Capsule Biology Function
Structure and Function of Bacterial Cells (page 4)
(This chapter has 10 pages)
© Kenneth Todar, PhD
The Cell Envelope: capsules, cell walls and cell membranes
The cell envelope is a descriptive term for the several layers of material that envelope or enclose the protoplasm of the cell. The cell protoplasm (cytoplasm) is surrounded by the plasma membrane, a cell wall and a capsule. The cell wall itself is a layered structure in Gram-negative bacteria. All cells have a membrane, which is the essential and definitive characteristic of a "cell". Almost all procaryotes have a cell wall to prevent damage to the underlying protoplast. Outside the cell wall, foremost as a surface structure, may be a polysaccharide capsule or glycocalyx.
Figure 9. Profiles of the cell envelope the Gram-positive and Gram-negative bacteria. The Gram-positive wall is a uniformly thick layer external to the plasma membrane. It is composed mainly of peptidoglycan (murein). The Gram-negative wall appears thin and multilayered. It consists of a relatively thin peptidoglycan sheet between the plasma membrane and a phospholipid-lipopolysaccharide outer membrane. The space between the inner (plasma) and outer membranes (wherein the peptidoglycan resides) is called the periplasm.Capsules
Most procaryotes contain some sort of a polysaccharide layer outside of the cell wall polymer. In a general sense, this layer is called a capsule. A true capsule is a discrete detectable layer of polysaccharides deposited outside the cell wall. A less discrete structure or matrix which embeds the cells is a called a slime layer or a biofilm. A type of capsule found in bacteria called a glycocalyx is a thin layer of tangled polysaccharide fibers which occurs on surface of cells growing in nature (as opposed to the laboratory). Some microbiologists refer to all capsules as glycocalyx and do not differentiate microcapsules.Figure 10. Bacterial capsules outlined by India ink viewed by light microscopy. This is a true capsule, a discrete layer of polysaccharide surrounding the cells. Sometimes bacterial cells are embedded more randomly in a polysaccharide matrix called a slime layer or biofilm. Polysaccharide films that may inevitably be present on the surfaces of bacterial cells, but which cannot be detected visually, are called glycocalyx.
Figure 11. Negative stain of Streptococcus pyogenes viewed by transmission electron microscopy (28, 000X). The halo around the chain of cells is the hyaluronic acid capsule that surrounds the exterior of the bacteria. The septa between dividing pairs of cells may also be seen. Electron micrograph of Streptococcus pyogenes by Maria Fazio and Vincent A. Fischetti, Ph.D. with permission. The Laboratory of Bacterial Pathogenesis and Immunology, Rockefeller University.
Capsules are generally composed of polysaccharide; rarely they contain amino sugars or peptides (Table 4).
Table 4. Chemical composition of some bacterial capsulesBacterium Capsule composition Structural subunits Gram-positive Bacteria Bacillus anthracis polypeptide (polyglutamic acid) D-glutamic acid Bacillus megaterium polypeptide and polysaccharide D-glutamic acid, amino sugars, sugars Streptococcus mutans polysaccharide (dextran) glucose Streptococcus pneumoniae polysaccharides sugars, amino sugars, uronic acids Streptococcus pyogenes polysaccharide (hyaluronic acid) N-acetyl-glucosamine and glucuronic acid Gram-negative Bacteria Acetobacter xylinum polysaccharide (cellulose) glucose Escherichia coli polysaccharide (colonic acid) glucose, galactose, fucose glucuronic acid