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BACTERIAL CELL
BACTERIAL CELL
The illustration shows a generalised bacterium
with many of the main components illustrated.
No real organism would have all of these
features.
The top half of the diagram shows a Gram +ve bacterium
with its thick peptidoglycan outer wall closely apposed to
the inner plasma membrane. The lower half of the cell
shows a Gram -ve bacterium with its inner plasma
membrane, thin intermediate peptidoglycan layer and
external membrane. The outer portion of the external
membrane is a lipopolysaccharide layer. This layer
comprises core components that form the outer layer of
the membrane and (usually) side chains that radiate off.
These side chains are called O-antigens. They are absent
in Yersinia pestis. In this diagram, parts of the two types of
wall are covered in a regular arrangment of proteins called
an S-layer. A good example of an S-layer is shown in our
diagram of anthrax.
DETAILED BACTERIAL CELL WALL PICTURES:
Gram positive Bacterial Cell Wall
Gram negative Bacterial Cell Wall
CAPSULE
A generalised capsule is shown covering part of the Gram +ve and Gram -ve
regions. Capsules can prevent drying, help in adhesion and help to ward off
attack from viruses and host cells.
SURFACE FEATURES
Several flagella are shown. These are spiralised protein tubes that have a
motor at their base. This motor anchors the flagella into the cell wall and its
rotation causes the flagella to propel the bacterium along, like the propeller
of a boat. Various flagellar arrangements are possible, from a single
flagellum at one pole to numerous flagella radiating from all over the
bacterial surface. Straighter protein tubes called pili are also shown. These
can help in attachment of bacteria to host cells or other substrates. Pili are
also used for gene transfer.
GENOME
The genetic material is a skein of circular DNA localised as the nucleoid.
The nucleoid lacks a nuclear membrane (a defining characteristic of
prokaryotic cells). Peeping out from the upper right part of the nucleoid is a
plasmid - a separate piece of DNA. Plasmids replicate independently of the
nucleoid DNA and can be exchanged between cells (the means of passing
on, for example, drug resistance).
CYTOPLASM
The bacterial cytoplasm is shown filled with ribosomes. These are
somewhat smaller than their eukaryotic counterparts and many are shown
linked into polysomes. Infoldings of the Plasma membrane are common
and are often associated with the nucleoid. In this picture such an infolding
can be seen around the middle of the cell and it is termed a mesosome.
HELICAL FILAMENTS
Helically arranged protein filaments are shown winding around just
beneath the plasma membrane. They are thought to guide the production
of the cell wall to create the shape of the bacterium. They are absent in
spherical forms. The purple helix (MreB) controls cell width and the bluish
one (Mbl) controls length. They are composed of actin like molecules. This
form of prokaryotic cytoskeleton was recently described in the journals
CELL and NATURE (references: CELL, Vol. 104, 913-922, March 23,
2001. Nature, Vol. 413, September 2001).
ANTHRAX - Bacillus anthracis
PLAGUE - Yersinia pestis
Bacteria Cell Structure
They are as unrelated to human beings as living things can be, but bacteria are
essential to human life and life on planet Earth. Although they are notorious for
their role in causing human diseases, from tooth decay to the Black Plague,
there are beneficial species that are essential to good health.
For example, one species that lives symbiotically in the large intestine
manufactures vitamin K, an essential blood clotting factor. Other species are
beneficial indirectly. Bacteria give yogurt its tangy flavor and sourdough bread
its sour taste. They make it possible for ruminant animals (cows, sheep, goats)
to digest plant cellulose and for some plants, (soybean, peas, alfalfa) to convert
nitrogen to a more usable form.
Bacteria are prokaryotes, lacking well-defined nuclei and membrane-bound
organelles, and with chromosomes composed of a single closed DNA circle.
They come in many shapes and sizes, from minute spheres, cylinders and
spiral threads, to flagellated rods, and filamentous chains. They are found
practically everywhere on Earth and live in some of the most unusual and
seemingly inhospitable places.
• In the late 1600s, Antoni van Leeuwenhoek became the first to study bacteria
under the microscope. During the 19th century, the French scientist Louis
Pasteur and the German physician Robert Koch demonstrated the role of
bacteria as pathogens (causing disease). The 20th century saw numerous
advances in bacteriology, indicating their diversity, ancient lineage, and general
importance. Most notably, a number of scientists around the world made
contributions to the field of microbial ecology, showing that bacteria were
essential to food webs and for the overall health of the Earth's ecosystems.
The discovery that some bacteria produced compounds lethal to other
bacteria led to the development of antibiotics, which revolutionized the field
of medicine.
• There are two different ways of grouping bacteria. They can be divided into
three types based on their response to gaseous oxygen. Aerobic bacteria
require oxygen for their health and existence and will die without it. Anerobic
bacteria can't tolerate gaseous oxygen at all and die when exposed to it.
Facultative aneraobes prefer oxygen, but can live without it.
• The second way of grouping them is by how they obtain their energy.
Bacteria that have to consume and break down complex organic compounds
are heterotrophs. This includes species that are found in decaying material as
well as those that utilize fermentation or respiration. Bacteria that create their
own energy, fueled by light or through chemical reactions, are autotrophs.