The bacterium Clostridium tetani is the causative agent of tetanus. This bacteria is common in the soil and in animal manures. Typically, Clostridium tetani (also called C. tetani) causes human infection via a dirty, skin wound. This bacterial species has distinguishing characteristics that make it easily recognizable clinically and in the laboratory.
Gram Stain Positive
Bacteria are treated with dyes to examine their shapes and cell wall characteristics under the microscope; determining these features helps identify them. The dying procedure is called a Gram stain. C. tetani is Gram-positive, which means its cell wall takes up the purple Gram stain dye. A high concentration of a cell wall protein-sugar complex (called peptidoglycan) and the absence of an outer membrane surrounding the bacterium cause the uptake of the dye.
Racquet Shaped
C. tetani is rod-shaped with one bulbous end. The bulb represents a spore, a walled-off infectious particle within the bacterium. The terminal spore gives C. tetani a distinctive racquet shape.
Forms Spores
C. tetani makes spores--a dormant form of the bacteria, which is resistant to the elements. C. tetani spores are common in the environment. The bacterium remains in its spore form until it gains entry into an environment where it can become active and reproduce.
Indefinite Infectivity
The spores produced by C. tetani are so resistant to the elements that they are able to remain infective in the natural environment indefinitely.
Resistance to Disinfectants
C. tetani spores are resistant to many common disinfectants, specifically those where the primary sterilizing ingredient is alcohol, phenol or formalin. However, the spores can be killed by iodine, hydrogen peroxide and extreme heat.
Obligate Anaerobes
Bacteria are characterized by the type of gaseous environment in which they grow best. C. tetani is an obligate anaearobe, which means it cannot survive the oxygen concentration of ambient air. C. tetani thrives in a high carbon dioxide, low oxygen content environment. This characteristic of C. tetani helps explain why deep wounds are a favored location for C. tetani--because deep wounds protect the bacteria from the ambient air. It is important to note that the C. tetani spore survives without difficulty in an ambient air environment.
Toxin Production
The most distinguishing clinical characteristic of C. tetani is its production of a nervous system toxin called tetanospasmin, more commonly called tetanus toxin. This substance causes the painful muscle spasms and contractions of the disease tetanus.
Extracellular Enzyme Production
C. tetani produces and secretes an array of enzymes that break down substances in the environment, including proteins, fats and complex sugars. By breaking down these substances, the bacteria are able to convert material in the environment into chemicals they can use for energy.
Broad Fermentation Capacity
C. tetani is able to metabolize a wide variety of compounds for energy. The metabolic characteristics of C. tetani help identify it in the laboratory.
References
- "Principles and Practice of Infectious Diseases, Sixth Edition"; Gerald L. Mandell, M.D., et al; 2004
- Centers for Disease Control and Prevention, Pink Book: Tetanus
- Todar's Online Book of Bacteriology: Pathogenic Clostridia, Including Botulism and Tetanus
- Merck Manual: Tetanus
- Mayo Clinic: Tetanus
