The Pathobyte Series: Diphtheria – Silent Throat Killer and Its Deadly Toxin

BugSpeaks

Mar 25, 2026

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4 min read

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First isolated in 1884, Corynebacterium diphtheriae is a versatile Gram-positive bacterium capable of causing a wide array of human illnesses. Driven by an advanced arsenal of virulence factors and toxins like the diphtheria toxin, it effortlessly evades host immunity, spreading rapidly through respiratory droplets and close contact. Infections range from localized pharyngeal lesions featuring a leathery pseudomembrane to life-threatening systemic conditions, including myocarditis and polyneuropathy. Accurate diagnosis leverages classic laboratory cultures on selective tellurite media and modern tools like PCR testing. While treatment typically requires targeted antibiotic therapy using penicillin alongside urgent antitoxin administration for toxigenic strains, robust prevention relies heavily on routine toxoid immunization protocols.

History

The clinical entity of diphtheria was first clearly described by the French physician Pierre Bretonneau in 1826, who coined the term 'diphthérite' from the Greek word for leather, referring to the characteristic tough throat membrane. Friedrich Löffler isolated the causative bacillus in 1884, now known as C. diphtheriae.

Pathogenesis

C. diphtheriae is a gram-positive, non-spore-forming, non-motile bacillus that colonises the upper respiratory tract. Its pathogenicity is driven almost entirely by diphtheria toxin (DT), encoded by the tox gene located on the lysogenic corynephage β. Only toxigenic strains carry this phage. The toxin is a two-fragment (A-B) exotoxin: the B fragment binds to the heparin-binding epidermal growth factor precursor on host cell surfaces, enabling receptor-mediated endocytosis, while the A fragment catalyses ADP-ribosylation of elongation factor-2 (EF-2), irreversibly halting eukaryotic protein synthesis. This mechanism underlies local pseudomembrane formation in the pharynx and systemic complications including myocarditis, polyneuropathy, and renal tubular necrosis. Locally, the organism elicits a fibrinous, inflammatory exudate forming the classic greyish-white pseudomembrane, tightly adherent and bleeding upon attempted removal.

Gram-positive- A category of bacteria that possess a thick outer cell wall, which stains purple during standard laboratory identification tests.

Non-spore-forming- Bacteria that do not create hard, protective internal shells (spores) to survive extreme environments, typically making them easier to kill with heat or disinfectants.

Non-motile- Microbes that lack microscopic tails or hairs, meaning they cannot move or swim on their own.

Pathogenicity- The inherent ability of a microbe (like a bacterium or virus) to cause disease or inflict damage inside a host body.

Diphtheria toxin (DT)- A highly poisonous protein released by specific diphtheria bacteria that shuts down cell production, causing massive tissue damage.

Exotoxin- A toxic substance manufactured by a living bacterium and secreted out into the surrounding tissues or bloodstream to harm the host.

Pseudomembrane formation- The growth of a thick, greyish-white layer of dead tissue, bacteria, and immune cells over the throat or airway, which can cause choking.

Pharynx- The medical term for the throat, which acts as a highway connecting the mouth and nose to the esophagus and windpipe.

Myocarditis- A dangerous inflammation of the heart muscle that can weaken the organ and severely disrupt its normal rhythm.

Polyneuropathy- Widespread damage affecting multiple nerves across the body, often leading to muscle weakness, numbness, or tingling.

Renal tubular necrosis- Severe damage to the tiny filtering tubes within the kidneys, which can cause sudden kidney failure.

Transmission

Diphtheria is transmitted primarily via respiratory droplets from infected individuals or asymptomatic carriers. Close, prolonged contact such as household exposure significantly elevates infection risk. Fomite transmission has also been documented, though less common. Cutaneous diphtheria, characterised by infected skin ulcers, can transmit through direct contact with wound secretions and is endemic in tropical and resource-limited settings. The incubation period typically ranges from 2 to 5 days (range: 1–10 days). Importantly, individuals may carry and transmit C. diphtheriae without exhibiting clinical disease, sustaining community transmission.

Asymptomatic carriers- Healthy individuals who harbor and can spread a dangerous bacterium without showing any symptoms or feeling sick themselves.

