Bacterial structure

1. WELCOME

2. Basic bacteriology
&
Bacterial structure
Dr. Umma Israt Jahan Sani
MBBS, M. Phil (Microbiology)

3. Infectious agents for human beings
Prokaryotic: Eukaryotic: Others
-Bacteria -Algae -Virus
-protozoa -Prions
-Fungi
-Helminths
-Slime molds

4. Difference between Prokaryotic and
Eukaryotic cells:
Structure Prokaryotic
cells
Eukaryotic
cells
DNA within a nuclear membrane No Yes
Mitotic division No Yes
DNA associated with histones No Yes
Chromosome number One More than one
Membrane bound organelles No Yes
Size of ribosome Smaller Larger
Cell wall containing peptidoglycan Yes No

6. Koch’s postulate:
• 1877 Robert Koch proposed a series of postulates
that have been applied broadly to link many specific
bacterial species with particular diseases.
 The organism must be isolated from every patient
with the disease.
 The organism must be isolated free from all other
organisms and grown in pure culture in vitro.

7.  The pure organism must cause the disease in a
healthy, susceptible animal.
 The organism must be recovered from the inoculated
animal.
An additional fifth criterion was antibody to the
causative organism should be demonstrable in
patient’s serum.

8.  limitations of Koch’s postulates:
-Causative agents cannot be identified in genetic &
auto immune disease.
-Some microbes cannot be cultured in artificial
media e.g. M. leprae, T. pallidum.
-Some microbes cannot produce disease in
experimental animal e.g. N. gonorrhoeae.

9. Molecular Koch’s postulates:
• It was a modification of Koch’s postulates by the
microbiologist Stanley Falkow (1988).
• He stated that gene (coding for virulence) of a
pathogenic microorganism that contributes to the
disease should satisfy all the criteria of Koch’s
postulates rather than the microorganism itself.
 The virulence trait under study should be associated
much more with pathogenic strains of the species
than with nonpathogenic strains.

10.  Inactivation of the gene associated with the
suspected virulence trait should substantially
decrease pathogenecity.
 Replacement of the mutated gene with the normal
wild type gene should fully restore pathogenecity.
 The gene should be expressed at some point during
the infection and disease process.
 Antibodies or immune system cells directed against
the gene products should protect the host.

11. Structure of Bacteria
Essential structures: Nonessential structures:
-Cell wall -Capsule
-Plasma membrane -Pilus/Fimbria
-Ribosome -Flagellum
-Nucleiod -Spore
-Mesosome -Plasmid
-Periplasm -Granule
-Glycocalyx

12. Essential structure:
• Nucleiod:
-No definite structure
-No true nucleus, no nuclear membrane
-No mitotic apparatus
-Single long molecule of double stranded DNA
-No introns in DNA
• Ribosome:
- Site of protein synthesis.
-Composed of rRNA and ribosomal proteins.
-70S in size with 50S and 30S subunit.

13. • Cell membrane:
-Phospholipid bilayer & protein
 Functions:
-Selective permeability &
transport of solutes.
-Electron transport & oxidative
phosphorylation.
-Excretion of hydrolytic enzymes.
-Biosynthetic function.

14. • Cell wall:
-Tough & rigid structure surrounding the bacterium.
 Functions:
-Osmotic protection & gives rigid support.
-Maintains shape of bacteria.
-Helps in cell division.
-Protects cell from toxic substances & is the site of
action of several antibiotics.
-Various components are antigenic.
-Contains certain virulence factors(e.g endotoxin)

15. Gram positive cell wall: It consist of-
i. Thick multilayered peptidoglycan
ii. Special components:Teichoic
and lipoteichoic acids.
iii. Other components:
Certain Gram positive cell wall
contains antigens
(e.g. polysaccharide and protein)

16. i) Peptidoglycan:
• Single bag-shaped giant macromolecule
• Highly cross linked
• Synonomously called-murien or cell wall
• 2%- 40% of the dry weight of cell wall
• Complex polymer consisting of Glycan & Polypeptide
• It has 3 parts: -Backbone
-A set of identical tetrapeptide side
chain to N acetyl muramic acid
-A set of identical peptide cross bridge

