‫‪Structure of Bacteria‬‬
‫كلية العلوم الطبيه التطبيقيه‬
Size of Bacteria
Average bacteria 0.5 - 2.0 um in diameter
RBC is 7.5 um in diameter
Shapes and Cellular arrangement of Bacteria
1. Coccus (spherical shape)
–
–
–
–
Diploid
No cellular arrangemnt
Chain = Streptoccus
Cluster = Staphylococcus
2. Bacillus
– Chain = Streptobacillus
3. Coccobacillus
4. Vibrio = curved
5. Spirillum
6. Spirochete
Shapes of Bacteria
Spherical Bacteria
Coccus (diploid & Chain)
Coccus (Cluster)
Bacillus (Rod-Shaped Bacteria)
Shape of Bacillus :
- Long bacillus
- Short bacillus
- Thin bacillus
- Fat bacillus
- Cigar shape
Cellular arrangement:
-No cellular arrangement
- Diploid
- Chains
Bacillus (Rod-Shaped Bacteria)
Spiral-Shaped Bacteria
Prokaryotic vs. Eukaryotic Cells
• Prokaryotic cells
– No Nucleus
– No Organelles
– Cell Wall of
peptidoglycan
– Binary Fission
– 1 circular
chromosome
• Eukaryotic Cells
– Nucleus
– Organelles
– If cell wall,
Cellulose or chitin
– Mitosis
– Linear
chromosomes
Eukaryote Cell Structure
Bacterial Cell Structure
Bacterial cell structure includes:
Essential components:
basic structures present in all bacteria, e.g. cell
wall, cytoplasmic membrane, cytoplasm and
nuclear material.
Non essential components:
present in some bacteria species, e.g. capsules,
fimbriae, flagella and spores.
Bacterial Cell Structure
Bacterial Cell Structure
•
•
•
•
•
•
•
•
•
•
Flagella
Pili
Capsule
Plasma Membrane
Cell Wall
Cytoplasm
Inclusions
Ribosomes
Chromosome
Plasmid
Flagella
• Motility – movement
- Flagella are the organs of motility
• Swarming occurs with some bacteria
- Proteus species most evident
• They arise from cytoplasm and extrude through
cell wall.
They are formed of flagellin protein which is
antigenic (H antigen).
They can not be stained by Gram stain.
They have special flagellar stain.
Chapter 4
Motility
• Almost all Spiral bacteria are motile
• About 1/2 of Bacilli are motile
• Almost all Cocci are non-motile
Flagella Arrangement
A. Monotrichous (1 flagellum at one end)
B. Lophotrichous (tuft at one end)
C. Amphitrichous 2 flagella (1 at each end)
D. Peritrichous (all around bacteria)
Flagella Arrangement (cont)
Monotrichous
Lophotrichous
Amphitrichous
Peritrichous
Chapter 4
Pili (Fimbriae)
• Short protein appendages formed of protein
called pillin which is antigenic
– smaller than flagella
Function
1- Adhere bacteria to surfaces (Adhesion is a
virulence factor)
– E. coli has numerous types: K88, K99, F41, etc.
2- F-pilus; used in conjugation (Sex pili)
– Exchange of genetic information
3- Flotation
Chapter 4
F-Pilus for Conjugation
Capsule or Slime Layer
• It is present only in some bacteria outside the cell wall.
• It is gelatinous in nature.
The capsule may be
- Glycocalyx polysaccharide as in pneumococci,
meningococci
- polypeptide as in Bacillus anthracis
- hyaluronic acid as in streptococc.
• Adherence of bacteria to surface
- Streptococcus mutans and enamel of teeth causing dental caries.
• The capsule is antigenic.
• Prevents Phagocytosis
- The capsule has antiphagocytic function
so it determines the virulence of many
bacteria.
Cell Wall- Deficient Bacteria:
It may be naturally occurring or induced
Naturally occurring:
e.g. Mycoplasma is the only group of bacteria
that exist naturally without cell wall
- It has no defined shape due to lacking the rigid
cell wall.
- It is resistant to antibiotics which inhibit
bacterial cell wall synthesis, e.g. penicillin.
Induced:
removal of the bacterial cell wall may be
accomplished by hydrolysis with lysozyme or by
blocking
peptidoglycan
biosynthesis
with
antibiotics.
This process liberates various abnormal forms
called spheroplasts, protoplasts and L-forms.
Prokaryotes - Cell Wall
From the peptidoglycan inwards all bacteria are very similar. Going further out,
the bacterial world divides into two major classes (plus a couple of odd types).
