Clostridial toxins compared: tetanus, botulinum, alpha, C. difficile
Tetanus and botulinum toxins cut the same SNARE proteins but give opposite pictures. Compare the clostridial toxins by their target and their clinical picture.
Compare the mechanisms of tetanus, botulinum, C. perfringens alpha and C. difficile toxins and link each to its clinical picture.
The short version
- Tetanus and botulinum toxins are both zinc-dependent proteases that cut SNARE proteins, so the transmitter is not released.
- Botulinum toxin acts at the neuromuscular junction: no acetylcholine, so flaccid paralysis.
- Tetanus toxin travels backwards to the spinal cord and brainstem and silences inhibitory interneurons (GABA and glycine): spastic paralysis.
- C. perfringens alpha toxin is a phospholipase C (lecithinase) that bursts cells and causes gas gangrene.
- C. difficile toxins A and B glucosylate Rho proteins and cause pseudomembranous colitis.
Two patients, one genus: the shared SNARE machinery
Picture two patients infected by the same genus of bacteria. One has a locked jaw and a back that arches off the bed. The other is an infant, floppy, with a weak cry and drooping eyelids.
The two pictures are opposite, yet the two toxins behind them cut the same kind of protein. With the clostridial toxins compared side by side, the answer lies in where each toxin acts.[3][4]
Clostridia are gram-positive, spore-forming anaerobes, and their spores survive in soil.[1][2][4]
A nerve ending releases its transmitter when a vesicle fuses with the membrane. Fusion needs the SNARE proteins: synaptobrevin, syntaxin and SNAP-25.[3][4]
Tetanus toxin and botulinum toxin are both zinc-dependent proteases that cut SNARE proteins, so the transmitter is not released.[1][3][4]
Botulinum toxin: the neuromuscular junction and flaccid paralysis
Botulinum toxin acts at the periphery. It enters cholinergic nerve endings, chiefly the motor nerve ending at the neuromuscular junction, and cuts the SNAREs there, so acetylcholine is not released. No acetylcholine means flaccid paralysis; the same block at autonomic nerve endings helps explain features such as constipation.[2][3][4]
The picture begins with cranial nerve palsies and descends, typically with no fever, a clear mind and no sensory loss.[2][7]
In infant botulism, swallowed spores colonise the gut and make toxin. The baby is constipated, feeds poorly and goes floppy.[2][7]
Tetanus toxin: retrograde transport and the lost brakes
Tetanus toxin starts at a wound, where spores germinate. It enters motor nerve endings and travels backwards along the axon to the spinal cord and brainstem.[1][4][6]
There it crosses the synapse into the inhibitory interneurons that normally restrain the motor neurons, and cuts synaptobrevin. Those cells stop releasing GABA and glycine.[4][5][6]
Remove the brakes, and motor neurons fire unchecked: spastic paralysis.
The clinical picture is a locked jaw (trismus), a fixed grin (risus sardonicus) and an arched back (opisthotonos), with spasms set off by stimuli.[1][4]
Flaccid versus spastic paralysis: botulinum and tetanus compared
| Feature | Botulinum toxin | Tetanus toxin |
|---|---|---|
| Shared mechanism | Zinc-dependent protease cutting SNAREs | Zinc-dependent protease cutting SNAREs |
| Where it acts | Motor nerve ending at the neuromuscular junction | Inhibitory interneurons in the spinal cord and brainstem |
| Transmitter not released | Acetylcholine | GABA and glycine |
| Paralysis | Flaccid | Spastic |
| Picture | Cranial nerve palsies, descending; no fever, clear mind, no sensory loss | Trismus, risus sardonicus, opisthotonos; spasms set off by stimuli |
Botulinum blocks the signal at the muscle, giving flaccid paralysis. Tetanus blocks the brakes in the cord, giving spastic paralysis.[3][4][5]
Alpha toxin: lecithinase and gas gangrene
The other two clostridial toxins leave the SNAREs alone. Clostridium perfringens makes alpha toxin, a phospholipase C, also called lecithinase. It splits lecithin in cell membranes, so cells burst.[8]
In a dirty wound, that is gas gangrene: dying muscle, with gas in the tissue that crackles under the fingers.[8]
C. difficile toxins A and B: Rho glucosylation and pseudomembranous colitis
Clostridioides difficile, formerly Clostridium difficile, makes toxins A and B. Toxin A is the enterotoxin; toxin B is the cytotoxin.[9]
Both glucosylate Rho proteins, which collapses the actin skeleton of the cells lining the gut. The result is pseudomembranous colitis, typically after antibiotics have cleared the normal gut flora.[4][9]
| Toxin | Mechanism | Clinical picture |
|---|---|---|
| Botulinum | Cuts SNAREs at the neuromuscular junction | Flaccid paralysis |
| Tetanus | Cuts synaptobrevin in inhibitory interneurons | Spastic paralysis, trismus |
| C. perfringens alpha | Phospholipase C (lecithinase) splits lecithin | Gas gangrene |
| C. difficile A and B | Glucosylate Rho proteins | Pseudomembranous colitis |
How to tell the clostridial toxins apart
When a clinical picture is described, work from the picture back to the toxin. Start with the muscles, then ask where the toxin could have acted.[3][4]
- Is the paralysis flaccid or spastic?Flaccid paralysis points to botulinum toxin, where acetylcholine is not released at the neuromuscular junction. Spastic paralysis points to tetanus toxin, where the brakes in the cord are lost.
