bactect

1. GROUP 5
PRESENTATION
DESCRIBE THE MECHANISM OF BACTERIAL
PATHOGENECITY

2. MEMBERS
• MARGARET TUNU
• NICOLE AKINYI
• JOB MANDELA
• OSCAR TILEN
• GILLIAN CHEPKORIR
• CHRISTOPHER MURIITHI

3. Host-Parasite Relationships: Pathogenesis of Infections
In any host-pathogen encounter, there are two determinants of the
outcome:
1. Virulence of the parasite
2. Resistance of the host

4. In some cases, the host-pathogen
relationship is very complex:
-Commensal but opportunistic will take advantage of weakened host and invade
tissues setting up a potentially life-threatening infection.
Examples include motile Aeromonads- natural inhabitants of intestine but
cause septicemia when fish is immune suppressed.
Bacteria cause disease by 2 basic mechanisms:
Direct damage of host cells.
Indirectly by stimulating exaggerated host inflammatory/immune
response.

5. Virulence factors are molecular components
expressed by a pathogen that increases its ability to
cause disease.
Virulence factors can be divided into two categories:
• 1. Those that cause damage to the host (toxins).
• 2. Those that do not directly damage the host but promote
colonization and survival of infecting bacteria.

6. A. Bacterial toxins
1. Exotoxin: protein molecule liberated from intact living
bacterium.
a. They are antigenic and can elicit protective antitoxic antibodies. Many of
these toxins can be converted to nontoxic immunizing agents termed
toxoids.
b. Three roles of exotoxins in disease:
i. Ingestion of preformed toxin (botulism).
ii. Colonization of wound or surface followed by toxin production
(cholera and
diphtheria toxins).
iii. Exotoxin produced by bacteria in tissues to aid growth and
spread (Clostridium perfringens alpha-toxin).

7. c. Types of
exotoxins:
i. A-B toxins (intracellular acting)
1) Composed of two parts: A and B portions
2) The B portion mediates binding to a specific host cell receptor.
3) After binding to the host cell, the A portion is translocated into host cells
and has biological activity against an intracellular target
or

8. 4) Examples:
a) Diphtheria toxin: ADP-ribosylation of host EF-2; host cells are killed by blocking
translation.
b) Cholera toxin: ADP-ribosylation of a cAMP regulatory protein, which causes loss
of ion regulation, water loss, diarrhea.
c) Shiga toxin cleaves host rRNA, which blocks translation and kills the host cell.
d) Clostridium botulinum- large subunit targets neurons, small subunit cleave snare
proteins inhibiting neurotransmitter release from neurons- causes paralysis BoNT-
E in fish (most toxic substance known).

9. ii. Membrane disrupting (surface damaging)
1) Cause damage or disruption of plasma membranes, which leads to osmotic lysis and
cell death. Many were originally termed “hemolysins” because they lyse RBCs.
2) Three types of membrane disrupting toxins:
a) Enzymes that hydrolyze phospholipids: phospholipase, sphingomyelinase.
b) Toxins with detergent-like surfactant activity that disrupt by membrane
solubilization.
c) Pore forming toxins (the most common): proteins that insert in the host membrane
and form a hydrophilic pore.
Aeromonas produces up to 4 hemolysins- aerolysin A (AeroA) and Heat labile hemolysin
AHH1- work synergistically, also some aeromonads produce the pore forming toxin RtxA.

10. iii
.
Superantigens
1) Toxins that bind directly to MHC II on macrophages (without being
processed) and form a crosslink with T cell receptors.
2) Crosslinking causes stimulation of up to 1 in 5 T cells in the body
(normal antigens cause stimulation of 1 in 10,000).
3) Excessive IL-2 production results from the massive stimulation of T
helper cells.
4) Stimulation of other cytokines by IL-2 lead to shock.
Example: staphylococcal toxic-shock syndrome

11. iv. Extracellular enzymes: break down host macromolecules.
They play an important role in disease development by providing a nutrients
or aiding in dissemination. Can cause extensive tissue damage.
Examples:
a) Coagulase – clots fibrin, thus protecting the bacteria.
b) Hyaluronidases and proteases – aid in the spread of bacteria by degrading
extracellular matrix.
c) Collagenase – aids in dissemination.
d) DNase – reduces viscosity of debris from dead cells (may help escape DNA
net by neutrophil).
A. hydrophila - Express diverse extracellular enzymes can contribute to virulence
including collagenase, elastase, enolase, lipases (heat stable lipase, pla and Plc), metallo
protease, and serine protease, Rnase R.

