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Different Size, Shape and Arrangement of Bacterial
Cells
Last updated: February 9, 2022 by Sagar Aryal
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).
Size of Bacterial Cell
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.
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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 µm long by 80 µm 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 gramnegative 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).
Shape of Bacterial Cell
The three basic bacterial shapes are coccus (spherical), bacillus (rod-shaped),
and spiral (twisted), however pleomorphic bacteria can assume several shapes.
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.
Arrangement of Cocci
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
Arrangement of Bacilli
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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Flagella – Introduction, Types, Examples, Parts, Functions and
Flagella Staining- Principle, Procedure and Interpretation
Differences Between Cilia and Flagella
Differences Between Bacteria and Viruses
Negative Staining- Principle, Reagents, Procedure and Result
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Flagella – Introduction, Types, Examples, Parts,
Functions and Flagella Staining- Principle, Procedure
and Interpretation
Last updated: August 15, 2019 by Sagar Aryal
Introduction of Flagella
Flagella are the complex filamentous cytoplasmic structure protruding
through cell wall. These are unbranched, long, thread like structures, mostly
composed of the protein flagellin, intricately embedded in the cell
envelope. They are about 12-30 nm in diameter and 5-16 µm in length. They
are responsible for the bacterial motility. Motility plays an important role in
survival and the ability of certain bacteria to cause disease.
Types and Examples of Flagella
There are 4 types of flagellar distribution on bacteria-
1. Monotrichous
– Single polar flagellum
– Example: Vibrio cholerae
2. Amphitrichous
– Single flagellum on both sides
– Example: Alkaligens faecalis
3. Lophotrichous
– Tufts of flagella at one or both sides
– Example: Spirillum
4. Peritrichous
– Numerous falgella all over the bacterial body
– Example: Salmonella Typhi
Parts of Flagella
Each flagellum consists of three distinct parts- Filament, Hook and Basal Body.
The filament lies external to the cell.
Hook is embedded in the cell envelope.
Basal Body is attached to the cytoplasmic membrane by ring-like structures.
Functions of Flagella
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Movements
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Sensation
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Signal transduction
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Adhesion
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For cells anchored in a tissue, like the epithelial cells lining our air
passages, this moves liquid over the surface of the cell (e.g.,
driving particle-laden mucus toward the throat).
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Flagella are generally accepted as being important virulence
factors
Principle of Flagella Staining
A wet mount technique for staining bacterial flagella is simple and is useful
when the number and arrangement of flagella are critical in identifying species
of motile bacteria.
Procedure of Flagella Staining
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Grow the organisms to be stained at room temperature on blood
agar for 16 to 24 hours.
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Add a small drop of water to a microscope slide.
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Dip a sterile inoculating loop into sterile water
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Touch the loopful of water to the colony margin briefly (this
allows motile cells to swim into the droplet of water).
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Touch the loopful of motile cells to the drop of water on the slide.
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Cover the faintly turbid drop of water on the slide with a cover
slip. A proper wet mount has barely enough liquid to fill the space
under a cover slip. Small air spaces around the edge are
preferable.
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Examine the slide immediately under 40x for motile cells.
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If motile cells are seen, leave the slide at room temperature for 5
to 10 minutes.
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Apply 2 drops of RYU flagella stain gently on the edge of the
cover slip. The stain will flow by capillary action and mix with the
cell suspension.
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After 5 to 10 minutes at room temperature, examine the cells for
flagella.
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Cells with flagella may be observed at 100x.
Staining
Observe the slide and note the following:
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Presence or absence of flagella
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Number of flagella per cell
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Location of flagella per cell
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Differences Between Cilia and Flagella
Last updated: June 23, 2018 by Sagar Aryal
Flagella are the complex filamentous cytoplasmic structure protruding
through cell wall. These are unbranched, long, thread like structures, mostly
composed of the protein flagellin, intricately embedded in the cell envelope.
Cilia are slender, microscopic, hair-like structures or organelles that extend
from the surface of nearly all mammalian cells (multiple or single).
S.N.
Characteristics
Cilia
Flagella
1
Definition
Cilia are short, hair like appendages extending from the surface
of a living cell.
Flagella are long, threadlike appendages on the surface of a
living cell.
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Number
Numerous
Less in Number
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Length
Short and hair like organelle (5-10µ)
Long wipe like organelle (150µ)
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Occurrence
Occurs throughout the cell surface.
Presence at one end or two ends or all over the surface.
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Cross section
Nexin arm present.
Nexin arm absent
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Density
Many (hundreds) per cell
Few (less than 10) per cell
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Beating
Cilia beat in a coordinated rhythm either simultaneously
(synchronous) or one after the other (metachronic).
They beat independent of each other.
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Motion
Rotational, like a motor, very fast moving
Wave-like, undulating, sinusoidal, slow movement compared to
cilia
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Found in
Eukaryotic cells
Eukaryotic and prokaryotic cells
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Energy Production
Cilia use ‘kinesin’ which has an ATPase activity that produces
energy to perform the movement.
