Virulence and Pathogenesis

1. Kailash
M.Sc Biotechnology
CDLU (SIRSA)

2. Virulence
• Virulence is defined by the ability
of a microorganism to cause
disease in the host.
• The development of a disease and
the chain of events leading to that
disease is called Pathogenesis.

3. • Food can carry
microorganisms that are
virulent and are able to cause
disease in the individuals who
consume the food.
• Microorganisms possess a
number of different virulence
mechanisms that help to
evade host defenses, enter
host cells, and disable or lyse
the host cells.

4. • The consumer of the food, in turn, is protected by a number of components in
the body including mucosal immunity.
• Virulence mechanisms continue to evolve and be transferred among different
microorganisms through a process known as lateral or horizontal gene
transfer.

5. • The ability of bacteria to cause disease is described in terms of the number
of infecting bacteria, the route of entry into the body, the effects of host
defense mechanisms, and intrinsic characteristics of the bacteria called
virulence factors.
• Many virulence factors are so-called effector proteins that are injected into
the host cells by special secretion machines such as the type 3 secretion
system. (Type three secretion system is a protein appendage found in
several Gram-negative bacteria).

6. • Host-mediated pathogenesis is often important because the host can
respond aggressively to infection with the result that host defense
mechanisms do damage to host tissues while the infection is being
countered.
• The virulence factors of bacteria are typically proteins or other molecules that
are synthesized by enzymes.
• These proteins are coded for by genes in chromosomal DNA, bacteriophage
DNA or plasmids. Certain bacteria employ mobile genetic elements and
horizontal gene transfer.

7. • Therefore, strategies to combat
certain bacterial infections by
targeting these specific virulence
factors and mobile genetic
elements have been proposed.
Bacteria use quorum sensing to
synchronise release of the
molecules. These are all proximate
causes of morbidity in the host.

8. Methods by which bacteria cause disease
• Adhesion
• Colonization
• Invasion
• Immune response inhibitors
• Toxins

9. Adhesion
• Many bacteria must first bind to host cell surfaces.
• Many bacterial and host molecules that are involved in the adhesion of
bacteria to host cells have been identified.
• Often, the host cell receptors for bacteria are essential proteins for other
functions. Due to presence of mucous lining and of anti-microbial substances
around some host cells, it is difficult for certain pathogens to establish direct
contact-adhesion.

10. • Different microorganism utilizes different structure for attachment. For
examples; Staphylococcus mutans attaches to the surface of teeth by its
sticky capsule. Other bacteria such as coli, Neisseria spp, Shigella attaches
with the help of pilli.
• Some examples of adhesions are:
• Capsule
• Pili
• Hemagglutinin spike of viruses
• Lipoteichoic acid
• Biofilm producing glycocalyx

11. adhesion

12. Colonization
• Some virulent bacteria produce special proteins that allow them to colonize
parts of the host body.
• Helicobacter pylori is able to survive in the acidic environment of the human
stomach by producing the enzyme urease.
• Colonization of the stomach lining by this bacterium can lead to gastric ulcers
and cancer. The virulence of various strains of Helicobacter pylori tends to
correlate with the level of production of urease.

13. colonization

14. Invasion
• Some virulent bacteria produce proteins that either disrupt host cell
membranes or stimulate their own endocytosis or macro-pinocytosis into host
cells.
• These virulence factors allow the bacteria to enter host cells and facilitate
entry into the body across epithelial tissue layers at the body surface.

15. invasion

16. Immune response inhibitors
Many bacteria produce virulence factors that inhibit the host's immune system
defenses.
For example, a common bacterial strategy is to produce proteins that bind host
antibodies. The polysaccharide capsule of Streptococcus pneumoniae
inhibits phagocytosis of the bacterium by host immune cells

17. Toxins
• Some microorganism produces toxic chemical.
• Toxins are of two types; exotoxin and endotoxin
i. Exotoxin:
• Toxins which are released outside the bacterial cell is called exotoxins
• Exotoxin is protein in nature.
• In general, exotoxins are highly toxic and lethal dose is low.
• Both Gram Positive and Gram Negative bacteria produces exotoxin

18. • Some examples are:
• Neurotoxin: Botulinum toxin; produced by Clostridium botulinum, tetanus
toxin; produced by Clostridium tetani
• Enterotoxin: cholera toxin; produced by Vibrio cholerae, heat stable and
heat labile toxin; produced by coli
• Cytotoxin: Dephtheria toxin; produced by Corynebacterium dephtheriae
• Hemolysin: lyse RBCs
• Leucosidin: lyse WBCs

19. ii. Endotoxin:
• Toxins which are not released outside of bacteria cell is called endotoxin.
• Lipopolysaccharide (LPS) present in outer membrane of Gram Negative
bacterial cell wall is an example of endotoxin.
• Endotoxin is less toxic than exotoxin and lethal dose is high.
• Endotoxin causes infection or disease by inducing fever, blood poisoning and
septic shock

20. THANKYOU