Clostridium tetani
Spore-forming obligate anaerobic Gram-positive rod; causative agent of tetanus. Disease is caused entirely by one exotoxin — tetanospasmin (TeNT), encoded on the plasmid pE88. TeNT is a zinc-dependent metalloprotease that undergoes retrograde axonal transport to inhibitory interneurons, where its light chain cleaves the Gln⁶³–Phe⁷⁶ peptide bond of VAMP-2 (synaptobrevin-2) on GABA- and glycine-containing synaptic vesicles. VAMP-2 cleavage prevents SNARE complex formation and neurotransmitter release, silencing inhibitory interneurons and allowing unchecked α-motor neuron firing → spastic paralysis, trismus (lockjaw), risus sardonicus, opisthotonos, and potentially fatal laryngospasm or respiratory failure. The bacterium remains localised to the wound — it does not invade or bacteraemically disseminate. Tetanus is 100% preventable by tetanus toxoid vaccination; every individual must maintain personal immunity (no herd immunity applies).
Classification & Structure
| Gram stain | Gram-positive; thick peptidoglycan; no outer membrane |
| Morphology | Slender rod, 0.5–1.7 µm × 2.1–18.1 µm; distinctive "drumstick" or "tennis racket" appearance due to spherical terminal endospores that swell the cell end |
| Motility | Peritrichous flagella; motile in early log phase; non-motile in sporulation; obligate anaerobe (redox potential <−50 mV required for germination and vegetative growth) |
| Toxins | Tetanospasmin (TeNT; 150 kDa precursor; processed to 100 kDa HC + 50 kDa LC by disulphide bond) — encoded on 74 kb plasmid pE88; tetanolysin (TetZ; haemolysin; local tissue necrosis; not essential for disease) |
| Spores | Terminal, spherical endospores; resistant to boiling (>1 h), desiccation, UV, and most disinfectants; killed by autoclaving (121°C, 15 min) or 2% glutaraldehyde; ubiquitous in soil, especially agricultural/horse-manured soil; survive years in environment |
Pathogenesis
1 · Wound germination — establishing the anaerobic niche
Spores enter puncture wounds, lacerations, crush injuries, burns, surgical wounds, or umbilical stumps (neonates). The critical requirement is local anaerobiosis — created by tissue necrosis, devitalised tissue, coexisting aerobic bacteria consuming oxygen, or foreign bodies (nails, splinters, soil). Spores germinate, vegetative bacilli multiply, and tetanospasmin is secreted as a single-chain 150 kDa precursor. Bacterial proteases (or trypsin-like enzymes) nick the precursor into the 100 kDa heavy chain (HC) and 50 kDa light chain (LC) linked by a disulphide bond. The organism does not disseminate; bacteraemia does not occur.
2 · Neuromuscular junction binding and endocytosis
The TeNT heavy chain C-terminal domain (HC-C) binds polysialogangliosides (GT1b, GD1b) on α-motor nerve terminals at the neuromuscular junction — dictating neurospecific tropism. Binding triggers endocytosis into acidic vesicular compartments within the motor axon terminal. Once inside endosomes, TeNT begins its journey toward the CNS.
3 · Retrograde axonal transport to inhibitory interneurons
TeNT-containing endosomes undergo retrograde axonal transport at ~10 mm/h toward the spinal cord and brainstem — a journey taking hours to days depending on the distance from the wound to the CNS. This explains why proximal wounds (face, neck) have shorter incubation periods and worse prognosis. At the CNS, TeNT undergoes transcytosis from motor axon terminals into the cytoplasm of inhibitory interneurons (Renshaw cells, GABAergic and glycinergic interneurons). The HC-N domain mediates endosomal pore formation at low pH, translocating the LC into the inhibitory interneuron cytoplasm.
4 · VAMP-2 cleavage — silencing inhibitory neurotransmission
The light chain (LC) is a zinc endopeptidase that cleaves the Gln⁶³–Phe⁷⁶ bond in VAMP-2 (synaptobrevin-2) — the v-SNARE on synaptic vesicles carrying GABA and glycine. Cleaved VAMP-2 cannot form SNARE complexes; vesicles cannot fuse with the presynaptic membrane → no GABA or glycine release. Unlike botulinum toxin (which blocks ACh at the NMJ → flaccid paralysis), TeNT acts centrally on inhibitory interneurons → spastic paralysis. Unchecked α-motor neuron firing produces trismus, opisthotonos, laryngospasm, and respiratory failure.
5 · Autonomic instability — sympathetic chain involvement
TeNT also impairs inhibitory interneurons in the sympathetic chain and medulla oblongata. In the second week of severe tetanus, profound autonomic instability emerges: sympathetic hyperactivity (hypertension, tachycardia, diaphoresis, hyperpyrexia, peripheral vasoconstriction) alternating with parasympathetic surges (bradycardia, hypotension, hypersalivation). Cardiac arrhythmias secondary to autonomic instability are a leading cause of ICU death, particularly in Grade IV tetanus.
6 · No dissemination — immune evasion by compartmentalisation
The bacterium remains in the wound and does not invade bloodstream or lymph nodes. The immune system mounts a local acute inflammatory response at the wound site but cannot reach already-fixed TeNT in neuronal tissue. Once bound and internalised by neurons, TeNT is inaccessible to circulating antitoxin antibodies — explaining why TIG (tetanus immunoglobulin) can only neutralise unbound toxin in blood/lymph, not toxin already fixed to neurons. This compartmentalised infection makes post-exposure prophylaxis critical (early debridement + TIG).
