Capsule The capsule is a gelatinous layer, typically composed of polysaccharide, that covers the entire bacterium. 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 approximately 95 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. 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.
(3) Capsular polysaccharides are used as antigens in certain vaccines because they are capable of eliciting protective anti bodies. For example, the capsular polysaccharide of S. pneumoniae, Neisseria meningitidis, and Haemophilus influenzae is the immunogen in the current vaccine against these bacteria.
(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
Flagella are long, whip-like 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.
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. Many rods have flagella, but most cocci do not and are therefore nonmotile. Spirochetes move 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 anti bodies against flagellar proteins.
Pili (Fimbriae)
Pili are hairlike filaments that extend from the cell surface. They are shorter and straighter than flagella and are composed of subunits of pilin. They are found mainly on gram-negative organisms.
Pili have two important roles:
(1) They mediate the attachment of bacteria to specific receptors on the human cell surface, a necessary step in the initiation of infection for some organisms.
(2) A specialized kind of pilus, the sex pilus, forms the attachment between the donor and the recipient bacteria during conjugation.
Glycocalyx (Slime Layer)
The glycocalyx is a polysaccharide secreted by many bacteria that 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. Its medical importance is illustrated by the finding that glycocalyx-producing strains of P. aeruginosa cause respiratory tract infections in cystic fibrosis patients. Glycocalyx-producing strains of Staphylococcus epidermidis and viridans streptococci cause endo carditis. The glycocalyx also mediates adherence of certain bacteria to the surface of teeth. This plays an important role in the formation of plaque.
