Bacterial Morphology and
Structure
Bacterial cells Morphology
• Cocci: sphere, 1μm
• Bacilli: rods , 0.5-1 μm in width -3 μm in length
• Spiral bacteria: 1~3 μm in length and 0.30.6 μm in width.
• Unit for measurement Micron or micrometer,
μm:
• 1μm=10-3mm
• Size: Varies with kinds of bacteria, and also
related to their age and external environment
Bacterial Shapes and Arrangements
Structure of Bacteria
Essential structure
• Cell wall
• cell membrane
• Cytoplasm
• Nuclear material
Particular structures
• Capsule
• flagella
• pili
• Spore
Gram Stain
• 1884: Christian Gram: First publication for the Gram stain method.
• Gram staining is a common technique used to differentiate two large
groups of bacteria based on their different cell wall constituents.
The Gram stain procedure distinguishes between Gram positive
and Gram negative groups by coloring these cells red or violet.
Cell Wall of Bacteria
Cell Wall: common peptidoglycan
layer
• A back bone of N-acetyl glucosamine and Nacetyl muramic acid: Both discovered in Gram
positive and Gram negative bacteria.
• A set of identical tetrapeptide side chain
attached to N-acetyl-muramic acid: different
components and binding modes in Gram
positive and Gram negative bacteria.
• A set of identical peptide cross bridges: only in
Gram positive bacteria
Special Components of Gram Positive
Cell Wall
• Teichoicacid
Special Components of Gram
Negative Cell Wall
• lipopolysaccharide
Functions of Cell Wall
• Maintaining the cell's characteristic shape
• The rigid wall compensates for the flexibility of the
phospholipid membrane
• keeps the cell from assuming a spherical shape
• Countering the effects of osmotic pressure
• Providing attachment sites for bacteriophages
• Providing a rigid platform for surface appendages- flagella,
fimbriae, and pili all emanate from the wall and extend
beyond it
• Play an essential role in cell division
• Be the sites of major antigenic determinants of
The cell surface
• Resistance of Antibiotics
Wall-less forms of Bacteria
• When bacteria are treated with 1) enzymes that are
lytic for the cell wall e.g. lysozyme or 2) antibiotics
that interfere with biosynthesis of peptidoglycan,
wall-lles bacteria are often produced
• Usually these treatments generate non-viable
organisms. Wall-less bacteria that can not replicate
are referred to as sphero-plasts (when an outer
membrane is present) or protoplasts(if an outer
membrane is not present).
• L-form bacteria, also known as L-phase bacteria, L-phase
variants, and cell wall-deficient (CWD) bacteria,
are strains of bacteria that lack cell walls
Cell membrane
• Site of biosynthesis of DNA, cell wall polymers and membrane lipids.
Selective permeability and transport of solutes into cells
• Electron transport and oxidative phosphorylation
• Excretion of hydrolytic exoenzymes
Mesosomes
Mesosomes are folded invaginations in the plasma
membrane of bacteria that are produced by the chemical fixation
techniques used to prepare samples for electron microscopy.
Although several functions were proposed for these structures in the
1960s, and are no longer considered to be part of the normal structure of
bacterial cells.
•
Cytoplasm
Composed largely of water, together with proteins, nucleic
acid, lipids and small amount of sugars and salts
•
Ribosomes: numerous, 15-20nm in diameter with 70S; Distributed throughout the
cytoplasm; sensitive to streptomycin and erythromycin site of protein synthesis
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Plasmids: extra-chromosomal genetic elements
Inclusions: sources of stored energy, e,g volutin
Plasmid
• Plasmids are small，circular/line，
• extrachromosomal，double-stranded DNA molecules。
• They are capable of self-replication and contain genes that confer
some properties，such as antibiotic resistance，virulence factors。
• Plasmids are not essential for cellular survival.
Cell Inclusions and Storage Granules
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Bacteria have different methods of nutrient
storage that are employed in times of plenty, for
use in times of want.
Inclusions are aggregates of various compounds
that are normally involved in storing energy reserves
or building blocks for the cell
Inclusions accumulate when a cell is grown in
the presence excess nutrients and they are
often observed under laboratory conditions.
Nucleus
• Lacking nuclear membrane
• absence of nucleoli, hence
known as nucleic material or nucleoid
• one to several per bacterium.
Capsules and slime layers
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These are structures surrounding the outside of the cell envelope.
They usually consist of polysaccharide; however, in certain bacilli they are
composed of a polypeptide (polyglutamic acid).
They are not essential to cell viability and some strains within a species will
produce a capsule, whilst others do not.
Capsules are often lost during in vitro culture.
Functions
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Attachment
Protection from phagocytic
engulfment.
Resistance to drying.
Depot for waste products.
Reservoir for certain nutrients.
Protection
A bacterial capsule is similar, but is a well
ordered structure that is resistant to washing off
Flagella
• Some bacterial species are mobile and possess
locomotory organelles - flagella.
• Flagella consist of a number of proteins including flagellin
• The diameter of a flagellum is thin, 20 nm, and
long with some having a length 10 times the diameter of cell.
• Due to their small diameter, flagella
cannot be seen in the light microscope unless a special stain is applied.
• Bacteria can have one or more flagella arranged in clumps
or spread all over the cell.
Functions
• Identification
• of Bacteria
• Pathogenesis
• Motility of bacteria
Flagellum Structure
Pili
• Pili are hair-like projections of the cell
• They are known to be receptors for certain
bacterial viruses
• Chemical nature is pilin
• Classification and Function
a. Common pili or fimbriae: fine , rigid numerous,
related to bacterial adhesion
b. Sex pili: longer and coarser, only 1-4, related to
bacterial conjugation
Endospores(spores)
• A bacterial spore is a spore or spore- like structure
produced by bacteria
• Spore formation in bacteria is not a method of
reproduction but simply a method of surviving in an
unfavorable conditions
• Produced when starved
• Contain calcium dipicolinate
• DPA, Dipicolinic acid
• E.g.Bacillus and Clostridium
• Dormant cell
• Resistant to adverse conditions
- high temperatures
- organic solvents
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