Host Immune Response
Disease Spectrum
| Presentation | Severity | At-Risk Groups | Mortality |
|---|---|---|---|
| Localised tetanus | Muscle rigidity near the wound only; mild; good prognosis; may progress to generalised | Partially immunised; distal extremity wounds | <1% |
| Cephalic tetanus | Head/face wounds; cranial nerve palsies (especially facial nerve VII); local trismus; can progress to generalised | Facial/ear/eye wounds; otitis media (rare entry route) | ~15–30% |
| Generalised tetanus (Ablett I–IV) | Descending: trismus → risus sardonicus → neck stiffness → dysphagia → opisthotonos → intercostal/abdominal rigidity → laryngospasm. Autonomic instability in Grade III–IV | Unvaccinated adults; injection drug users; wound-neglected patients; immigrants from low-coverage countries | 10–15% (ICU); >50% (no ICU) |
| Neonatal tetanus | Onset 3–14 days post-birth; contaminated umbilical stump; inability to suck (trismus) → generalised spasms; high mortality without intensive care | Neonates of unvaccinated mothers; home deliveries with non-sterile cord cutting | 70–80% (without ICU); 10–20% (with full ICU) |
Treatment & Prophylaxis
| Tetanus Immunoglobulin (TIG) | 3,000–6,000 U IM; neutralises unbound circulating toxin; does not reverse already-fixed TeNT in neurons; must be given as early as possible after diagnosis; human TIG preferred over equine (lower anaphylaxis risk). |
| Wound debridement | Removes the toxin source (vegetative C. tetani and spores); devitalised tissue must be excised; critical for stopping ongoing toxin production; do not close wound primarily. |
| Metronidazole | 500 mg IV/PO q6–8h × 7–10 days; kills vegetative C. tetani; superior to penicillin in clinical trials (penicillin is a GABA-A antagonist and may worsen spasms); IV route for patients with dysphagia/ileus. |
| Benzodiazepines (diazepam, midazolam) | GABA-A receptor agonists; first-line ICU sedation for spasm control; titrated to spasm frequency; high doses often required; IV infusion preferred in severe cases; diazepam suppositories used in resource-limited settings. |
| Magnesium sulphate | IV infusion; reduces sympathetic hyperactivity and spasm frequency; used as alternative or adjunct to benzodiazepines; widely used in LMICs (low cost, effective for autonomic instability). |
| Mechanical ventilation / tracheostomy | Laryngospasm and chest wall rigidity are common indications for early intubation; tracheostomy for prolonged ventilatory support (>10–14 days expected); neuromuscular blocking agents (vecuronium) for refractory spasms. |
| Cardiovascular management | Labetalol or esmolol for sympathetic hypertension/tachycardia; morphine for sympathetic crisis; atropine for bradycardia; epidural bupivacaine for severe autonomic instability (specialist centres). |
| Tetanus toxoid vaccine (prevention) | Formaldehyde-inactivated tetanospasmin; formulations: DTP, DTaP, Td, TT, Tdap. Primary series: 3 infant doses + 2 childhood boosters (4–6 y, 11–12 y). Duration: ≥10 years/dose; complete 5-dose series likely lifelong. Boosters every 10 years or post-significant wound. Maternal Tdap each pregnancy → placental IgG → neonatal protection. Wound prophylaxis algorithm: clean minor wound + ≥3 prior doses ≥5 years ago → no intervention; tetanus-prone wound + unknown/incomplete → TIG + booster. |
| Resistance | No antibiotic resistance relevant to clinical tetanus treatment; metronidazole uniformly effective; resistance to tetanus toxoid not reported (TeNT is invariant). |
Connections
References
- Brüggemann H, Bäumer S, Fricke WF, et al. The genome sequence of Clostridium tetani, the causative agent of tetanus disease. Proc Natl Acad Sci USA. 2003;100(3):1316-21. doi:10.1073/pnas.0335853100 · PubMed 12552129
- Montecucco C, Schiavo G. Structure and function of tetanus and botulinum neurotoxins. Q Rev Biophys. 1995;28(4):423-72. doi:10.1017/S0033583500003292 · PubMed 8771234
- Lalli G, Bohnert S, Deinhardt K, Verastegui C, Schiavo G. The journey of tetanus and botulinum neurotoxins in neurons. Trends Microbiol. 2003;11(9):431-7. doi:10.1016/S0966-842X(03)00210-5 · PubMed 12948668
- Rodrigo C, Fernando D, Rajapakse S. Pharmacological management of tetanus: an evidence-based review. Crit Care. 2014;18(2):217. doi:10.1186/cc13797 · PubMed 24661523
- World Health Organization. Tetanus vaccines: WHO position paper, February 2017. Wkly Epidemiol Rec. 2017;92(6):53-76. who.int/publications/i/item/who-wer9206
- Popoff MR. Tetanus in animals. J Vet Diagn Invest. 2020;32(2):184-191. doi:10.1177/1040638720906645 · PubMed 32089082
Contribute to the Pathogen Atlas
This entry covers C. tetani biology, TeNT mechanism, and clinical management. Planned expansions include structural biology of the TeNT-VAMP-2 complex, wound prophylaxis decision algorithms, and neonatal tetanus elimination programme data. Every entry follows the same schema: structured frontmatter, peer-reviewed citations, and cross-atlas links.