Morphological and Tinctorial Characteristics
Clostridium tetani is a large, straight bacillus with rounded ends. The microorganism is motile due to flagella distributed over the entire perimeter of the cell (peritrichous). In smears, the bacteria lie haphazardly or form chains. They stain Gram-positive (G+).
The hallmark of Clostridium tetani is its ability to form terminal spores whose diameter significantly exceeds the width of the vegetative cell, giving the bacterium its characteristic "drumstick" appearance. The spores are extremely resistant to environmental factors: they withstand low temperatures down to -60 °C, survive boiling for up to 6 hours, and are resistant to phenol and formalin. They can only be reliably eradicated by autoclaving at 130 °C for 20 minutes, whereas vegetative forms are killed by boiling within just 5 minutes.
Cultural and Biochemical Properties
The bacterium is an obligate anaerobe, meaning it grows and multiplies exclusively in oxygen-free conditions at a temperature of 37 °C.
On solid nutrient media (nutrient agar, gelatin), the pathogen can form two types of colonies:
- S-forms: smooth, transparent, lenticular.
- R-forms: rough, grayish-yellow, resembling "fluff balls" with a dense center in an agar column.
When inoculated on blood agar, growth is accompanied by a zone of hemolysis. In liquid Kitt-Tarozzi medium, the microorganism grows slowly, causing uniform turbidity and releasing a specific unpleasant odor.
The biochemical activity of C. tetani is extremely low. The bacterium is inert to most carbohydrates, and its proteolytic properties are limited to the slow breakdown of proteins with the formation of ammonia, indole, and volatile fatty acids. The pathogen produces gelatinase and a renin-like enzyme.
Pathogenicity Factors
The primary weapon of the pathogen is tetanus exotoxin. It is identical across all 10 serovars of the bacterium and is released upon lysis of the microbial cell. The toxin consists of two fractions:
- Tetanospasmin (neurotoxin): The main factor of pathogenesis. It binds to the neuromuscular junction and reaches the motor neurons of the spinal cord and brainstem via retrograde axonal transport. There, it blocks the release of inhibitory neurotransmitters, leading to uncontrolled clonic and tonic muscle contractions.
- Tetanolysin: Destroys erythrocytes and exerts a membrane-toxic effect on the tissues of the heart, lungs, liver, and kidneys, significantly worsening the patient's general condition.
Epidemiology and Clinical Presentation
Tetanus is a universally distributed sapronosis. The natural reservoir of infection is soil, where spores enter with human and animal feces (the pathogen is a normal inhabitant of the intestine). Transmission occurs via contact (wound route) when infected soil enters damaged tissues, creating anaerobic conditions. An infected person is not contagious to others.
The incubation period lasts from 1 day to several weeks (averaging 6–14 days). An important prognostic rule: the shorter the incubation period, the more severe the infection. The disease begins acutely with the classic triad:
- Trismus: Spasm of the masticatory muscles (the first symptom).
- "Sardonic smile" (risus sardonicus): Uncontrolled spasm of the facial mimetic muscles.
- Opisthotonos: Severe spasm of the back and limb muscles, causing the body to arch backward.
Diagnosis and Prophylaxis
Post-infection immunity is not established after tetanus because the lethal dose of the toxin is significantly below the threshold required to activate an immune response.
Diagnosis includes bacterioscopy (searching for "drumsticks") and bacteriological culture on Kitt-Tarozzi medium. The gold standard for detecting the exotoxin is the biological assay in white mice using the neutralization test (NT).
The foundation of protection against infection is routine immunization. It begins at 3 months of age with vaccines containing tetanus toxoid (DTaP, DT), followed by revaccination every 10 years. In case of injuries, emergency specific prophylaxis is administered: injection of toxoid combined with specific human immunoglobulin or heterologous horse serum (administered using Besredka's desensitization method).