12. 2. Endotoxin- released when cells die: lipopolysaccharide (LPS) produced by
gram- negative bacteria. In gram-positive bacteria peptidoglycan and
teichoic acids.
a. LPS is bound by LPS binding proteins in plasma, which then binds CD14. This complex
binds Toll-like receptor 4 (TLR4) on macrophages and monocytes. TLR2 binds teichoic
acids. TLR1 binds peptidoglycan.
b. Macrophages and monocytes release cytokines (IL-1, IL-6, IL-8, TNF alpha,
Platelet Activating Factor), which subsequently trigger prostaglandin and
leukotriene release.
c. The complement and coagulation cascades are activated.
e. endotoxic shock occurs when bacterial products reach high enough levels in the blood
to trigger complement activation, cytokine release, and coagulation cascade activation
in many parts of the body. Circulatory system collapse followed by multiple organ
system failure occurs.

13. B. Bacterial invasion of host
tissues
1. Host damage is caused during invasion by either:
a. direct disruption of function.
b. an exaggerated immune response that compromises tissue function.
2. The invasive bacteria are classified as:
a. Facultative Intracellular Parasites
i. FIPs are not confined to cells.
ii. Some can multiply in professional phagocytic cells.
iii. When a balance is established between the bacterium and phagocyte,
the bacteria may survive in this intracellular state for months or years
(example: Mycobacterium).
b. Obligate Intracellular Parasites; can only propagate inside host
cells.
Examples include chlamydia and rickettsia
c. Extracellular parasites, which cause tissue damage while they are
outside phagocytes and other cells and do not have the ability to
survive long periods in cells.

14. 3. Steps in bacterial
invasion:
a. Motility
iii.
i. Flagella are the best characterized; adapted for low viscosity
fluids.
ii. Other types of motility: corkscrew type (Spirochetes--best in viscous
solutions), gliding motility (Flavobacterium columnare and
cytophagas, myxobacteria--movement over solid surfaces).
Chemotaxis is directional swimming using a gradient (especially
nutrients).
A. hydrophila produce lateral flagella for surface movement and polar
flagella for movement in suspension. Glycosylation of polar flagella
involved in biofilm formation, binding to cells and mucosal adherence.

15. b.
Adherence
iii.
i. Two common strategies: fimbriae and monomeric protein adhesins.
ii. Fimbriae (pili): receptors are usually carbohydrate residues of glycoproteins
or glycolipids. Attachment is more fragile. Highly specific binding, often
mediated by adhesins, can be blocked by antibodies, often specific for host
tissue type/location.
Monomeric protein adhesins: mediated by cell surface proteins, tighter
binding to host cell, may recognize proteins on host cell surface, may follow
looser fimbrial attachment.
Aeromonas-bundle-forming pilus (encoded by bfp) is a critical internal
colonizing factor.

16. c. Invasion of host cells (intracellular
pathogens)
i. Some invasive bacteria have mechanisms for entering host cells that
are not naturally phagocytic.
ii. Two types of bacterial-mediated invasion:
a. Zippering: bacteria present ligands on their surface allowing them to
bind to host cells and initiate the entry process. It is similar to FcR- and
CR3- mediated phagocytosis, which is characterized by the formation of
inclusion shaped by the bacteria they ingest (Yersinia pestis Ail).
b. Triggering: bacteria inject effectors into host cells via T3SS to
regulate phagocytosis (Salmonella).
iii. Following attachment to host cells, pathogens cause changes in host
cell cytoskeleton (actin) that cause the pathogen to be internalized.

17. iv. Some pathogens can utilize actin fibers intracellularly to move through host
cells (transcytosis).
v. Invasins may also mediate uptake of bacteria into professional phagocytic
cells in a way that bypasses normal phagosome formation.

18. d. Manipulation of host cell
functions
i. Bacterial pathogens are often very manipulative of host cell
functions; both extracellular and intracellular pathogens will
cause host cells to perform functions favorable to the
pathogen.
a. For example, leukotoxin produced by Mannheimia
haemolytica (extracellular pathogen) induces cytokine
secretion.
b. Listeria monocytogenes (intracellular pathogen) produces
a protein that mobilizes actin to propel bacteria through
the cell and into neighboring cells.

19. ii. Some bacterial pathogens have a specialized type III secretion system (TTSS) that
forms a
needle-like structure that injects effector proteins directly into the host cell cytoplasm.
a. In some cases, these effector proteins serve as receptors in the host
membrane for bacterial attachment.
b. In some cases, these effector proteins can mobilize cytoskeleton to cause
phagocytosis.
c. In some cases, effector proteins can induce or prevent apoptosis.
Aeromonas express type II, III and VI
secretion systems III and VI can inject
effector proteins into host cells (II is for
extracellular release of proteins).

20. 4. Obtaining
nutrients
a. Pathogenic bacteria have intricate methods to obtain all essential
nutrients.
b. Obligate intracellular bacteria have complex nutrient
requirements and parasitize the living cell for an extended period.
c. Host cytoplasm is a very nutrient rich environment.
i. Extracellular pathogens often lyse cells to obtain nutrients.
ii. Intracellular pathogens will either escape from phagosomes to
enter the nutrient rich cytoplasm or modify the vacuole so they can
get nutrients from the cytoplasm (example: E. ictaluri).

