Bacterial Cell Morphology

Introduction to Bacteria
 Bacteria constitute a large domain of prokaryotic
microorganisms.
 They are unicellular
 They are prokaryotic cells because they do not
have well developed nucleus.
 Despite their simplicity they contain a well
developed cell structure which is responsible for
many of their unique biological properties.
 The most elemental structural property of bacteria
is its cell morphology.

What is ‘Morphology’ of Bacteria?
The morphology of bacteria
describes the external appearance
of bacterial cells including their
shape, arrangement and size.

In the year 1872 scientist Cohn classified bacteria to 4 major
types depending on their shapes as follows –
A. COCCI –
They are unicellular and spherical or elliptical in shape.
They may either remain as a single cell or may aggregate
together for various configurations as follows -
 Monococcus –They are also called Micrococcus and are
represented by single, discrete, round cells.
Example – Micrococcus flavus
 Diplococcus – The cells of Diplococcus divides once in a
particular plane and after division the cells remain attched to
each other. Example – Diplococcus pneumonia
Classification of Bacteria on the
basis of morphology

 Streptococcus – Here the cells divide repeatedly in one plane to
form a chain of cells.
Example – Streptococcus pyogenes
 Tetrad – This consists of four round cells, which defied in two
planes at right angles to
one another.
Example – Gaffkya tetragena.
 Staphylococcus – Here the cells are divided into three planes
forming a structure like
bunches of grapes giving an irregular configuration.
Example – Staphylococcus aureus
 Sarcina – In this case the cells divide in three planes but they form
a cube like configuration
consisting of eight or sixteen cells but they have a regular
shape.
Example – Sarcina lutea

B) BACILLI– They are rod shaped or
cylindrical which either remain singly or
in pairs.
Example – Bacillus cereus
C) VIBRIO– They are curved, comma shaped
and are represented by a single genus.
Example – Vibrio cholerae
D) SPIRILLA - They are spiral or spring like
with multiple curvature and terminal
flagella..
Example – Spirillum voluntans

Overview of Various structures of Bacterial cell

Bacterial cell wall
 The bacterial cell wall is uniquely
characterized by the presence of
peptidoglycan made up of a
polysaccharide backbone of
alternating N-Acetylmuramic acid
(NAM) and N-acetylglucosamine
(NAG) residues.
 It is located immediately outside of
the cell membrane and is responsible
for the rigidity ,determination of cell
shape and transport of substances
due to porous nature.

Bacterial cell membrane
Composed of a phospholipid bilayer and thus has
all of the general functions of a cell membrane such
as acting as a permeability barrier for most
molecules.
Unlike eukaryotes, bacterial membranes (with
some exceptions e.g. Mycoplasma and
methanotrophs) generally do not contain sterols.
However, many microbes do contain structurally
related compounds called hopanoids which likely
fulfill the same function.
Since the bacteria lack membrane bound
organelles all the membrane associated functions
are performed in the plasma membrane.

Gram positive & Gram negative bacteria
Apart from the general view, these two
forms of bacteria differ greatly in structure
of cell wall and cell membrane:
Gram-positive cell walls are thick and
the peptidoglycan (also known as
murein) layer constitutes almost 95% of
the cell wall in some gram-positive
bacteria and as little as 5-10% of the cell
wall in gram-negative bacteria.
The presence of teichoic acid is also an
exclusive feature of cell wall of gram
positive bacteria.

 In addition to the cell membrane and the
cell wall, the cellular envelope of Gram
negative bacteria has an additional
outer membrane located outside the
peptidoglycan cell wall.
 The outer membrane of gram negative
bacteria is not a phospholipid bilayer,
as the phospholipid is confined to it's
inner leaflet.
 The outer leaflet consists of glycolipids,
principally lipopolysaccharide (LPS)
which can sensitize the human innate
immune system.

Some of the proteins of outer
membrane act as porins for
passive diffusion of small
molecules such as mono- and
disaccharides and amino acids
across it.
 The outer and inner membrane
delimit an aqueous cellular
compartment called the
periplasm which is densely
packed with proteins and it is
more viscous than the cytoplasm.

GRAM STAINING
The primary stain (crystal violet) binds to peptidoglycan present in both gram-positive and gram-
negative cell walls, so initially, all the bacterial cells stain violet.
 Gram's iodine (iodine and potassium iodide) is applied as a mordant or fixative.
Alcohol or acetone is then used to decolorize the cells. Gram-negative bacteria have much less
peptidoglycan in their cell walls, so this step essentially renders them colorless, while only some of
the color is removed from gram-positive cells.
A counterstain safranin is applied. Both gram-positive and gram-negative bacteria pick up the pink
stain, but it is not visible over the darker purple of the gram-positive bacteria, which remain
purple, while gram-negative bacteria shows pink colouration.