- Look for the supporting featuresBotulism descends from cranial nerve palsies, with typically no fever, a clear mind and no sensory loss. Tetanus gives trismus, risus sardonicus and opisthotonos, with spasms set off by stimuli.
- Is the problem local tissue or gut?Dying muscle with crackling gas in the tissue of a dirty wound is gas gangrene from alpha toxin. Colitis after antibiotics have cleared the normal gut flora is C. difficile toxins A and B.
Neither alpha toxin nor the C. difficile toxins touch the SNAREs, so a mechanism that involves SNARE cleavage narrows the answer to botulinum or tetanus toxin.[4][8][9]
Practice question
Original · USMLE styleA 34-year-old man steps on a rusty nail while gardening. Ten days later he has a stiff jaw that opens only a finger's width and painful spasms of the back. His temperature is 37.2 °C. He is alert and has never been vaccinated against tetanus.
Common questions
How do tetanus and botulinum toxins differ if both cut SNARE proteins?
They act in different cells. Botulinum toxin acts at the motor nerve ending and blocks acetylcholine release, giving flaccid paralysis. Tetanus toxin reaches inhibitory interneurons in the cord and blocks GABA and glycine release, giving spastic paralysis.
What is the mechanism of C. perfringens alpha toxin?
Alpha toxin is a phospholipase C, also called lecithinase. It splits lecithin in cell membranes, so cells burst, and in a dirty wound this produces gas gangrene.
What do C. difficile toxins A and B do?
Toxin A is the enterotoxin and toxin B is the cytotoxin. Both glucosylate Rho proteins, which collapses the actin skeleton of gut lining cells and causes pseudomembranous colitis.
What are trismus, risus sardonicus and opisthotonos?
They are the signs of tetanus: a locked jaw (trismus), a fixed grin (risus sardonicus) and an arched back (opisthotonos), with spasms set off by stimuli.
How does infant botulism develop?
Swallowed spores colonise the gut and make toxin. The baby is constipated, feeds poorly and goes floppy.
Transcript
Read the full narration, by chapter666 words
0:00The case: a rigid patient and a floppy infant
Two patients, and the same genus of bacteria. One has a locked jaw and a back that arches off the bed. The other is an infant, floppy, with a weak cry and drooping eyelids. Opposite pictures, yet both toxins cut the same kind of protein. How? Today: compare the clostridial toxins by their target and their clinical picture.
0:23Clostridia and the shared machinery: SNARE proteins and zinc proteases
Clostridia are gram-positive, spore-forming anaerobes, and their spores survive in soil. Start with the shared machinery. A nerve ending releases its transmitter when a vesicle fuses with the membrane, and fusion needs the SNARE proteins: synaptobrevin, syntaxin and SNAP 25. Tetanus and botulinum toxins are both zinc-dependent proteases that cut SNARE proteins, so the transmitter is not released. What differs is where they act.
0:54Botulinum toxin: the neuromuscular junction, flaccid paralysis, infant botulism
Botulinum toxin acts at the periphery. It enters the motor nerve ending at the neuromuscular junction and cuts the SNAREs there, so acetylcholine is not released. No acetylcholine means flaccid paralysis. It begins with cranial nerve palsies and descends, typically with no fever, a clear mind and no sensory loss. In infant botulism, swallowed spores colonise the gut and make toxin; the baby is constipated, feeds poorly and goes floppy.
1:27Tetanus toxin: retrograde transport and the lost brakes
Tetanus toxin starts at a wound, where spores germinate. It enters motor nerve endings and travels backwards along the axon to the spinal cord and brainstem. There it crosses the synapse into the inhibitory interneurons that normally restrain the motor neurons, and cuts synaptobrevin, so those cells stop releasing GABA and glycine. Remove the brakes, and motor neurons fire unchecked: spastic paralysis.
1:55The pictures side by side: trismus, risus sardonicus, opisthotonos
The picture is a locked jaw, called trismus, a fixed grin, risus sardonicus, and an arched back, opisthotonos, with spasms set off by stimuli. So botulinum blocks the signal at the muscle, giving flaccid paralysis; tetanus blocks the brakes in the cord, giving spastic paralysis.
