane.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/corynebacterium-diphtheriae/)

## Related

- [Clostridium tetani](https://durgical.com/en/microbiology/clostridium-tetani/)
- [Streptococcus pyogenes](https://durgical.com/en/microbiology/streptococcus-pyogenes/)

## Related conditions

- [Diphtheria](https://durgical.com/en/conditions/diphtheria/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

**Contact**
Gown and gloves; dedicated equipment. For C. difficile, soap-and-water hand washing.

**Droplet**
Surgical mask within about a metre; single room.

**Airborne**
N95 respirator and a negative-pressure room.

[← Back to the group: Gram-positive rods](https://durgical.com/en/microbiology/groups/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](https://www.who.int/news-room/fact-sheets/detail/diphtheria) (WHO, 2025); [About Diphtheria](https://www.cdc.gov/diphtheria/about/index.html) (CDC, 2024); [Difteria](https://www.paho.org/es/temas/difteria) (OPS/OMS, 2026); [Protocolo de vigilancia en salud pública difteria](https://www.ins.gov.co/buscador-eventos/Lineamientos/Pro_Difteria.pdf) (Instituto Nacional de Salud (INS), Colombia, 2024).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

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Source: https://durgical.com/en/microbiology/corynebacterium-diphtheriae/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/corynebacterium-diphtheriae/