Capsule, Flagella, Pili (Fimbriae), Glycocalyx (Slime Layer)

Capsule, Flagella, Pili (Fimbriae), Glycocalyx (Slime Layer)

Capsule, Flagella, Pili (Fimbriae), Glycocalyx (Slime Layer)


The capsule is a gelatinous layer covering the entire bacterium. It is composed of polysaccharide, except in the
anthrax bacillus, which has a capsule of polymerized d-glutamic acid. The sugar components of the polysaccharide
vary from one species of bacteria to another and frequently determine the serologic type (serotype) within a species.

  • For example, there are 84 different serotypes of Streptococcus pneumoniae, which are distinguished by the antigenic differences of the sugars in the polysaccharide capsule.

The capsule is important for four reasons:

(1) It is a determinant of virulence of many bacteria since it limits the ability of phagocytes to engulf the bacteria. Negative charges on the capsular polysaccharide repel the negatively charged cell membrane of the neutrophil and prevent it from ingesting the bacteria. Variants of encapsulated bacteria that have lost the ability to produce a capsule are usually nonpathogenic.

(2) Specific identification of an organism can be made by using antiserum against the capsular polysaccharide. In the presence of the homologous antibody, the capsule will swell greatly. This swelling phenomenon, which is used in the clinical laboratory to identify certain organisms, is called the quellung reaction.

(3) Capsular polysaccharides are used as antigens in certain vaccines because they are capable of eliciting protective antibodies. For example, the purified capsular polysaccharides of 23 types of S. pneumoniae are present in the current vaccine.

(4) The capsule may play a role in the adherence of bacteria to human tissues, which is an important initial step in causing infection.


Flagella are long, whiplike appendages that move the bacteria toward nutrients and other attractants, a process called chemotaxis. The long filament, which acts as a propeller, is composed of many subunits of a single protein, flagellin, arranged in several intertwined chains. The energy for movement, the proton motive force, is provided by adenosine triphosphate (ATP), derived from the passage of ions across the membrane.

  • Flagellated bacteria have a characteristic number and location of flagella: some bacteria have one, and others have many; in some, the flagella are located at one end, and in others, they are all over the outer surface.
  • Only certain bacteria have flagella. Many rods do, but most cocci do not and are therefore non-motile.
  • Spirochetes move by using a flagellum like structure called the axial filament, which wraps around the spiral-shaped cell to produce an undulating motion.

Flagella are medically important for two reasons:

(1) Some species of motile bacteria (e.g., E. coli and Proteus species) are common causes of urinary tract infections. Flagella may play a role in pathogenesis by propelling the bacteria up the urethra into the bladder.

(2) Some species of bacteria (e.g., Salmonella species) are identified in the clinical laboratory by the use of specific antibodies against flagellar proteins.

Pili (Fimbriae)

Pili are hair-like filaments that extend from the cell surface.

  • They are shorter and straighter than flagella and are composed of subunits of pilin, a protein arranged in helical strands.
  • They are found mainly on gram-negative organisms.

Pili has two important roles:

(1) They mediate the attachment of bacteria to specific receptors on the human cell surface, which is a necessary
step in the initiation of infection for some organisms. Mutants of
Neisseria gonorrhoeae that do not form pili are

(2) A specialized kind of pilus, the sex pilus, forms the attachment between the male (donor) and the female (recipient) bacteria during conjugation.

Glycocalyx (Slime Layer)

The glycocalyx is a polysaccharide coating that is secreted by many bacteria.

  • It covers surfaces like a film and allows the bacteria to adhere firmly to various structures (e.g., skin, heart valves, prosthetic joints, and catheters).
  • The glycocalyx is an important component of biofilms. The medical importance of the glycocalyx is illustrated by the finding that it is the glycocalyx-producing strains of P. aeruginosa that cause respiratory tract infections in cystic fibrosis patients, and it is the glycocalyx-producing strains of Staphylococcus epidermidis and viridans streptococci that cause endocarditis.
  • The glycocalyx also mediates adherence of certain bacteria, such as Streptococcus mutans, to the surface of teeth.
  • This plays an important role in the formation of plaque, the precursor of dental caries.



Capsule, Flagella, Pili (Fimbriae), Glycocalyx (Slime Layer)


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