2:16Alpha toxin: lecithinase and gas gangrene
The other two clostridial toxins leave the SNAREs alone. Clostridium perfringens makes alpha toxin, a phospholipase C, also called lecithinase. It splits lecithin in cell membranes, so cells burst. In a dirty wound, that is gas gangrene: dying muscle, with gas in the tissue that crackles under the fingers.
2:39C. difficile toxins A and B: Rho and pseudomembranous colitis
Clostridioides difficile, formerly Clostridium difficile, makes toxins A and B. Toxin A is the enterotoxin; toxin B is the cytotoxin. Both glucosylate Rho proteins, which collapses the actin skeleton of the cells lining the gut. The result is pseudomembranous colitis, typically after antibiotics have cleared the normal gut flora. Your turn.
3:05Your turn: a rusty nail and a stiff jaw
A 34-year-old man steps on a rusty nail while gardening. Ten days later he has a stiff jaw that opens only a finger's width, painful spasms of the back, and a temperature of 37.2 degrees Celsius. He is alert and has never been vaccinated against tetanus. Which of the following best explains his muscle findings? A, cleavage of SNARE proteins in motor neuron endings, blocking release of acetylcholine. B, cleavage of SNARE proteins in inhibitory interneurons, blocking glycine and GABA release. C, hydrolysis of lecithin in muscle cell membranes by a bacterial phospholipase. D, glucosylation of Rho proteins, collapsing the actin skeleton of muscle cells. E, ADP-ribosylation of a G protein, raising cyclic AMP in muscle cells. Pause here and decide. A is out: that is botulinum toxin, and it causes flaccid paralysis, but this man is rigid. C is out: that is alpha toxin, which kills tissue locally, the gas gangrene picture. D is out: that is the C. difficile toxin, acting on the gut lining. E is out: that is the cholera-type second messenger mechanism, not a clostridial neurotoxin.
4:34The answer: B, and why the others fail
So the answer is B. A contaminated wound, a locked jaw and spasms with a clear mind point to tetanus toxin, which silences the inhibitory interneurons. Three things to keep.
4:47High-yield recap and the next lesson: Staphylococcus vs Streptococcus
One: botulinum and tetanus toxins both cut SNAREs; botulinum at the neuromuscular junction gives flaccid paralysis, tetanus in inhibitory interneurons gives spastic paralysis. Two: alpha toxin is a lecithinase, and it makes gas gangrene. Three: C. difficile toxins A and B glucosylate Rho proteins and cause pseudomembranous colitis. Next, identifying staphylococci and streptococci in the lab.
Sources
Numbers in the text, e.g. [1], point here- LITFL. Tetanus. litfl.com/tetanus/
- LITFL. Botulism. litfl.com/botulism/
- Botulinum Neurotoxins: History, Mechanism, and Applications. Europe PMC PMC12323300. europepmc.org/article/PMC/PMC12323300
- Bacterial neurotoxins and large clostridial toxins. Europe PMC PMC13450570. europepmc.org/article/PMC/PMC13450570
- Transsynaptic migration of tetanus toxin to inhibitory interneurons. Europe PMC PMC3153206. europepmc.org/article/PMC/PMC3153206
- Tetanus review (Lancet Infectious Diseases 2025). Europe PMC PMC7617864. europepmc.org/article/PMC/PMC7617864
- Botulism case and guideline sources: CDC 2021 botulism guidelines (PMC8112830) and infant botulism (PMC8703831). europepmc.org/article/PMC/PMC8112830 · europepmc.org/article/PMC/PMC8703831
- C. perfringens gas gangrene case (PMC6592470); phospholipase C and lecithinase (PMC10240202, EMBO Rep); clostridial myonecrosis case report (PMC12414490). europepmc.org/article/PMC/PMC6592470 · europepmc.org/article/PMC/PMC10240202 · europepmc.org/article/PMC/PMC12414490
- C. difficile toxins A and B, pseudomembranous colitis (PMC12291952). europepmc.org/article/PMC/PMC12291952
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, cropped, outlined, labelled): neuromuscular synapse, baby, muscle, colon. Spinal cord, anterior horn cells, motor nerve and muscle: DBCLS (Database Center for Life Science) via Bioicons, CC BY 4.0, adapted. Bacillus rods: NIAID NIH BioArt, public domain. All other drawings (SNARE diagrams, scissors, faces, lipid bilayer, enterocyte lattice, tables) are original code. No AI-generated teaching images.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-MICRO-013.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
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- EP 03Endotoxin vs exotoxin: lipid A, A-B toxins and toxoids5:46
- EP 04Bacterial toxins that raise cAMP: cholera, pertussis, anthrax5:54
- EP 05Clostridial toxins compared: tetanus, botulinum, alpha, C. difficile5:22
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- EP 07Encapsulated organisms and the spleen: asplenia and conjugate vaccines5:08
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