Cutaneous diphtheria- A type of diphtheria infection that targets the skin, usually causing painful, slow-healing open sores or ulcers.

Incubation period- The window of time between initially catching a germ and when the very first symptoms of the disease start showing up.

Community transmission- The widespread, unchecked sharing of an infectious disease among people within a local area or neighborhood.

Signs and Symptoms

Clinical diphtheria typically manifests as pharyngeal/tonsillar diphtheria, the most common form, characterised by: Low-grade fever, sore throat, and malaise in the prodromal phase (1–2 days), Formation of a tough, greyish-white pseudomembrane over the tonsils/pharynx that bleeds on removal, 'Bull neck' appearance due to cervical lymphadenopathy and soft-tissue oedema, Hoarseness, stridor, and potential airway obstruction in laryngeal involvement (croup), Myocarditis (in ~10–25% of cases), presenting as arrhythmias or heart block, typically 1–2 weeks after onset, Neuropathy (palatal palsy, oculomotor palsy, peripheral motor neuropathy) weeks after illness onset. Cutaneous diphtheria presents as chronic, non-healing ulcers with a membranous base.

Pharyngeal/tonsillar diphtheria: A traditional diphtheria infection concentrated in the throat and tonsils, marked by a severe sore throat and difficulty breathing.

Lymphadenopathy: Swollen or inflamed lymph nodes, which frequently enlarge into tender lumps as the immune system tries to fight off an invasion.

Soft-tissue oedema: An abnormal build-up of fluid in the body's soft tissues, leading to noticeable swelling and puffiness (such as a swollen "bull neck").

Neuropathy: A general medical term for nerve damage or dysfunction, which often causes pain, weakness, or numbness.

Diagnosis

Diagnosis is primarily clinical but must be confirmed microbiologically. Throat and nasopharyngeal swabs should be obtained before antibiotic initiation and cultured on selective media, Löffler's serum slope, and tellurite medium (e.g., Hoyle's medium), where C. diphtheriae produces characteristic grey-black colonies. Gram staining reveals gram-positive 'Chinese letter' or palisade arrangements. Toxigenicity must be confirmed using the Elek immunodiffusion test (gold standard) or PCR for the tox gene. The WHO recommends immediate treatment upon clinical suspicion without awaiting laboratory confirmation, given the rapid progression and high mortality.

Selective media: A specialized laboratory gel or broth designed to grow only a specific type of microbe while stopping other unwanted bacteria from sprouting.

Toxigenicity: The specific capacity or strength of a bacterial strain to produce harmful toxins that damage the host.

Immunodiffusion test: A laboratory technique (such as the Elek test) used to see if a bacterium is actively producing a specific toxin by watching how it reacts with antibodies on a gel plate.

PCR: A highly sensitive lab method that acts like a molecular copy machine, making millions of copies of a germ's DNA so it can be quickly identified.

Treatment

Treatment involves two parallel strategies: toxin neutralisation and bacterial eradication. Diphtheria Antitoxin (DAT), derived from equine serum, neutralises circulating (unbound) toxins and is the cornerstone of treatment; it must be administered promptly, as it cannot reverse toxins already bound to tissues. Dosing ranges from 20,000 to 100,000 IU intramuscularly or intravenously depending on disease severity, after testing for hypersensitivity. Antibiotics erythromycin (40–50 mg/kg/day orally for 14 days) or penicillin G eliminate the organism, terminate toxin production, and prevent carriage; penicillin is the preferred agent per WHO guidelines. Patients should be isolated, and supportive care, airway management, cardiac monitoring, and nutritional support are critical in severe cases. Contacts require prophylaxis with antibiotics and booster vaccination regardless of prior immunisation status.

Toxin neutralisation: A protective medical or immune response where specific antibodies bind tightly to a toxin, permanently blocking its ability to harm human cells.

Erythromycin: A common antibiotic frequently used to clear bacterial infections and stop bacteria from manufacturing harmful proteins.

Penicillin: The classic, widely used antibiotic family that destroys bacteria by breaking down and rupturing their protective cell walls.

Prophylaxis: Any preventative medical treatment, medication, or action taken specifically to stop a disease from taking root or spreading.

