Virulence & Pathogenicity

1. BACTERIAL VIRULENCE
PRESENTED BY:
Dr. Sumesh Kumar Dash
MODERATOR:
Dr. Sunita Kabi

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INTRODUCTION

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• Pathogen – a microorganism capable of causing disease.
• Pathogenicity – Ability of microorganism to induce
disease.
• Virulence – Capacity of bacteria to cause disease
TERMINOLOGY

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• Different strains of same species - varying degree of virulence
• Highly, low, avirulent
• Virulent strains may undergo spontaneous or induced variation
 Exaltation
 Attenuation
VIRULENCE

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Extra cellular factor
1. Fimbriae or Pilli
2. Capsule
3. Afimbrial
Intra cellular factor
1. Toxins
2. Enzyme
3. Secretion system
4. Iron
VIRULENCE FACTOR

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FIMBRIAE or PILLI
CAPSULE
AFIMBRIAL
EXTRA CELLULAR FACTOR

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• Gram negative and many gram
positive bacteria has numerous
thin, rigid, rod-like structures
called fimbriae or pili.
• Made of protein.
• Mediate attachment of bacteria to
cell surface.
FIMBRIAE

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• Type I (Mannose sensitive)
Hemagglutination of untreated RBCs.
E.g. E.coli ,klebsiella ,Serratia, salmonella spp.
• Type II
No adhesiveness
• Type III (Mannose resistant)
Adhesive to RBCs treated with tannic acid/heated to 70 ° C
E.g. klebsiella, serratia
• Type IV (Mannose resistant)
P.aeruginosa
• Type VI
No adhesiveness
E.g. Non fimbriated strains of K.ozaenae
Types

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MAJOR ADHESION FACTORS
• Colonization factor antigens (CFA I, II
and IV)
HOST CELL RECEPTOR
• Sialoglycoprotein (CFA/I)
• Asialo ganglioside GM1 (CFA/II)
• Glycosphingolipid sulphatide (CFA/IV)
PREFERENTIAL GUT SITE
• Jejunal
• Duodenal mucosa
ETEC
Knutton, S.; McConnell, M.M.; Rowe, B.; McNeish, A.S. Adhesion and ultrastructural properties of human enterotoxigenic
Esherichia coli producing colonization factor antigens III and IV. Infect. Immun. 1989, 57, 3364±3371

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• Loose, relatively unstructured
network of polymers that covers the
surface of an organism.
• Prevents desiccation, complement
mediated bacterial cell lysis
• Protects from phagocytosis & action
of lysozyme and bacteriophages
• Eg- Pneumococcus, Meningococcus,
H.influenzae, Klebsiella,
Pseudomonas, Bacillus etc
CAPSULE

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Specialised molecule helps in attachment.
• Lipoteichoic acid ( GPC)
• Protein F , M protein ( Streptococcus)
• Cell surface lectin ( Chlamydia)
• Surface protein (E. coli, Salmonella, Shigella)
• Exopolysaccharide (Streptococcus mutans )
AFIMBRIAL

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 TOXINS
 ENZYME
 SECRETION SYSTEM
 IRON
INTRA CELLULAR FACTOR

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Molecules that are produce by micro organism and
released in order to affect, at distance, a target cell and
tissue in the infected host.
TOXINS

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Bacterial Toxins

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EXOTOXIN ENDOTOXIN
NATURE Proteins LPS
SOURCE
Secreted both by gram-
positive and negative bacteria
Cell wall o f gram
negative bacteria
RELEASED BY
Actively secreted by the
bacteria
Cell lysis
HEAT Labile Stable
EFFECT
Specific action on particular
tissues
Nonspecific (fever,
shock, etc.)
FATAL DOSE
Highly toxic
Fatal in low dose
Weekly toxic
Rarely fatal
ANTIGENIC Highly Poorly
VACCINE
Toxoid forms are used as
vaccine, e.g. tetanus toxoid
No effective vaccine is
available using
endotoxin

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• Typically proteins secreted or release by bacteria during growth.
• Usually produce by single bacterial species of a genus preferably by
virulent strains.
• Strongly antigenic.
• Toxoid are prepared.
EXOTOXIN

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Consist of 2 component
Subunit A
• Responsible for the enzymatic activity.
• Enzymatically active but lack binding & cell entry capability.
Subunit B
• Facilitates binding to a specific receptor and transferring the enzyme.
• Bind to target cells & even block the binding of the native toxin, but they
are nontoxic
Structure

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TOXIN SUBUNIT
Botulinum toxin A/B
Diphtheria toxin A-B
Cholera toxin A-5B
Shiga toxin A-5B
Shiga like toxin A-5B
Pertussis toxin A-5B
Anthrax toxin LF 2A+B

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2 Mechanism. In both case, a large protein molecule insert and cross the
membrane lipid bilayer.
DIRECT ENTRY
• Subunit B binds to a specific receptor on the target cell
• Formation of a pore in the membrane.
• Through which subunit A is transferred into the cell cytoplasm
ALTERNATIVE MECHANISM
• Native toxin binds to the target cell
• Structure is taken into the cell by the process of receptor-mediated
endocytosis (RME )
Attachment & Entry

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• Most potent biological toxin
• Comprise 7 distinct toxins (A to G)
• B-subunit binds to neuroreceptor
gangliosides on cholinergic neuron.
• A-subunit irreversibly inhibit release
of Ach
• Result in paralysis & death
Botulinum toxin

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LETHAL DOSE: 0.1ug/kg
STRUCTUR: 2 subunit
ENTRY: Receptor-mediated endocytosis
MOA: A-subunit inhibit the cell protein synthesis by inactivating EF-2
Diphtheria Toxin

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Shiga toxin

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Cholera toxin

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ENDOTOXIN

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HYALURONIDASE
• Spreading Factor
• Digests hyaluronic acid
(Cement substance between cells)
COLLAGENASE
• Breaks down collagen.
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ENZYME

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IgA PROTEASES
• Split IgA at specific bonds.
• Inactivates Ab activity

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Coagulase
• Coagulates blood proteins.
• Providing a “hiding place” for
bacteria within a clot.
Kinases
• Such as staphylokinase and
streptokinase.
• Kinase enzymes convert inactive
plasminogen to plasmin which
digests fibrin and release of
bacteria in to the blood.

