Under an electron microscope, the structure of the bacterial cells looks like a capsule. It has the following components like outside the cell membrane and internal to the cell membrane. (Fig).
Fig: Ultrastructure of Bacterial cell
Capsule:
It is the outer layer of the bacteria cell. Depending on the
chemical nature capsules are thick or thin, and rigid or flexible. It is the most
important virulence factor of bacteria which is visualized by negative staining
technique. Generally, it is composed of 98% water and 2% polysaccharide or
glycoprotein/polypeptide or both. Only some bacterial species possess capsules.
The capsule is usually made of polysaccharide (e.g. pneumococcus), occasionally
polypeptide (e.g. anthrax bacilli) and hyaluronic acid (e.g. streptococcus).
Most bacterial capsules are composed of polysaccharides, especially with
homopolysaccharides which are usually synthesized outside the cell from
disaccharides by exocellular enzymes. The capsule of acetic acid bacteria cell
is composed of homopolysaccharide (hemicellulose).
Glucan is synthesized by Streptococcus mutans which are
composed of several types of sugar and are termed heteropolysaccharides. A few
capsules are polypeptides like Bacillus anthracis, which is composed entirely
of a polymer of glutamic acids.
There are two types of capsules,
1. Macrocapsule: Thickness of 0.2 µm or more, visible
under a light microscope.
2. Microcapsule: Thickness is less than 0.2 µm,
visible under Electron microscope.
Functions:
1. Preventive nature: Capsular polysaccharides bind a significant
amount of water making the cell resistant to drying and also preventing
attachment of bacteriophage on the cell surface.
2. Protection: It protects from mechanical injury,
temperature, drying, etc.
3. Attachment: Capsule helps in attachment on the surface.
For example: Streptococcus mutants that cause dental carries attach on teeth
surface by its capsule.
4. Anti-phagocytic: Capsule resists phagocytosis by WBCs.
5. Source of nutrition: It acts as a resource organ of
nutrition during insufficient nutrient supply.
6. Repulsion: The same charge-capsulated bacteria repel each
other.
Cell wall:
The cell wall is the rigid wall situated outside of the
plasma membrane. It provides the shape of the bacteria as well as provides
protection from osmotic lysis. The cell wall is made up of peptidoglycan. The
gram-positive bacteria contain a thick homogeneous peptidoglycan or murein
layer (20 to 80 nm thickness) outside the plasma membrane whereas gram-negative
bacteria have a 2-7 nm peptidoglycan layer surrounded by 7-8 nm thick outer
membrane. Peptidoglycan is a polymer consisting of two sugar molecules viz.
N-acetylglucosamine and N-acetylmuramic acid. Apart from that it also contains
several amino acids.
Flagella:
Flagella are slender and rigid hair-like structures. They
are about 20 nm cross and up to 15 to 20 µm long. The eukaryotic flagellum is a
long, rod-like structure, surrounded by an extension of the cell membrane like
a sheath. The bulk of the structure is a filament called an axoneme.
Monotrichous bacteria have one polar flagellum whereas amphitrichous bacteria
have either singe or duster of flagellum at both poles. Lophotrichous bacteria
have a cluster of flagella at one end. Peritrichous bacteria have surrounded
lateral flagella.
Flagellar Ultra-Structure: Transmission electron
microscope studies revealed three regions of flagella which are:
(1) The outermost region is the filament which is extended
from the cell surface to tips.
(2) Basal bodies consist of small central rods inserted into
cells
(3) A short curved segment, the flagellar hook which links
the filament to basal bodies and acts as flexible coupling.
The filament is a hollow, rigid cylinder made up of protein
subunits called flagellin. Depending on the bacterial species, its molecular weight varies from 30,000 dalton to 6,000 dalton.
Gram-negative bacteria have four rings connected to a central rod. Two rings (M and S rings) are embedded in the cell membrane, and the other two rings (L and P rings) are associated with the cell wall, which is made up of lipopolysaccharide and peptidoglycan, respectively.
Gram-positive bacteria have only two rings in their basal
body. M-ring which is the inner ring, connected to the plasma membrane, and the
S-ring which is the outer ring, connected to the peptidoglycan.