These are:
Gram-positive
Gram-negative
Images: PHIL Public Health Image Library
Cell Wall
- Outermost component of bacterial cell
- Common in all bacteria except mycoplasma which has no
cell wall
Function:
1- It maintains the shape of the cell (cocci,bacilli…
etc).
2- It supports the weak cytoplasmic membrane.
3- It plays a role in cell division.
4- It is responsible for passive diffusion of fluid and
nutrients and for staining properties of the
organism.
Cell Wall
lipopolysaccharide
teichoic acid
protein
peptidoglycan
phospholipid
Gram-negative
Gram-positive
Diagrams of the cell wall structure of Gram-negative (left) and Gram-positive
bacteria. Key: peptidoglycan layer (yellow); protein (purple); teichoic acid
(green); phospholipid ( brown); lipopolysaccharide (orange). (Used by
permission of P. Sforza)
Gram (+) Cell Wall
•
•
•
•
•
NAM
N-acetylmuramic acid
NAG
N- acetylglucosamine
tetrapeptide side chains
pentaglycine crossbridges
teichoic acid
Gram (-) Cell Wall
•
•
•
•
•
NAM
NAG
Tetrapeptide side chains
pentaglycine
2nd Outer membrane
– Lipopolysaccharides (LPS)
• Lipid A
• O Antigen
Chapter 4
In Gram- positive bacteria:
The cell wall consists mainly of peptidoglycan
and teichoic acids.
In Gram- negative bacteria:
The cell wall consists of peptidoglycan, outer
membrane, periplasmic space, lipoprotein,
and lipopolysaccharides.
Gram-negative bacteria
Gram-positive bacteria
Cell wall of Gram positive bacteria:
It is formed of two layers:
1- Peptidoglycan:
It is a thick layer which constitutes up to50%
of cell wall thickness.
It is formed of 40 sheets of alternating Nacetyl
muramic
acid
and
N-acetyl
glucosamine.
The sheets are connected by identical short
chain of 4 amino acids (tetrapeptides) attached
to muramic acids and a set of identical cross
linking peptide bridges.
The peptidoglycan layer is responsible for
rigidity of the cell wall.
Some antibiotics effect directly e.g. Penicillin
2- Teichoic acid:
It is formed of polymers of glycerol or ribitol
phosphate.
It is found in the cell wall and cytoplasmic
membrane.
It is the major surface antigen of Gram-
positive bacteria.
Gram Positive cell wall
Cell wall of Gram negative bacteria:
It is composed of:
1- Peptidoglycan:
It is a thin layer which constitutes up to 10.5%
of Gm negative cell wall.
It is formed of 1- 2 sheets of N- acetyl
muramic acid and N- acetyl glucosamine
connected by identical tetrapeptides attached
to muramic acid and a set of identical cross
linking peptide bridges.
2- Outer membrane:
•
It is formed of bilayered phospholipids that
resemble
in
composition
that
of
cell
membrane.
•
It has special channels, consisting of
proteins called porins which allow passive
diffusion
of
low
molecular
weight
compounds like sugar, and amino acids.
3- Periplasmic space:
It is the space between the inner cytoplasmic
membrane and outer membrane where the
1-2 sheets of peptidoglycan layer are present.
It also contains gel-like solution of protein.
4- Unusual lipoproteins layer:
It stabilizes the outer membrane layer with
tetrapeptide side chains of peptidoglycan.
5- Lipopolysaccharide (LPS)
- Attached to outer membrane
- Consist of 3 parts:
i.
Inner lipid A
ii. Middle polysaccharides Core.
iii. Outer polysaccharides side chains
eg. O Antigen of E. coli, Salmonella
LPS structure
• O Antigen of Salmonella and E. coli
– 2,000 different O Ags of Salmonella
– 100’s different O Ags of E. coli
• E. coli O157
• O Ags differ in Sugars, not Lipid A
• Endotoxin Fever causing
– Toxin nomenclature
• Endo- part of bacteria
• Exo- excreted into environment
• Only in G-ve bacteria
• Appearance of Colonies
– Mucoid = Smooth (lots of LPS or capsule)
– Dry = Rough (little LPS or capsule)
Lipopolysaccharide LPS
• Functions
– Toxic; kills mice, pigs, humans
• G- septicemia; death due to LPS
– causes fever
• DPT vaccination always causes fevers
– stimulates immunity
- Heat Resistant; hard to remove
Gram negative cell wall
G+. G• G+
– Thicker cell wall
– Teichoic Acids
• G– Endotoxin - LPS
• Which are more sensitive to Penicllin?
