Clostridium tetani

Clostridium tetani is a gram-positive, spore-forming, obligate anaerobic bacillus found in soil and animal feces. Its terminal spore gives the organism a distinctive tennis racket or drumstick appearance under the microscope. Spores enter the body through puncture wounds and other deep, devitalized tissue where oxygen is scarce — the classic rusty nail scenario — then germinate and produce toxin.

Disease is driven entirely by the exotoxin tetanospasmin, one of the most potent toxins known. The toxin travels by retrograde axonal transport to the central nervous system, where it cleaves SNARE proteins and blocks release of the inhibitory neurotransmitters GABA and glycine from Renshaw cells in the spinal cord. Without inhibition, motor neurons fire unopposed, causing spastic paralysis: lockjaw (trismus), the fixed grin of risus sardonicus, arched-back opisthotonus, and painful generalized spasms.

The mechanism contrasts sharply with Clostridium botulinum, whose toxin also cleaves SNARE proteins but instead blocks acetylcholine release at the neuromuscular junction, producing flaccid paralysis. Tetanus is prevented by the tetanus toxoid vaccine — a formaldehyde-inactivated toxin that induces protective antitoxin antibodies — with boosters maintaining immunity; unvaccinated wound patients receive passive immunization with tetanus immune globulin.

Clostridium tetani is high-yield microbiology on the USMLE Step 1 exam. Test questions focus on the toxin's mechanism — SNARE cleavage blocking GABA and glycine release — the spastic-versus-flaccid contrast with botulism, the organism's morphology and anaerobic physiology, and toxoid-based prevention.