▶Capsule or Slime Layer
Structure: Many bacteria synthesize a large amount of extracellular polymer in their natural environments. When the polymer forms a condensed, well-defined layer closely surrounding the cell, it is called the capsule as in the pneumococcus. If the polymer is easily washed off and does not appear to be associated with the cell in any definite fashion, it is referred to as a slime layer as in Leuconostoc. The glycocalyx is a network of polysaccharide extending from the surface of bacteria and other cells.
▶Composition of capsules and slime layers
Capsules and slime layers usually are made up of polysaccharide (for example pneumococcus) or of the polypeptide in some bacteria (for example, Bacillus anthracis). Some bacteria may contain both a capsule and a slime layer (for example, Streptococcus salivarius). Bacteria secrete large amounts of slime produce mucoid growth on agar, which gives a stringy consistency when touched with the loop.
Streptococcus pneumoniae, several groups of streptococci, Neisseria meningitidis, Klebsiella, Haemophilus influenzae, Yersinia and Bacillus.
Slime has little affinity for basic dyes and is not visible in Gram stained smears. Special capsule staining techniques are available, usually employing copper salts as mordants. Capsules may be readily demonstrated by negative staining in wet films with India ink, when they are seen as clear halos around
the bacteria, against a black background
▶Demonstration of Capsule
i. Gram stain.
ii. Special capsule staining techniques.
iii. India ink staining (negative staining).
iv. Electron microscope.
v. Serological methods.
i. Gram stain: Capsules and slime are usually not visible in films stained by ordinary methods except as clear haloes surrounding the stained smear. Slime has little affinity for basic dyes and is not visible in Gram-stained smears.
ii. Special capsule staining techniques: Special capsule staining techniques are available, usually employing copper salts as mordants.
iii. India ink staining (negative staining): The most reliable method of demonstration is by ‘negative’ staining in wet films with India ink and the capsule appears as a clear halo around the bacterium, against a dark background in the film.
iv. Electron microscope: They can also be studied with an electron microscope.
v. Serological methods: Capsular material is antigenic and may be demonstrated by serological methods. Capsules may also be visualized by reaction with capsule-specific antibodies which causes a
characteristic swelling of the capsule. When a suspension of a capsulated bacterium is mixed with its
specific anticapsular serum and examined under the microscope, the capsule becomes very prominent and appears ‘swollen’ due to an increase in its refractivity. It is known as the capsule-swelling
reaction or Quellung reaction (Quellung—(Ger) swelling), described by Neufeld (1902) was widely employed for the typing of pneumococci in the presulphonamide days when lobar pneumonia used to
be treated with specific anticapsular sera.
▶Use of capsule-swelling reaction:
This phenomenon is seen in and allows rapid identification of capsular serotypes of Streptococcus pneumoniae, Neisseria meningitidis, several groups of streptococci, Klebsiella, Haemophilus
influenzae, Yersinia and Bacillus.
▶Functions of Capsule
i. Virulence factor: Capsules often act as a virulence factor by protecting the bacterium from ingestion
by phagocytosis, and noncapsulate mutant of these bacteria are nonvirulent. Repeated subcultures in
vitro lead to the loss of capsule and also of virulence.
ii. Protection of the cell wall: In protecting the cell wall attack by different kinds of antibacterial
agents, e.g. bacteriophages, colicines, complement, lysozyme and other lytic enzymes.
iii. Identification and typing of bacteria: Capsular antigen is specific for bacteria and can be used for identification and typing of bacteria