Cytoplasmic matrix
Cytoplasmic matrix is a crystallo-colloid complex.
It is composed of
 80 %water &
 20% solid substances
Solid substances are carbohydrate, enzyme, lipid,
protein, inorganic ions and low molecular weight
components.
Membrane bound cell organelles (mitochondria,
endoplasmic reticulum etc.) are absent.
It is rich in ribosomes.
Cytoplasmic streaming is absent.

Cytoskeleton System
Bacteria lack a true cytoskeleton system.
But they do have some Protein which
functions like the eukaryotic cytoskeletal
protein:
Tubulin Homologues:- Two types of
tubulin homologues FtsZ and BtubA/B, have
been identified.
FtsZ I essential for bacterial
cytokinesis, assembles at the cell division
site into circumferential ring, the Z-ring,
located at the inner surface of plasma
membrane.

 Actin Homologues:- Three best studied
actin like cytoskeletal proteins are MreB,
ParM & MamK.
 The extended coil structure of MreB
proteins are located on the undersurface of
plasma membrane and are important in
regulation of cell shape, chromosome
segregation and cell polarity by
localizing the proteins.
 ParM function as a partitioning protein in
plasmid segregation.
Intermediate filament homologues :-
Crescentin is the only cytoskeletal
Intermediate filament thus far identified in
bacterial cell. It is reponsible for the shape of
the comma shaped organism C.
crescentus.

Bacterial Cytoskeletal Proteins
Other than the homologues proteins, there are
some unique bacterial cytoskeletal proteins
present in the bacterial cytoplasmic matrix,
which are:
MinD :- It prevents polymerization of FtsZ
at cell poles (specially in many rod shaped
bacteria).
ParA:- It segregates chromosome and
plasmid (observed in many species including
vibrio cholerae).

Internal membrane system
Internal membrane system are basically circular to villiform
specilization of plasma membrane that develops as an ingrowth
from the cell membrane. This structure is called mesosome.
It has the shape of vesicle, tubule and lamelle.
It is abundant in gram positive bacteria than gram negative
bacteria.
The recent study confirmed that hydrogen peroxide is
involved in meosome formation and excess H2O2
accumulation is associated with mesosome size.
May form in bacteria as a defensive mechanism of
enviornmental harsh condition.
Depending on location and function, mesosome can be
divided in two types.
• Septal or central mesosome.
• Lateral or peripheral mesosome.

Septal mesosome
Connects nucleoid with plasma
membrane (long invagination).
It takes part in replication of
nucleoid by providing point of
attachment of the replicated one.
It helps in cell wall formation by
secreting enzymes such as
dehydrogenase.
At the time of cell division, plasma
membrane grow in the region where the
septal mesosome is present. So that it
provides membrane for rapid
elongation

Lateral mesosome
Lateral mesosome shows a shallow
penetration into the cytoplasm and
thus is not connected with nucleoid.
It contains respiratory enzymes and
therefore is called “chondriod” and
believed to be equivalent to the
mitochondria of eukaryote.
It plays a role in electron transport
chain, phosphorylation and
oxidation-reduction reactions.

Internal membrane system
 Many bacteria have internal membrane
system quite different from mesosome.
They may be spherical vesicle, flattened
vesicle or tubular membrane.
 Plasma membrane infoldings can become
extensive and complex in photosynthetic
bacteria such as the cyanobacteria and
purple bacteria or in bacteria with very
high respiratory activity.
 Its function may be to provide larger
membrane surface for greater metabolic
activity.

Inclusion Bodies
 May occur freely inside the Cytoplasm
(e.g Cyanophycean Granules, Volutin
Or Phosphate Granules) or covered by
2-4nm thick non-lipid, non-unit protein
membrane (e.g Gas Vacuoles, Sulphur
Granule and Carboxysome).
 On the basis of their nature, Inclusion
Bodies are of 3 types
~ Gas Vacuoles.
~ Inorganic Inclusion.
~ Food Reserve

Gas Vacuoles
 These are gas storing vacuoles,found in
Cyanobacteria,Purple and Green Bacteria,
which helps to protect the bacteria from
harmful radiation.
 Gas Vacuole consists of variable number of
hexagonal,hollow and cylindrical Gas
Vesicles.Each Gas Vesicle is surrounded by a
single non-unit, non-lipid protein membrane
having ribs or folds.
 The membrane is impermeable to water
but permeable to atmospheric gases.

Fig: Sulphur granules
Fig: Polyphosphate granules
Inorganic Inclusion
 Because of the ability to pick up different colour with
basic dyes, they are called Metachromatic Granules.
 Two common type of Inorganic inclusion are –
 Volutin Granules are Polymetaphosphates which
function as storage reserve of Phosphate.
 Sulphur Granules occur in bacteria living in sulphur
rich medium which pick up Hydrogen Sulphide for
obtaining reducing power in photosynthesis.
 Aquaspirillum magnetotacticum contains
Magnetosomes which are vesicles having magnetite.
The cytoskeletal protein MamK helps to form
magnetosome chain. Magnetosome helps the bacteria
to orientate themselves along geomagnetic lines.

