Different Size, Shape and Arrangement of Bacterial Cells
- برجاء قراءة الصفحة الى اخرها لمعرفة اجهزة الاليزا وPCR و أجهزة تحليل CBC واجهزة تحليل الهرمونات ودلالات الاورام
Bacteria
are prokaryotic, unicellular microorganisms, which lack chlorophyll
pigments. The cell structure is simpler than that of other organisms as
there is no nucleus or membrane bound organelles.
Due to the presence of a rigid cell wall, bacteria maintain a definite shape, though they vary as shape, size and structure.
When
viewed under light microscope, most bacteria appear in variations of
three major shapes: the rod (bacillus), the sphere (coccus) and the
spiral type (vibrio). In fact, structure of bacteria has two aspects,
arrangement and shape. So far as the arrangement is concerned, it may
Paired (diplo), Grape-like clusters (staphylo) or Chains (strepto). In
shape they may principally be Rods (bacilli), Spheres (cocci), and
Spirals (spirillum).
The
average diameter of spherical bacteria is 0.5-2.0 µm. For rod-shaped or
filamentous bacteria, length is 1-10 µm and diameter is 0.25-1 .0 µm.
- E. coli , a bacillus of about average size is 1.1 to 1.5 µm wide by 2.0 to 6.0 µm long.
- Spirochaetes occasionally reach 500 µm in length and the cyanobacterium
- Oscillatoria is about 7 µm in diameter.
- The bacterium, Epulosiscium fishelsoni , can be seen with the naked eye (600 mm long by 80 mm in diameter).
- One
group of bacteria, called the Mycoplasmas, have individuals with size
much smaller than these dimensions. They measure about 0.25 µ and are
the smallest cells known so far. They were formerly known as
pleuropneumonia-like organisms (PPLO).
- Mycoplasma gallicepticum, with a size of approximately 200 to 300 nm are thought to be the world smallest bacteria.
- Thiomargarita namibiensis
is world’s largest bacteria, a gram-negative Proteobacterium found in
the ocean sediments off the coast of Namibia. Usually it is 0.1—0.3 mm
(100—300 µm) across, but bigger cells have been observed up to 0.75 mm
(750 µm).
Thus
a few bacteria are much larger than the average eukaryotic cell
(typical plant and animal cells are around 10 to 50 µm in diameter).
The
three basic bacterial shapes are coccus (spherical), bacillus
(rod-shaped), and spiral (twisted), however pleomorphic bacteria can
assume several shapes.
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Shape of Bacterial Cell
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- Cocci (or coccus for a single cell) are round cells, sometimes slightly flattened when they are adjacent to one another.
- Bacilli (or bacillus for a single cell) are rod-shaped bacteria.
- Spirilla
(or spirillum for a single cell) are curved bacteria which can range
from a gently curved shape to a corkscrew-like spiral. Many spirilla
are rigid and capable of movement. A special group of spirilla known as
spirochetes are long, slender, and flexible.
Cocci
bacteria can exist singly, in pairs (as diplococci ), in groups of four
(as tetrads ), in chains (as streptococci ), in clusters (as
stapylococci ), or in cubes consisting of eight cells (as
sarcinae). Cocci may be oval, elongated, or flattened on one side. Cocci
may remain attached after cell division. These group characteristics
are often used to help identify certain cocci.
1. Diplococci
The cocci are arranged in pairs.
Examples: Streptococcus pneumoniae, Moraxella catarrhalis, Neisseria gonorrhoeae, etc.
2. Streptococci
The cocci are arranged in chains, as the cells divide in one plane.
Examples: Streptococcus pyogenes, Streptococcus agalactiae
3. Tetrads
The cocci are arranged in packets of four cells, as the cells divide in two plains.
Examples: Aerococcus, Pediococcus and Tetragenococcus
4. Sarcinae
The
cocci are arranged in a cuboidal manner, as the cells are formed by
regular cell divisions in three planes. Cocci that divide in three
planes and remain in groups cube like groups of eight.
Examples: Sarcina ventriculi, Sarcina ureae, etc.
5. Staphylococci
The cocci are arranged in grape-like clusters formed by irregular cell divisions in three plains.
Examples: Staphylococcus aureus
The cylindrical or rod-shaped bacteria are called ‘bacillus’ (plural: bacilli).
1. Diplobacilli
Most bacilli appear as single rods. Diplobacilli appear in pairs after division.
Example of Single Rod: Bacillus cereus Examples of Diplobacilli: Coxiella burnetii, Moraxella bovis, Klebsiella rhinoscleromatis, etc.
2. Streptobacilli
The bacilli are arranged in chains, as the cells divide in one plane.
Examples: Streptobacillus moniliformis
3. Coccobacilli
These are so short and stumpy that they appear ovoid. They look like coccus and bacillus.
Examples: Haemophilus influenzae, Gardnerella vaginalis, and Chlamydia trachomatis
4. Palisades
The
bacilli bend at the points of division following the cell divisions,
resulting in a palisade arrangement resembling a picket fence and
angular patterns that look like Chinese letters.
Example: Corynebacterium diphtheriae
Arrangement of Spiral Bacteria
Spirilla
(or spirillum for a single cell) are curved bacteria which can range
from a gently curved shape to a corkscrew-like spiral. Many spirilla
are rigid and capable of movement. A special group of spirilla known as
spirochetes are long, slender, and flexible.
1. Vibrio
They are comma-shaped bacteria with less than one complete turn or twist in the cell.
Example: Vibrio cholerae
2. Spirilla
They
have rigid spiral structure. Spirillum with many turns can
superficially resemble spirochetes. They do not have outer sheath and
endoflagella, but have typical bacterial flagella.
Example: Campylobacter jejuni, Helicobacter pylori, Spirillum winogradskyi, etc.
3. Spirochetes
Spirochetes
have a helical shape and flexible bodies. Spirochetes move by means of
axial filaments, which look like flagella contained beneath a flexible
external sheath but lack typical bacterial flagella.
Examples: Leptospira species (Leptospira interrogans), Treponema pallidum, Borrelia recurrentis, etc.
Others Shapes and Arrangements of Bacteria
1. Filamentous Bacteria
They
are very long thin filament-shaped bacteria. Some of them form
branching filaments resulting in a network of filaments called
‘mycelium’.
Example: Candidatus Savagella
2. Star Shaped Bacteria
Example: Stella
3. Rectangular Bacteria
Examples: Haloarcula spp (H. vallismortis, H. marismortui)
4. Pleomorphic Bacteria
These
bacteria do not have any characteristic shape unlike all others
described above. They can change their shape. In pure cultures, they can
be observed to have different shapes.
Examples: Mycoplasma pneumoniae, M. genitalium, etc.
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Retain crystal violet dye
and stain blue or purple
Can be decolorized to accept
counterstain (safranin) and stain pink or red