Environmental Engineering Reference
In-Depth Information
known as microcapsules can be detected only by electron microscopy
(Singleton and Sainsbury, 1994). The production of capsules is determined
largely by genetics as well as environmental conditions and depends on the
presence or absence of capsule-degrading enzymes and other growth factors.
Varying in composition, capsules are mainly composed of water; the organic
contents are made up of complex polysaccharides, nitrogen-containing sub-
stances, and polypeptides. Capsules confer several advantages when bacteria
grow in their normal habitat; for example, they help to (1) prevent desiccation,
(2) resist phagocytosis by host phagocytic cells, (3) prevent infection by bac-
teriophages, and (4) aid bacterial attachment to tissue surfaces in plant and
animal hosts or to surfaces of solid objects in aquatic environments. Capsule
formation often correlates with pathogenicity (Spellman, 2007).
Flagella
Many bacteria are motile, and this ability to move independently is usually
attributed to a special structure, the
flagella.
(singular: flagellum). Depending
on species, a cell may have a single flagellum (see
Figure 6.2
) (
monotrichous
bacteria;
trichous
means “hair”), one flagellum at each end (
amphitrichous
bac-
teria;
amphi
means “on both sides”), a tuft of flagella at one or both ends
(
lophotrichous
bacteria;
lopho
means “tuft”), or flagella that arise all over the
cell surface (
peritrichous
bacteria;
peri
means “around”). A flagellum is a
threadlike appendage extending outward from the plasma membrane and
cell wall. Flagella are slender, rigid locomotor structures, about 20 µm across
and up to 15 or 20 µm long. Flagellation patterns are very useful in identi-
fying bacteria and can be seen by light microscopy, but only after the fla-
gella have been stained with special techniques designed to increase their
thickness. The detailed structure of flagella can be seen only in an electron
microscope. Bacterial cells benefit from flagella in several ways. Flagella can
increase the concentration of nutrients or decrease the concentration of toxic
materials near the bacterial surfaces by causing a change in the flow rate of
fluids. They can also disperse flagellated organisms to areas where colony
formation can take place. The main benefit of flagella to organisms is the
improved ability to flee from areas that might be harmful.
Cell Wall
The main structural component of most procaryotes is the rigid
cell wall
.
Functions of the cell wall include: (1) providing protection for the delicate
protoplast from osmotic lysis (bursting); (2) determining the shape of a cell;
(3) acting as a permeability layer that excludes large molecules and various
antibiotics and plays an active role in regulating the intake of ions by the cell;
and (4) providing a solid support for flagella. Cell walls of different species
may differ greatly in structure, thickness, and composition. The cell wall
accounts for about 20 to 40% of the dry weight of a bacterium.
