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
the several layers of material that envelope or enclose the protoplasm
of the cell. The cell protoplasm (cytoplasm) is surrounded by
membrane, a cell wall and a capsule. The cell wall
is a layered structure in Gram-negative bacteria. All cells have a
which is the essential and definitive characteristic of a "cell".
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
Figure 9. Profiles of the cell
envelope the Gram-positive and Gram-negative bacteria. The
wall is a uniformly thick layer external to the plasma membrane. It is
composed mainly of peptidoglycan (murein). The Gram-negative wall
thin and multilayered. It consists of a relatively thin peptidoglycan
between the plasma membrane and a phospholipid-lipopolysaccharide outer
membrane. The space between the inner (plasma) and outer membranes
the peptidoglycan resides) is called the periplasm.
Most procaryotes contain
some sort of a
layer outside of the cell wall polymer. In a general sense, this layer
is called a capsule. A true capsule is a discrete
layer of polysaccharides deposited outside the cell wall. A less
structure or matrix which embeds the cells is a called a slime
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.
10. Bacterial capsules
outlined by India ink viewed by light microscopy. This is a true
a discrete layer of polysaccharide surrounding the cells. Sometimes
cells are embedded more randomly in a polysaccharide matrix called a
layer or biofilm. Polysaccharide films that may inevitably be present
the surfaces of bacterial cells, but which cannot be detected visually,
are called glycocalyx.
Figure 11. Negative stain of
pyogenes viewed by transmission electron microscopy (28,000X). The
halo around the chain of cells is the hyaluronic acid capsule that
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.
Laboratory of Bacterial Pathogenesis and Immunology, Rockefeller
Capsules are generally
rarely they contain amino sugars or peptides (Table 4).
of some bacterial capsules
||D-glutamic acid, amino
||sugars, amino sugars,
Capsules have several
have multiple functions in a particular organism. Like fimbriae,
slime layers, and glycocalyx often mediate adherence of cells
surfaces. Capsules also protect bacterial cells from engulfment by
predatory protozoa or white blood cells (phagocytes), or from attack by
antimicrobial agents of plant or animal origin. Capsules in certain
bacteria protect cells from perennial effects of drying or
Capsular materials (e.g. dextrans) may be overproduced when bacteria
fed sugars to become
reserves of carbohydrate for subsequent metabolism.
Figure 12. Colonies of Bacillus
anthracis. The slimy or mucoid appearance of a bacterial colony is
usually evidence of capsule production. In the case of B. anthracis,
the capsule is composed of poly-D-glutamate. The capsule is an
determinant of virulence to the bacterium. In the early stages of
and infection the capsule protects the bacteria from assaults by the
and phagocytic systems.
Some bacteria produce
slime materials to
and float themselves as colonial masses in their environments. Other
produce slime materials to attach themselves to a surface or substrate.
Bacteria may attach to surface, produce slime, divide and produce
within the slime layer, and construct a biofilm, which becomes
enriched and protected environment for themselves and other bacteria.
A classic example of
biofilm construction in
is the formation of dental plaque mediated by the oral
mutans. The bacteria adhere specifically to the pellicle of the
by means of a protein on the cell surface. The bacteria grow and
a dextran capsule which binds them to the enamel and forms a biofilm
300-500 cells in thickness. The bacteria are able to cleave sucrose
by the animal diet) into glucose plus fructose. The fructose is
as an energy source for bacterial growth. The glucose is polymerized
an extracellular dextran polymer that cements the bacteria to tooth
and becomes the matrix of dental plaque. The dextran slime can be
to glucose for use as a carbon source, resulting in production of
acid within the biofilm (plaque) that decalcifies the enamel and leads
to dental caries or bacterial infection of the tooth.
Figure 13. (Left) Dental plaque
revealed by a harmless red dye. http://www.medicdirect.co.uk/DentalHealth
(Right) Human dental plaque. Transmission
electron micrograph by Marilee Sellers, Northern Arizona University. http://www4.nau.edu/electron/TEM_img.htm
characteristic of capsules
be their ability to block some step in the phagocytic process and
prevent bacterial cells from being engulfed or destroyed by phagocytes.
For example, the primary determinant of virulence of the pathogen Streptococcus
pneumoniae is its polysaccharide capsule, which prevents ingestion
of pneumococci by alveolar macrophages. Bacillus anthracis survives
phagocytic engulfment because the lysosomal enzymes of the phagocyte
initiate an attack on the poly-D-glutamate capsule of the bacterium.
such as Pseudomonas aeruginosa, that construct a biofilm made
extracellular slime when colonizing tissues, are also resistant to
which cannot penetrate the biofilm.