Fimbriae and Pili:
Many Gram negative bacilli contain short, fine, hair like,
non-flagellar appendages called fimbriae. Pili are similar appendages which are
about 1-10 cells. The filament of the pilus is straight and the diameter is 7
nm to 10 nm, larger than the diameter of fimbriae. It is made up of pilin
protein. The molecular weight is 17,000. Pili are non-motile but adhesive
structures. They enable the bacteria to stick firmly to other bacteria, to a
surface, or to some eukaryotic such as mould plants, plants, and animal cells.
They are genetically determined by sex factor or conjugative plasmid and hence
they help in the conjugation of male bacteria, in the attachment of pathogenic
bacteria to their host cell.
Spinae:
Some Gram-positive bacteria have tubular unicellular and
rigid appendage of single protein moiety, known as spinae. They help the
bacterial cell to tolerate environmental stress such as salinity, pH and
temperature, etc.
Cell Membrane:
A cell or plasma membrane is a thin structure that
completely surrounds the cell. It is about 8 nm thick. This structure is a critical
barrier separating the inside of the cell from the environment. The cell
membrane is also a highly selective barrier enabling the cell to concentrate a
specific metabolite and excrete waste material. Mostly biological membrane is
composed primarily of Phospholipids (about 20 to 30 percent) and proteins
(about 60 to 70 percent).
One major difference in the chemical composition of the membrane
between eukaryotic cells and prokaryotic cells is that eukaryotes have sterol
in their membrane depending on cell type, sterol can make up from 5-25% of the total
lipid of the eukaryotic membrane. Sterols are absent from membranes of all
prokaryotic cells (methanotrophs are a major exception). Sterols are rigid,
planner molecules, whereas fatty acids are flexible.
The main function of the cell membrane is selective
permeability and transport of solute, electron transport, and oxidative
phosphorylation in aerobic species, and excretion of hydrolytic exoenzymes.
Inclusion bodies:
Various types of inclusion bodies organic and inorganic
granules are present in the cytoplasmic matrix. They are mainly used for the storage
of energy and reduce osmotic pressure by tying up molecules in particulate
forms like polyphosphate granules, glycogen granules, etc. Some of the membrane-enclosed
inclusion bodies are like glycogen-sulfur granules, carboxysomes, etc.
In many cyanobacteria, green photosynthesis bacteria gas
vacuole is present as an organic inclusion body. Gas vacuoles are aggregates of
a huge number of small, hollow cylindrical structures and provide them
buoyancy.
Ribosomes:
Ribosomes are loosely attached to the plasma membrane (70S
ribosomes). They are made up of both protein and ribonucleic acid. They are the
site of protein synthesis. Plasma membrane ribosomes make proteins for
transport to the outside.
Nucleoid:
A prokaryotic chromosome is located in an irregularly shaped
region called nucleoid. The nucleoid is composed of 60% DNA, 30% RNA, and 10%
protein by weight. Prokaryotes contain a single circle of double-stranded DNA
but some have linear DNA chrosome.
Mesosome:
Mesosomes are present in the cell membrane of bacterial
cells that fold inward. They play a major role in cellular respiration. They
appear as a pocket formed by the cytoplasmic membrane, filled with vesicles,
tubules, or lamellae.
Some of the differences of gram positive and gram negative
bacteria are given in Table.
|
Sl. No. |
Gram Positive |
Gram Negative |
|
1. |
The cell wall
is made up of a peptide glycan layer which is thick. |
They have a
thin lipopolysaccharide exterior cell wall. |
|
2. |
Effect on
dyes: Retain the crystal violet dye, and change into purple during staining
identification. |
Effect on
dyes: Do not retain the crystal violet dye, and react only with a
counter-stain, generally stain pink. |
|
3. |
Effect on
antibiotics: Susceptible to the enzyme lysozyme and to penicillin |
Effect on
antibiotics: Resistant to penicillin and contains an endotoxin called
Lipopolysaccharide. |
|
4. |
The flagellum
has two supporting rings, in the peptidoglycan layer, and in the plasma
membrane. |
If present,
the flagellum has four supporting rings, namely the 'L' ring, 'P' ring, 'M'
ring, and 'S' ring. |
|
5. |
Teichoic
acids are present. |
Teichoic
acids are absent. |
|
6. |
Lipoproteins
are absent. |
Lipoproteins
are attached to the polysaccharide backbone. |
|
7. |
Periplasmic
space is absent. |
Periplasmic
space is present. |