• Alcohol/Acetone affects which more?
Cytoplasmic Membrane:
- phospholipids bilayer that contains protein.
- similar to eukaryotic cell membrane but does not
contain sterol except in Mycoplasma.
- Water can penetrate
- Flexible
- Not strong, ruptures easily
Osmotic Pressure created by cytoplasm
phospholipids bilayer
Cytoplasmic Membrane
Cytoplasmic Membrane
Functions of cytoplasmic membrane:
1- Selective permeability
2- Excretion of extracellular enzymes and toxins
(pathogenicity proteins) e.g. enzymes that
destroy harmful chemicals to bacteria e.g.
antibiotics.
3- Respiration
4- Plays an important role in cell division.
5- Excretion of hydrolytic enzymes which
digest large food molecules into subunits
small enough to penetrate the cell.
Cytoplasm:
- viscous watery solution or soft gel
- contains a variety of organic and inorganic
solute.
- No organelles (Mitochondria, Golgi, etc.)
a. Mesosomes:
- invaginations of cytoplasmic membrane
Functions of Mesosomes:
- site of attachment of DNA chromosome during cell
division.
- site of respiratory activity of the cell
- Increases surface area of the membrane, thus
increases efficiency of active transport.
b. Ribosomes:
- complex structure composed of 60% RNA and
40% protein
- site of protein synthesis in the cell
Ribosomes - protein synthesis
• Prokaryotic Ribosome
• Eukaryotic Ribosomes
• 70 S
• 80 S
– 50 S
– 30 S
– 60 S
– 40 S
Ribosomes
Ribosomes & Selective Toxicity
• Some antibiotics are aimed at the 70 S
ribosomes of bacterial cells
• Streptomycin, Neomycin, Erythromycin and
Tetracycline work by inhibiting protein
synthesis by disrupting the 70 S ribosome
c. Inclusion granules
• intracellular storage bodies
• vary in size, number & content
• bacterial cell can use them when
environmental sources are depleted
• Examples: glycogen, poly-bhydroxybutyrate, gas vesicles for floating,
sulfur and polyphosphate granules
c. Inclusion granules:
- Round granules observed in cytoplasm of
many bacteria.
- not permanent or essential structure
- either stored energy or nutrient reserve
concerned with cell metabolism
d. Plasmids:
Extra chromosomal double stranded circular DNA
They have the following characteristics:
i)
Dispensable: not necessary for life of the cell
ii) Autonomous: multiply independent of the host.
iii) Transmissible: can transfer to other bacteria by
conjugation, transformation, or transduction.
Inclusions
66
e. Bacterial Genome (nucleotides):
- single supercoiled circular double stranded DNA
molecule chromosome
- chromosome carries genes that control the bacterial
properties and pathogenecity
- nuclear body does not have nuclear membrane,
mitotic apparatus, or histones
Bacterial Spores (Endospores):
Some bacterial genera are capable of forming
highly resistant resting phase or endospores,
e.g. Bacillus group and Clostridium group.
Endospores
• Resistant structure
– Heat, irradiation, cold, dryness, disinfectants
– Boiling >1 hr still viable
• Takes time and energy to make spores
• oval or round
• Location important in classification
– Central, Subterminal, Terminal
Central
- Spore formation occurs in response to
unfavorable conditions
e.g.
depletion of nutrition
accumulation of metabolites
unsuitable temperature
- spores are formed outside the body, and can
not be stained by ordinary stains.
- Do not reproduce and exhibit absolute dormancy
- When the unsuitable conditions change, the spore
germinates to the vegetative form which can multiply
- Bacillus stearothermophilus -spores
Used for quality control of heat sterilization
equipment
- Bacillus anthracis - spores
Used in biological warfare
Stages of sporulation:
- The plasma membrane invaginates enclosing
section of cytoplasm that contains bacterial
chromosome, some ribosomes and other
cytoplasmic materials.
- It acquires a thick covering layer (cortex)
and hard protective coat formed
of calcium dipicolinate.
Endospore structure
Many layers from the outside:
1. exosporangium – thin delicate coating of protein
2. multi-layered spore coats – spore-specific proteins
3. cortex – loosely cross-linked peptidoglycan
4. core or spore protoplast – usual cell
• Unique substance found only in spore core – dipicolinic
acid
• Spores also high in calcium – most combined with
dipicolinic acid
Endospore structure
Chapter 4