C. tetani, an obligate anaerobic Gram-positive bacillus, causestetanus. Tetanus is an infectious disorder characterized by an increased muscle tone and spasms caused by the release of a neurotoxin, tetanospasmin, produced by C. tetani when it gets inoculated into humans.
C. tetani shows following features:
· C. tetani is a slender, Gram-positive bacillus measuring4–8 m in length. Young cultures are usually Gram-positive but old cultures are Gram- variable and even Gram-negative.
· It is a straight bacillus with parallel sides and rounded ends. The bacillus occurs in singles and occasionally in chains. The bacteria consist of round, terminal, and bulging spores giving drumstick appearance to the bacillus. The spores are rarely seen even in clinical specimens from lesions or in culture media.
· All strains of C. tetani except type VI are motile by the presence of flagella. Type VI strain bacilli do not contain any flagella, hence are non-motile.
· The bacterium is capsulated.
C. tetani is an obligate anaerobe. The bacteria are extremely sen-sitive to oxygen, hence can grow only in the absence of oxygen. The bacillus grows at an optimum temperature of 37°C and at pH of 7.4. C. tetani can grow on ordinary media and on media enriched with serum and blood.
a) RCM medium:C. tetanigrows well on RCM medium(Fig. 29-2). The bacteria produce turbidity with production of some gas in the medium. The meat, although is not digested, turns black on prolonged incubation.
b) Blood agar: On blood agar,C. tetaniproduces alpha-hemolysissurrounding the colonies. On prolonged incubation, the alpha-hemolysis becomes beta-hemolytic due to production of tetano-lysin, a hemolysin produced by the bacteria. Surface colonies tend to swarm over the entire surface of the agar. C. tetani produces an extremely fine translucent film of growth, which is difficult to visualize except at the edges of the colonies (Color Photo 29).
c) Gelatin stab culture:C. tetaniliquefies gelatin anaerobi-cally. Hence, the bacillus produces a fir-tree type of growth in gelatin stab culture under anaerobic incubation.
d) Nutrient agar slope: Inoculation of the bacteria into waterof condensation at bottom of the slope of a nutrient agar, fol-lowed by anaerobic incubation for 24 hours yields a pure colony of C. tetani at top of the slope in the tube. This method is known as Fildes technique and is employed as a routine method for isolating pure colonies of C. tetani.
tetani shows following reactions:
· C. tetani has mild proteolytic activity but completely lackssaccharolytic activity.
· It does not ferment any sugars. It does not produce H2S and does not reduce nitrates.
· It is indole positive, but MR and VP negative.
· It produces a greenish fluorescence on MacConkey medium, which contains neutral red.
Susceptibility to the physical and chemical agents: C. tetanispores of different strains show a variable heat resistance. Most spores are killed by boiling at 100°C for 10–15 minutes and by autoclaving at 121°C for 20 minutes. They are also killed by 1% aqueous solution of iodine and 10% of hydrogen peroxide. The spores are resistant to most antiseptics. They are not killed by 5% phenol or 0.1% mercuric chloride solution. They can survive in soil for years.
Typing: C. tetaniare classified into 10 serological types (typesI to X) based on agglutination. All strains produce the same toxin. The toxin is neutralized by the standard antitoxin.