17. • Backbone /glycan same in all bacterial species
constituted of the two amino sugar:
N-acetyl glucosamine &
N-acetyl muramic acid
joined together by β- 1,4 linkage

18. • Tetrapeptide side chain
& vary from species to species
Peptide cross bridge
• Tetrapeptide side chains:
position 1: L alanin
position 2: D glutamate
position 3: most variable
position 4: D alanine
(Gram positive cell wall usually have
L- lysin and most Gram
negative bacteria have diaminopimelic
acid at position 3 )

20. • Cross bridge:
In Gram positive bacteria- pentapeptide cross bridge
In Gram negative bacteria- direct

21. ii) Teichoic acid:
• Polymers of glycerol phosphate or ribitol
phosphates residues.
• Two types:
i) Wall Teichoic acid – linked to
peptidoglycan.
ii) Membrane Teichoic acid- linked
to membrane glycolipid.It intimately associated
with lipids and called lipoteichoic acid.

22. • Together with peptidoglycan, WTA & LTA make up a
network or matrix that provides function relating to
the elasticity, porocity, tensile strength &
electrostatic properties of the envelop.
• Function:
-Major surface antigen
-Helps in surface attachment
-Used as antigenic determinants.

23. Gram negative cell wall:
• Possess two membranes: inner cytoplasmic
membrane & outer membrane.
• Peptidoglycan layer is located between two
membranes.
• It has:
i) Periplasmic space
ii)A thin single layered peptidoglycan
iii)Outer membrane

25. I. Periplasmic space:
-Space between the inner and outer membrane.
-contains the peptidoglycan layer and
-Also contains many hydrolytic enzymes (e.g
alkaline phosphatase) and detoxifying enzymes (e.g
β- lactamase)
II) Peptidoglycan:
III) Outer membrane:
-bilayered structure; inner leaflet similar to
cytoplasmic membrane &outer leaflet contains
lipppolysaccaride(LPS)

26. • lipppolysaccaride(LPS): It consists of-
 O antigen:
-outer
polysaccharide
composed of 25
repeating subunits
of 3 to 5 sugars.
-Used as antigen to
identify the bacteria
-Neisseria genus has
Lipooligosaccharide.

27.  Core polysaccharide:
-Five sugars linked through ketodeoxyoctulonate
(KDO) to lipid A.
 Lipid A:
-Composed of phosphorylated glucosamine
disaccharide attached to a long chain fatty acid
-Responsible for toxic effects.
Lipoprotein:
-Cross-link the outer membrane and peptidoglycan
layer.
-Function is to stabilize the outer membrane and
anchor it to the peptidoglycan layer.

28. Differences between Gram positive and Gram
negative cell wall:
Characters Gram positive cell
wall
Gram negative cell
wall
Peptidoglycan layer Thicker; multilayer Thinner; single layer
Teichoic acid present Absent
Outer membrane Absent Present(Phospholipids
with saturated fatty
acid)
Periplasmic space Absent Present

29. Figure: Gram positive and Gram negative cell wall

30. Bacteria with defective cell wall:
• Protoplasts:
-Complete removal of the cell wall of Gram positive
bacteria results in the formation of protoplasts.
-Do not multiply and can not revert to normal
bacterial morphology by forming cell wall.
• Spheroplasts:
-Gram negative bacteria with damaged cell walls.
-Some cell wall material is retained by spheroplasts.
-In suitable environment they may multiply .

31. • L forms:
-These cell wall deficient forms of bacteria may
emerge spontaneously or by inhibition of cell wall
synthesis( during antibiotic therapy)
- Do not have regular size and shape due to lack of
rigid cell wall
- Capable of growing and multiplying in a suitable
culture medium.
- L forms may produce chronic infection and L forms
infections are resistant to antibiotic therapy.

