Tetanus

Tetanus is a life-threatening neurological disorder characterized by persistent muscle contractions and spastic paralysis. It is caused by Clostridium tetani, a motile, Gram-positive, rod-shaped, obligate anaerobe. These bacteria produce terminal endospores, giving them a distinctive “lollipop” or “tennis-racket” appearance. They thrive in anaerobic environments, such as those found in deep puncture wounds.

Once introduced into the body, the spores germinate into vegetative cells. These cells secrete two exotoxins: tetanolysin and tetanospasmin. Tetanolysin is a hemolysin that promotes local tissue damage and helps create an anaerobic environment for bacterial growth, although its exact role in disease progression remains unclear. The second toxin, tetanospasmin—also known as tetanus toxin—is primarily responsible for the neurological symptoms of the disease.

Tetanospasmin is a 150-kilodalton A-B type neurotoxin, composed of a heavy (B) and a light (A) chain connected by a disulfide bond. The heavy chain binds to neuronal membrane components, including gangliosides such as GD1b and GT1b, and GPI-anchored proteins located within lipid rafts. After binding, the toxin is internalized at the neuromuscular junction and transported retrogradely along axons via dynein motors to the central nervous system.

Acidification activates the heavy chain within endosomes to translocate the light chain into the cytosol. Reduction of the disulfide bond releases the light chain, a zinc-dependent metalloprotease. This enzyme cleaves synaptobrevin—also known as VAMP—a SNARE protein essential for synaptic vesicle fusion.

This disruption prevents the release of inhibitory neurotransmitters GABA and glycine from Renshaw interneurons. Without this inhibition, motor neurons become hyperactive, leading to continuous, uncontrolled muscle contractions. Significantly, tetanospasmin affects only motor pathways; sensory function remains intact.

Clinically, this unopposed motor activity manifests as muscle stiffness and spasms, including lockjaw (trismus), facial grimacing (risus sardonicus), generalized rigidity, and, in severe cases, life-threatening spasms of the respiratory muscles.

Tetanus highlights how a single bacterial toxin can profoundly disrupt neural function. Fortunately, the disease is preventable through routine immunization with the tetanus toxoid vaccine, underscoring the importance of maintaining up-to-date booster shots.