Bacteria is an ancient forms of cellular life. It is also the most widely dispersed, occupying every conceivable niche on
the planet.
Most bacteria have one of three general shapes: coccus (round), bacillus (rod), or spiral, based on the configuration
of the cell wall. Two types of spiral cells are the spirochetes and the spirilla.
Shape and arrangement of cells are key means of describing bacteria. Arrangements of cells are based on the
number of planes in which a given species divides. Cocci can divide in many planes to form pairs, chains, packets, or
clusters. Bacilli divide only in the transverse plane. If they remain attached, they form chains or palisades.
THE
BACTERIA
Bacterial
Form
and
Function
:
External
Structures
:
The external structures of bacteria include appendages (flagella, fimbriae, and pili) and surface coatings (the S layer
and the glycocalyx).
Flagella vary in number and arrangement as well as in the type and rate of motion they produce.
The
Boundary
Layer
of
Bacteria
:
The cell envelope is the complex boundary structure surrounding a bacterial cell. In gram-negative bacteria, the
envelope consists of an outer membrane, the cell wall, and the cytoplasmic membrane. Gram-positive bacteria have
only the cell wall and cytoplasmic membrane.
In a Gram stain, gram-positive bacteria retain the crystal violet and stain purple. Gram-negative bacteria lose the
crystal violet and stain red from the safranin counterstain.
Gram-positive bacteria have thick cell walls of peptidoglycan and acidic polysaccharides such as teichoic acid. The
cell walls of gram-negative bacteria are thinner and have a wide periplasmic space.
The outer membrane of gram-negative cells contains lipopolysaccharide (LPS). LPS is toxic to mammalian hosts.
The bacterial cytoplasmic membrane is typically composed of phospholipids and proteins, and it performs many
metabolic functions as well as transport activities.
Internal
Structures
:
The cytoplasm of bacterial cells serves as a solvent for materials used in all cell functions.
The genetic material of bacteria is DNA. Genes are arranged on large, circular chromosomes. Additional genes are
carried on plasmids.
Bacterial ribosomes are dispersed in the cytoplasm in chains (polysomes) and are embedded in the cytoplasmic
membrane.
Bacteria may store nutrients in their cytoplasm in structures called inclusions. Inclusions vary in structure and the
materials that are stored.
Packets in the cytoplasm called bacterial microcompartments are shells of protein packed with enzymes.
Some bacteria manufacture long actin- and tubulin-like filaments that help determine their cellular shape.
A few families of bacteria produce dormant bodies called endospores, which are the hardiest of all life forms,
surviving for hundreds or thousands of years.
The genera Bacillus and Clostridium are endospore formers, and both contain deadly pathogens.
Bacteria | 03