Storage Inclusion / Food Reserve
Storage of nutrients, metabolic
end product, energy, building
blocks.
 Glycogen Storage.
 Carbon Storage - Poly-beta-
hydroxy butyrate(PHB).
 Phosphate - Polyphosphate(
Volutin).
 Amino Acids - Cyanophycin
Granules.
 Carboxysome - occur in
Photosynthetic forms.

Ribosome
Small membrane less, submicroscopic
ribonucleoprotein entities having a size of 20nm × 14-
15nm.
 Ribosomes generally occur in helical groups called
Polysomes Or Polyribosomes.
 Bacterial ribosome is 70S type where the eukaryotic
ribosome is 80S type.
Subunits of 70S ribosome are- 30S ( one molecule of
16S rRNA) & 50S ( two molecules of 23S & 5S rRNA)
 It is a mechanism to synthesize several copies of same
protein.

Types of ribosome
Free Ribosomes
Occur free in the Cytoplasmic Matrix.
Synthesize protein for Intracellular use.
Fixed Ribosomes
Attached to the Plasma Membrane.
Synthesize protein for Transport to
outside.

 The nucleoid is an irregularly shaped
prokaryotic genetic material, composed of
about 60% DNA, 30 %RNA, 10% protein by
weight.
 For most bacteria, the nucleoid is simply a
region in the cytoplasm; it is not separated
from other components of the cytoplasm by a
membrane.
 Prokaryotes contain single, circle of double
stranded DNA
 Unlike the eukaryotes, prokaryotes use NAPs
or nucleoid associated proteins for
chromosomal supercoiling.
Nucleoid division
during cell division
Chromosome
released from
a gently lysed
E. coli cell.
Note how
tightly
packaged the
DNA must be
inside the cell.
Nucleoid

Plasmid
 Extra-chromosomal, small double
stranded DNA molecule, usually circular in
appearance.
 Relatively have fewer genes, generally less
than 30.
 Replication of plasmid is independent
from chromosomal replication and have no
link to any particular stage of cell cycle.
 Genetic information of plasmid is not
essential to the bacterium. However, many
plasmids carry genes that confer a selective
advantage to the bacterium in certain
environments.

Types of plasmid
Type Function Example Size (kbp)
Fertility plasmid Carry tra-gene for conjugation F-factor 95-100
Resistance
plasmid
Carry antibiotics resistance gene RP4 54
Col plasmid Carry genes that encode for antibacterial
polypeptide called bacteriocins, a protein that
destroy other strain of bacteria.
ColE1 9
Virulence plasmid Carry virulence gene Ti 200
Metabolic
plasmid
Carry genes for enzyme CAM 230

Depending on the ability to transfer to
other bacteria, plasmids may be
Conjugative plasmid: Help in initiating
conjugation
Non-conjugative plasmid: Incapable of
initiating conjugation and can only be
transferred with the assistance of conjugative
plasmid.
Episomes:
Plasmid of a bacteria or viral DNA that can
integrate itself into chromosomal DNA of
host organism and replicated as part of the
chromosome.
Plasmid are inherited during cell division and
some time are lost, called curing.

Plasmid
Components of plasmid:
 An origin of replication
 Multiple cloning sites
 An selectable marker (an
antibiotic resistance gene)
 Promoter site
Use of plasmid:
1. Genetically engineered plasmid (pBR322,pUC18) are used as cloning vector to make copies of
particular genes.
2. Plasmid are used to make a large amount of desired protein by growing bacteria containing plasmid
harboring the gene of interest. e.g. human insulin production using E.coli.

Pilli
• Fine, hairlike appendages that are
thinner and typically shorter than
flagella.
• Slender tubes composed of helically
arranged protein subunits (pilin)
and are about 3 to 10 nm in
diameter and up to several
micrometers long. Pili grow by
adding protein subunits to the base
of the pilus.

Types of Pilli
 The chaperone-usher pilus system:
 liner non covalent multi-subunit polymer that are prime virulence
factor of proteobacteria and contribute to the establishment and
persistence of the infection by mediating host and tissue specific
interaction.

 Type V pilus:
 Play a roll in bacterial addition, co-aggregation and biofilm formation.
 Type IV pilus:
 Multifunctional organelles (help in addition to host cell, biofilm
formation etc.) with a distinguishing ability to extend and retract by
reversable polymerization and depolymerization.
 Powers a flagella independent type of bacterial movement known as
twitching motility.
 Type IV secretion pilus:
 Help in 1. conjugation, 2. DNA uptake (competence).