Prevention

Vaccination is the most effective preventive measure. Diphtheria toxoid, formulated with tetanus toxoid and pertussis vaccine (DTP/DTaP/Tdap), is recommended by the WHO as part of the Expanded Programme on Immunisation. The primary series (3 doses) should begin at 6 weeks of age with booster doses at 18 months, 4–6 years, and in adolescence. Maintenance of >95% population coverage is needed to sustain herd immunity. In resource-limited settings or outbreak situations, mass immunisation campaigns are deployed. Surveillance systems for early case detection, strict isolation of confirmed cases, and contact tracing with chemoprophylaxis are essential public health tools. The WHO and CDC continue to report resurgences in regions with declining vaccination coverage, a stark reminder that diphtheria, though vaccine-preventable, remains an active threat wherever immunisation systems falter.

Tetanus toxoid- A safe, inactivated version of the tetanus toxin used in routine vaccines to safely train the immune system to defend against lockjaw.

Pertussis vaccine- A routine immunization designed to protect individuals against whooping cough, a highly contagious respiratory infection.

Herd immunity- A state where a large portion of a community becomes immune to a disease (usually through vaccination), making it incredibly hard for the germ to spread and protecting those who cannot be vaccinated.

Microbial Profile

Gram nature: Gram-positive
Spore Formation: Non-spore forming
Motility: Non-motile
Shape: Rod-shaped
Oxygen Requirement: Aerobic
Optimal Temperature: 37℃
Optimal pH: 7.2–8.0

Taxonomic Classification

Domain: Bacteria

Kingdom: Bacillati

Phylum: Actinomycetota

Class: Actinomycetes

Order: Mycobacteriales

Family: Corynebacteriaceae

Genus: Corynebacterium

Species: Corynebacterium diphtheriae

-Varsha V

Reference

Holmes R. K. (2000). Biology and molecular epidemiology of diphtheria toxin and the tox gene. The Journal of infectious diseases, 181 Suppl 1, S156–S167. https://doi.org/10.1086/315554

Burkovski, A. (2015). Pathogenesis of Corynebacterium diphtheriae and Corynebacterium ulcerans . In Human Emerging and Re-emerging Infections, S.K. Singh (Ed.). https://doi.org/10.1002/9781118644843.ch38

Jamal, S. B., Tiwari, S., Silva, A., & Azevedo, V. (2017). Pathogenesis of Corynebacterium diphtheriae and available vaccines; an overview. Glob. J. Infect. Dis. Clin. Res, 3, 20-24.

Valenzuela-Cardenas, M. B., & Goodfellow, S. M. (2025). Microbe Snapshot: Corynebacterium diphtheriae. Clinical Microbiology Newsletter.

Ningsih, I., & Safrullah, M. I. (2025). Pathogenesis, Diagnosis, and Examination of Corynebacterium diphtheriae. Indonesian Journal of Medical Chemistry and Bioinformatics, 3(2), 1.

Murphy JR. Corynebacterium Diphtheriae. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 32. Available from: https://www.ncbi.nlm.nih.gov/books/NBK7971/

Hoskisson, P. A. (2018). Microbe Profile: Corynebacterium diphtheriae–an old foe always ready to seize opportunity. Microbiology, 164(6), 865-867.

PATHOGEN

DIPHTHERIA

CORYNEBACTERIUM DIPHTHERIA

Frequently Asked Questions

Where does the name "diphtheria" come from?

It comes from the Greek word for leather, which describes the tough, leathery grey-white membrane that forms in the patient's throat.

Which major organs can be severely damaged by the systemic spread of the diphtheria toxin?

The toxin primarily targets and damages the heart (causing myocarditis), the nervous system (causing neuropathy), and the kidneys.

Can Diphtheria Antitoxin (DAT) reverse tissue damage that has already occurred?

No, the antitoxin can only neutralize free-floating, unbound toxins in the body, which makes early administration absolutely critical.

Should medical professionals wait for laboratory confirmation before treating a suspected case?

No, the WHO recommends starting treatment immediately upon clinical suspicion due to the infection's rapid progression and high mortality rate.

What population vaccination rate is required to successfully maintain herd immunity against diphtheria?

Communities must maintain a vaccination coverage rate of greater than 95% to sustain effective herd immunity and prevent resurgences.

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