21. d.Iron
I. Host tissues are very low in iron because it is bound to transferrin,
lactoferrin, ferritin, and heme.
II. Bacterial strategies for obtaining iron (often induced by low iron
conditions):
1. Siderophores--low MW compounds that chelate iron with very high affinity;
secreted and taken up by bacterial surface receptors.
2. Direct binding of host transferrin, lactoferrin, ferritin, or heme by bacterial surface
receptors.
3. Exotoxins that lyse host cells (can be used to obtain other nutrients as well).

22. 5. Evasion of host immune
response
a. Serum
resistance
iii.
i. Serum resistance is defined as the ability to prevent bacterial lysis by the
C5b-C9 membrane attack complex (MAC).
ii. Capsule mediates resistance to complement by:
1) preventing C3b binding.
2) promoting C3bH complex formation instead of C3bBb (mediated by sialic
acid in capsule-this inhibits complement cascade).
Lipopolysaccharide--binds C3b and C5b. However, O polysaccharide can
mediate resistance to complement by:
1) having sialic acid attached to promote C3bH formation.
2) having long O polysaccharide side chains that prevent MAC killing after C5b
binds (possibly too far from bacterial outer membrane).
iv. S-layer or outer membrane proteins
Aeromonas encodes an S-layer also and capsule, TagA cleaves C1-esterase inhibitor
imparting serum resistance.

23. b. Resistance to
opsonization/phagocytosis
i. Capsule:
1) prevents C3b-mediated opsonization (by the same mechanism used to
avoid complement-mediated killing)
2) prevents antibody-mediated opsonization by
masking (hyaluronic acid, sialic acid).
Aeromonas- capsule have anti-phagocytic activity, provide increased
resistance to the complement system, and increased adherence

24. b. Resistance to opsonization/phagocytosis
ii. LPS O polysaccharide can prevent opsonization if it has sialic acid
iii. S-layer
iv. Extracellular products: enzymes that inactivate C5a
chemoattractant (S. pyogenes), toxins that kill phagocytes
(leukotoxins) (Mannheimia haemolytica), inhibit migration, or reduce
oxidative burst.

25. c. Strategies for surviving
phagocytosis:
iii.
i. Escape from phagosome before fusion with lysosome (example:
Listeria monocytogenes, mediated by listeriolysin)
ii. Prevent phagosome-lysosome fusion-use type 3 secretion system to
influence trafficking
Express factors that allow survival in harsh phagolysosome conditions
(catalase, superoxide dismutase, surface polysaccharides, cell wall)

26. d. Evading antibody
i. Ig proteases
ii. Antigenic switching or phase variation
iii. Masking (sialic acid, hyaluronic acid, coating with
host proteins such as fibronectin).

27. 6. Virulence factors expression and release are coordinated by
intricate gene regulation and regulated protein function
a. Regulon-coordinated control of group of virulence factors
that are activated or deactivated in response to
environmental signal.
b. Allows bacterial pathogens to adapt to varying host
conditions.
c. Virulence gene expression can be triggered when a
pathogen senses environmental cues from the host
environment (examples: pH, iron concentration).

28. d. Virulence gene expression is sometimes triggered when a
pathogen detects sufficient bacterial numbers are present:
“quorum sensing”
i. Bacteria with quorum sensing capability secrete a small molecule (for
example, homoserine lactone)
ii. When the quorum sensing molecule reaches a critical concentration,
gene expression is stimulated.
iii. Sometimes quorum sensing regulates virulence genes.
Aeromonads have elaborate quorum sensing system that regulated biofilm
formation and virulence genes.

29. Biofilm
• Definition: a structured community of bacteria enclosed in a self-produced
polymeric matrix and adherent to an inert or living surface. Can provide
resistance to damage outside of host, can protect against immune
processes inside the host and can provide transient antibiotic resistance
• Resistance is due to:
• Slower growth rates of bacteria within biofilms
• Decreased diffusion of antibiotics through the biofilm (protective matrix)
• Accumulation of enzymes that contribute to
• resistance

30. Persistence in the presence of
antibiotics- regulated phenotypes
Persisters are non- or slow-growing reversible phenotypic variants of
the wild type, tolerant to bactericidal antibiotics.
i.
iii.
tolerance is due to inhibition of essential cell functions during antibiotic
stress,
resulting in inactivity of the antibiotic target.
ii. Persistence occurs in E. coli, Pseudomonas aeruginosa,
Mycobacterium tuberculosis, and Staphylococcus aureus.
Persistence requires coordinated metabolic changes; entry and exit from the
persister state is regulated by signal molecules (such as guanosine
pentaphosphate or ppGpp).

31. THANK YOU!!