There are many forms of
bacteria. A rod shaped bacterium with flagella and capsule will be considered
as a typical bacterium. The bacterial cell consists of (a) cell wall (b) cell
membrane (c) cytoplasm (d) genetic material.
Cell Wall
The bacterial cell is
surrounded by cell wall. The cell wall has no cellulose. It is composed of a
complex material called peptidoglycan (also known as murein). This substance is
found only in prokaryotes. It is a large molecule. It is composed of amino
acids and sugars. (Peptide, amino acid, glycan, sugar). The sugars form long
parallel chains to which are attached short side chains of four or five amino
acids. The cell wall gives shape and protection to the cell. Cell wall is
absent in Mycoplasmas.
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| Peptidoglycan |
Cell Membrane
It lies directly
beneath the cell wall. Bacterial membrane differs from eukaryotic membrane in
lacking sterols such as cholesterol. Plasma membrane is very delicate in
nature, any damage to it results in death of organism. Cell membrane regulates
transport of proteins, nutrients, sugar and electrons or the metabolite. Plasma
membranes contain enzymes for respiratory metabolites.
Slimy Capsule
Many bacteria are
enveloped by a slimy capsule, secreted by the cell. It is present outside the
cell wall and is an additional protective layer. It is composed of
polysaccharides. The capsule is a structure for defense. Bacteria may survive,
when the capsule is removed. The encapsulated form cause disease and the same
bacteria without capsule do not cause disease e.g. Diplococcus pneumonia
(bacteria causing pneumonia).
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| Structure of a typical bacterium |
Flagella
Many bacteria have fine
thread like outgrowth called flagella (sing: flagellum). It is composed of
flagelin, a non-contractile protein, originating from basal body, structure
just beneath cell membrane in the cytoplasm. The flagella are anchored in the
cell membrane and project out through the cell wall. The flagella are a structure
for movement. Prokaryotic flagella propel the cell by rotating rather whipping.
Bacilli and spirilla are motile bacteria and usually Cocci are non-motile.
Based on flagella bacteria are classified into different taxonomic group.
Atrichous means bacteria are without any flagella. Monotrichous means a single
flagellum is present at one end of the cell; Lophotrichous flagella means a
tuft of flagellum is present at one pole of the bacterium. Amphitrichous is a
condition when tuft of flagella at each of a two poles is present. In
Peritrichous form flagella surround the whole cell. Most of bacilli and spiral
shaped bacteria have flagella, Cocci very rarely have flagella.
Bacteria can detect and
move in response to chemical signals, with the help of flagella. The response
to chemical signal is called Chemotaxis.
Pili
(Singular: Pilus) are hollow protein filaments that are
anchored in the cell membrane and project through the cell wall. They can be
observed only by electron microscope and are found only on certain species of
gram negative bacteria. Pilus is made up of special protein called pilin. Pili
perform the following functions:
(a) Conjugation between
bacteria and subsequent transfer of genetic material
(b) Attachment on
surface such as the tissues of an infected person.
Mesosomes
In some bacteria there
is an enfolding of the cell membrane into the cytoplasm. This is called
mesosome. Mesosomes are in the form of vesicles, tubules or lamellae. Mesosomes
perform the following functions.
Functions of
Mesosomes
(a) Increase membrane
surface area, allowing the cell greater activity in respiration and active
transport.
(b) The formation of a
new cross wall occurs with the help of mesosome during cell division.
(c) Photosynthesis.
Cytoplasm:
The cytoplasm is dense. Small vacuoles and granules f stored food e.g. glycogen,
proteins, fats, are present in bacteria. About 90% of the cell is water.
Ribosomes are large in number and occur free in the cytoplasm. Mitochondria,
Golgi bodies, endoplasmic reticulum, microtubule and chloroplasts are absent in
bacteria.
Nucleoid:
The nuclear region of bacteria is not separated from the cytoplasm by a nuclear
membrane. It is seen in the electron microscope as an area lighter than the
cytoplasmic contents. Bacteria are haploid organisms. The nucleiod contains a
single chromosome. The chromosomes consist of a circular double helical DNA
molecule. The DNA is an extremely long molecule of DNA that is tightly folded
so as to fit inside the cell component.
Plasmids:
Plasmids are small circular pieces of double stranded DNA. They replicate when
the cell replicates. Plasmids can replicate independent of the chromosome. They
often contain drug resistant, heavy metals, disease and insect resistant genes
on them.
Endospores:
Only a few of many species of bacilli form spores.
Sporulation:
A single bacterium forms a single spore by a process called sporulation. During
sporulation the cell membrane encloses a section of the cytoplasm, containing
the entire bacterial chromosome, some ribosomes and other soluble cytoplasmic
materials. An impermeable thick wall develops around. Most of the water is
removed from the spore and metabolism stops. Because the spore develops within
the vegetative cell so it has been named as endospore. The endospores are very
resistant to adverse environmental conditions e.g. heat, radiation, chemicals,
etc.
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| Endospore |
Germination
During germination, the
cell takes up water and enlarges. At the
same time, the wall disintegrates and a vegetative cell emerges. Endospore
formation is not a means of reproduction since there is no increase in cell
number during the spore cycle.
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Germination of Bacteria
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Cysts
Cysts are dormant,
thick walled desiccation resistance forms and develop during differentiation of
vegetative cells, which can germinate under suitable condition. They are not
heat resistant.
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