Flagella
 Bacterial flagellum is a helical filamentous
appendages extended outwards from the plasma
membrane and cell wall, responsible for
motility.
 Rotational movement of flagella is responsible
for run and tumble movement of bacteria:
 counter clock wise movement of flagella:
run movement (forward movement)
 clock wise movement of flagella:
tumble movement (reorientation of the cell)

 Monotrichous: have one flagella if it
is located in one end then it called
polar flagella
 Amphitrichous: have single flagella in
both pole
 Lophotrichous: have a cluster of
flagella at one or both ends
 Peritrichous: Flagella are spread
evenly over the whole surface
Classification of bacteria according to
flagellal distribution

Structure of Flagella
Flagella composed of mainly two parts
1. Basal body
MS rings:
 located in cell membrane
 Composed of transmembrane protein
FliF
C ring:
 Attach to the cytoplasmic face of MS
rings
 Consist of 3 cytoplasmic proteins FliG,
FliM, FliN.
P ring:
 Embedded in the peptidoglycan layer.
 Made up of periplasmic protein FlgI
L ring:
 Embedded in the outer membrane
 Made up of lipoprotein FlgH
Stator unit:
H+ ion coupled MotAB complex
composed of MotA and MotB protein.
2.Rod:
 Proximal rod
composed of FliE,
FlgB, FlgC,and FlgF
 Distal rod composed
of FlgG.
 Rod act as drive shaft.
3.Hook:
 Links filament to its basal body.
 Composed of about 120 copies of
hook protein FlgE.
4.Filament:
 Made up of flagellin protein FliC.
 Undergoes polymorphic.
transformation for changing the
direction.

Assembly of bacterial flagella
 Starts from basal body rings.
 Components of Axial structure are
transported by type III protein
export apparatus
 Type III protein export
apparatus:
• Export gate complex: Composed
of FlhA,FlhB, FliP, FliQ and FliR
• Cytoplasmic ATPase ring
complex: Made up of FliH, FliI and
FliJ.

Chemotaxis: The movement of bacteria
toward or away from chemo attractants or
chemo repellents is called chemotaxis.
 Stator unit ( Mot AB complex) use
proton motive force as a fuel of energy fot
rotational movement.
 Influx of proton allow the Mot A to
associate with and dissociate from
FliG to drive the flagellar motor rotation.
 Rod transmits that torque to filament
through hook
Flagellar Movement

Endospores
Endospores is a heat resistant &
dehydrated structure.
It is capable of surviving for long periods
in an unfavourable environment ( as long
as 20 hrs) & potent to resist environmental
stresses such as ultraviolet radiation,
gamma radiation, chemical disinfectants
and desiccation.
Only found in Gram (+) Bacteria –
• Bacillus cereus • Bacillus anthracisw •
Clostridium tetani
• Clostridium botulinum• Clostridium
perfringens

Variations in endospores morphology
FIG: (1, 4) central endospore; (2, 3, 5) terminal endospore; (6) lateral endospore.

Structure of endospores
The arrangement of spore layers is
as follows:
Exosporium : It is a thin delicate
protein covering.
Spore coat ( acts like a sieve) ; It
lies beneath the exosporium
composed of several protein layers and
may be fairly thick, impermeable &
responsible for the spore’s resistance
to chemicals.

Structure of endospores
Spore cortex : It is present beneath the spore
coat, made up of peptidoglycan.
Core wall or spore cell wall : It is present
beneath the cortex and surrounds the
protoplast or spore core. The spore core has the
normal cell structures such as ribosomes and a
nucleoid , but is metabolically inactive. The
core contains the spore chromosomal DNA
which is encased in chromatin-like proteins
known as SASPs (small acid-soluble spore
proteins), that protect the spore DNA from UV
radiation and heat.

Contents of endospores
Up to 20% of the dry weight of the endospore consists of calcium
dipicolinate within the core, which is thought to stabilize the DNA.
Dipicolinic acid could be responsible for the heat resistance of the
spore, and calcium may aid in resistance to heat and oxidizing agents.
Small acid-soluble proteins (SASPs) are found in endospores. These
proteins tightly bind and condense the DNA, and are in part responsible
for resistance to UV light and DNA-damaging chemicals.

Staining procedures of endospores
Staining is used to determine the
morphology and the location of endospores.
A special stain technique called a Moeller
stain is used. That allows the endospore to
show up as red, while the rest of the cell
stains blue.
Another type of staining of endospores
through Schaeffer-Fulton procedure first
involves heating of bacteria with malachite
green that penetrates endospores and gives
greenish colour. After the malachite
treatment cell is washed with water & is
counterstained with safranin.

Comparison of prokaryotes & eukaryotes

Bacterial Cell Morphology

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