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ENZYMES ORGANISMS INVOLVED MECHANISM OFACTION
Hemolysins
Staphylococci
Streptococci
E. coli
Lyse erythrocyte, make iron
available for microbial growth
Phospholipase
Clostridium perfringenes
Destroys lecithin
(phosphatidylcholine) in
cell membranes.
Staphylococcus aureus Lyse red blood cells
Protein A
Staphylococcus aureus
Binds to IgA and prevent
the activation of
compliment system.

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• Protein secretion is a critical element of toxin mediated virulence
• Secretion of proteins or toxins occur in several pathway.
BACTERIAL SECRETION SYSTEM
TYPE ORGANISM TOXIN/PROTEINS
Type 1 E coli Hemolysin
Type 2 Vibrio cholerae Cholera toxin
Pseudomonas aeruginosa Exotoxin A
Type 3 Yersinia Yops
Type 4 B pertussis Pertussis toxin
H pylori CagA
Type 5 H pylori Vacuolating cytotoxin
Type 6 Pseudomonas aeruginosa, Vibrio cholerae
Type 7 Mycobacterium tuberculosis

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• Essential nutrient for growth and metabolism.
• Also affects the virulence.
• Extracellular iron chelators called Siderophores for scavenging iron.
REQUIREDMENT FOR IRON

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• C.jejuni & N.meningitidis obtain iron by siderophores produced by other
species.
• H.influenzae type B, H.parainfluenzae, S.aureus & S.epidermidis have
specific receptors for transferrin and lactoferrin on their surface which
bind these protein and remove bound iron.
• Bacteroid species remove iron by proteolytic cleavage of the chelator.

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• Route of transmission
• Infective dose
• Adherence
• Invasion
• Intracellular survival
• Pathogenicity island
DETERMINANT OF PATHOGENICITY

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• Important role in pathogenicity
• Any mode of entry.
• Introduced by the optimal routes.
ROUTE OF TRANSMISSION

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• Minimum inoculum size capable of initiating an infection
o Low Infective dose
o Large Infective dose
• Higher the virulence, lower is the infective dose
INFECTIVE DOSE

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NONSPECIFIC
Attractive forces, which allow approach of the bacterium to the eukaryotic
cell surface
• Hydrophobic and electrostatic attractions
• Atomic and molecular vibration
• Brownian movement
• Recruitment and trapping by biofilm
SPECIFIC ADHERENCE
• Permanent attachment.
• Irreversible noncovalent bond between adhesin & receptor molecule.
ADHERENCE

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• Complex communities of single or multiple species of microorganisms
that develop on abiotic & biotic surface held together by slime-like
matrix (EPS)
• Conelike or mushroom-shaped micro colonies adhering to each other and
to the surface.
• Water-filled channels surround the micro-colonies, allowing access
to nutrients, elimination of wastes, and inter-bacterial
communication.
• Nonmotile organisms readily from it.
• Bacteria in biofilms have 500 times more resistant to antibiotics
BIOFILM

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Stages

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After secured attachment, then microorganisms invade to tissue or organ by
• Disruption of skin mucosal
 Trauma
 Inhalation
 Childbirth
 Implantation of medical device
• Enzymes
• Toxins
• Cell wall proteins
INVASION

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(1) Repellents or toxins that inhibit chemotaxis.
(2) Capsules inhibit attachment by the
phagocyte
(3) Permit uptake but release factors that block,
subsequent triggering of killing mechanisms.
(4) Secrete catalase (e.g. staphylococci), which
breaks down hydrogen peroxide.
(5) Have highly resistant outer coats.
(6) Mycobacteria also release a
lipoarabinomannan, which blocks the ability
of macrophages to respond to the activating
effects of IFNy.
(7) Impaired antigen-presenting function.
(8) Escape from the phagosome to multiply in
the cytoplasm.
INTRACELLULAR SURVIVAL

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• Cluster of genes incorporated in pathogenic organism
• Acquired by horizontal gene transfer.
• Carriage of one or many virulence genes.
• Occupy large chromosomal regions.
• Unstable, prone to be transferred, or deleted.
• Different G+C content from host chromosome.
• Are associated with tRNA genes, which act as sites for recombination
into the DNA.
• They carry mobile genes (E.g. integrases), to enable insertion into host
DNA
PATHOGENICITY ISLAND

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• PAI are mostly inserted in the backbone genome of the host strain (dark grey bars)
in specific sites that are frequently tRNA or tRNA-like genes (hatched grey bar)
• Mobility genes, such as integrases (int), are frequently located at the beginning of
the island
• PAI harbor one or more genes that are linked to virulence (V1 to V4)
• Frequently interspersed with other mobility elements, such as ISc, Isd.
• PAI boundaries are frequently determined by DRs (triangle)
• PAI is a GC content different from that of the core genome.

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SUMMARY