32. Non essential structure:
I. Capsule:
-Many bacteria synthesize large amount of
extracelluar polymer which forms a condensed well
defined layer surrounding the cell.
- synthesized material is usually
polysaccharide(except Bacillus anthracis-
polypeptide)
-if this material is loosely bound and amorphous, it’s
called slime layer/ glycocalyx
-S. pneumoniae, B. anthracis etc are capsulated
organism

33. • Function:
-Major virulence factor-prevent phagocytosis
-Antigenic:
 Specific identification of bacteria
 Diagnosis of disease
 Vaccine production
-May help in adhesion
• Demonstration:
-By ordinary stain, negative stain & immunological
method

34. • Importance of Glycocalyx/slime layer:
-Helps in firm adherence
-Plays important role in the formation of plaque
II) Flagella:
-Thread like appendages composed of entirely of
protein
-Organ of locomotion
-1 to 20/ cells
-Originate in protoplasm

35. • Types(On the basis of arrangement):
Monotrichous: V.cholerae
Lophotrichous: H.pylori
Amphitrichous: Pseudomonas
Peritrichous: Escherichia coli

36. • Clinical significance of flagella:
1. Antigenic role:
-Some bacteria are identified in laboratory by using
specific antibodies against flagellar protein e.g
Salmonella species
2. Pathogenic role:
-It may play a role in pathogenesis by propelling the
bacteria up the urethra into the bladder causing UTI
e.g Escherichia coli, Proteus species

37. III) Pili/Fimbria:
• Rigid surface appendages.
• Shorter & finer than flagella.
• Composed of subunits of
protein called pilin.
• Originate in plasma membrane
of Gram negative bacteria
(Mainly).
• Types: -Ordinary pili
-Sex pili

38.  Ordinary pili:
-It plays a role in the adherence of symbiotic &
pathogenic bacteria to host cells.
-Antigenic in variation.
 Sex pili:
- Takes part in genetic transfer during conjugation.

39. IV) Plasmids:
-Plasmids are extra chromosomal genetic material that is
capable of self replication(Independently of bacterial
chromosome)
• Importance of plasmid:
Plasmid carry the genes for the following functions &
structures of medical importance
-Antibiotic resistance
-Resistance to heavy metals
-Resistance to UV light
-Synthesis of pili
-Formation of exotoxin

40. • Types of plasmid:
a) On the basis of capacity of transmission
i. Transmissible: It can be transferred from cell to cell
by conjugation
ii. Non-transmissible : They do not contain transfer
gene.
b) On the basis of genetic content:
i. R plasmid: transfer of drug resistant gene
ii. Colicenogenic plasmid : production of bacteriocin
iii. Virulence plasmid : Toxin in Anthrax & Enterotoxin

41. Spore:
• Spore is a resting state of bacteria
• Produced during unfavorable condition
• Produced by members of only two genera of bacteria
of medical importance, Bacillus and Clostridium
• Highly resistant to:
-Desiccation
-Heat
-Radiation&
-Chemical agents

42.  Structure of endospore:
1)Core: Spore protoplast
2)Spore wall: Inner
most layer surrounding
the spore.
3)Cortex: Thickest layer
of core envelop.
4)Coat: Composed of
Keratin like protein
5)Exosporium: Lipoprotein envelop

43. • Marked resistance is due to-
-Very little water content
-No metabolic activity
-Thick keratin-like coat
-Presence of large amount of Ca-dipicolinate
 Sporulation:
• The process of formation of spores from vegetative
stage of bacteria.
• It is not a method of reproduction.

44. • It is a complex process of seven stages:
i. Axial filament formation
ii. Septum formation
iii. Engulfment of forespores
iv. Cortex synthesis
v. Formation of protein coat and exosporium
vi. Maturation of spores
vii. Release

46.  Germination:
• Transformation of dormant spores into active
vegetative cells when grown in a nutrient-rich
medium.
• It comprises of three stages:
1. Activation
2. Germination
3. Outgrowth

47.  Shape and position of spores:
• Position:
Spores may be central,
subterminal or terminal.
• Shape:
-They may be oval or
spherical.

48. Thank you