Flagella are powered by the proton-motive force by the plasma
membrane.
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Functions
Helps in locomotion, feeding circulation, aeration, etc.
Help mainly in locomotion only.
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Examples
Cilia present in Paramecium
Flagella present in Salmonella
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Differences Between Bacteria and Viruses
Last updated: June 23, 2018 by Sagar Aryal
Although bacteria and viruses both are very small to be seen without a
microscope, there are many differences between Bacteria and Viruses.
Some of the Differences Between Bacteria and
Viruses are as follows:
S.N.
Characteristics
Bacteria
Viruses
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Size
Larger (1000 nm)
Smaller (20-400 nm)
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Cell Wall
Peptidoglycan or Lipopolysaccharide
No cell wall. Protein coat present instead.
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Ribosomes
Present
Absent
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Number of cells
One cell (Unicellular)
No cells
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Living/Non-Living
Living organisms
Between living and non-living things.
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DNA and RNA
DNA and RNA floating freely in cytoplasm.
DNA or RNA enclosed inside a coat of protein.
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Infection
Localized
Systemic
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Reproduce
Able to reproduce by itself
Need a living cell to reproduce
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Reproduction
Fission- a form of asexual reproduction
Invades a host cell and takes over the cell causing it to make
copies of the viral DNA/RNA. Destroys the host cell releasing
new viruses.
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Duration of illness
A bacterial illness commonly will last longer than 10 days.
Most viral illnesses last 2 to 10 days.
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Fever
A bacterial illness notoriously causes a fever.
A viral infection may or may not cause a fever.
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Cellular Machinery
Possesses a cellular machinery
Lack cellular machinery
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Under Microscope
Visible under Light Microscope.
Visible only under Electron Microscope.
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Benefits
Some bacteria are beneficial (Normal Flora)
Viruses are not beneficial. However, a particular virus may be
able to destroy brain tumors. Viruses can be useful in genetic
engineering.
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Treatment
Antibiotics
Virus does not respond to antibiotics.
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Examples
Staphylococcus aureus, Vibrio cholerae, etc
HIV, Hepatitis A virus, Rhino Virus, etc
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Diseases/Infections
Food poisoning, gastritis and ulcers, meningitis, pneumonia, etc
AIDS, common cold, influenza, chickenpox, etc
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Differences between DNA and RNA
Last updated: June 23, 2018 by Sagar Aryal
Here are 17 differences between DNA and RNA.
DNA
S.N.
RNA
1.
DNA stands for Deoxyribonucleic Acid. The sugar portion of
DNA is 2-Deoxyribose.
RNA stands for Ribonucleic Acid. The sugar portion of RNA is
Ribose.
2.
The helix geometry of DNA is of B-Form (A or Z also present).
The helix geometry of RNA is of A-Form.
3.
DNA is a double-stranded molecule consisting of a long chain of
nucleotides.
RNA usually is a single-strand helix consisting of shorter chains
of nucleotides.
4.
The bases present in DNA are adenine, guanine, cytosine and
thymine.
The bases present in RNA are adenine, guanine, cytosine and
uracil.
5.
DNA is self-replicating.
RNA is synthesized from DNA on an as-needed basis.
6.
Base Pairing
:AT (adenine-thymine)GC (guanine-cytosine).
Base Pairing
:AU (adenine-uracil)GC (guanine-cytosine).
7.
Purine and Pyrimidine bases are equal in number.
There is no proportionality in between the number of Purine and
Pyrimidine bases.
8.
DNA is susceptible to UV damage.
Compared with DNA, RNA is relatively resistant to UV damage.
9.
Hydrogen bonds are formed between complementary nitrogen
bases of the opposite strands (A-T, C-G).
Base pairing through hydrogen bonds, occurs in the coiled parts.
10.
DNA is found in the nucleus of a cell and in mitochondria.
Depending on the type of RNA, this molecule is found in a cell’s
nucleus, its cytoplasm, and its ribosome.
11.
DNA can’t leave the nucleus.
RNA leaves the nucleus (mRNA).
12.
The C-H bonds in DNA make it fairly stable, plus the body
destroys enzymes that would attack DNA. The small grooves in
the helix also serve as protection, providing minimal space for
enzymes to attach.
The O-H bond in the ribose of RNA makes the molecule more
reactive, compared with DNA. RNA is not stable under alkaline
conditions, plus the large grooves in the molecule make it
susceptible to enzyme attack.
13.
Renaturation after melting is slow.
It is quite fast.
14.
DNA is only two types: intra nuclear and extra nuclear.
Three different types of RNA: m-RNA, t-RNA and r-RNA.
15.
Its quantity is fixed for cell.
The quantity of RNA of a cell is variable.
16.
It is long lived.
Some RNAs are very short lived while others have somewhat
longer life.
17.
Functions:Long-term storage of genetic information;
transmission of genetic information to make other cells and new
organisms.
Functions:Used to transfer the genetic code from the nucleus to
the ribosomes to make proteins. RNA is used to transmit genetic
information in some organisms and may have been the
molecule used to store genetic blueprints in primitive organisms.