Thessaly master plan- WWF presentation_18.04.24.pdf
Bacterial Pathogenesis and Virulence.pdf
1. Institute of Basic and Biomedical Sciences
Medical Microbiology
Bacterial Pathogenesis and Virulence
Nundwe Mike | mith.n@yahoo.com
BSc. Biochemistry, MSc. Infectious Diseases
2. Introduction
• Recall that Pathogenesis is the development of a disease and the chain
of events leading to that disease
• The term pathogen refers to those microbes capable of causing disease,
especially if they cause disease in immunocompetent people.
• The term opportunistic pathogen refers to microbes that are capable
of causing disease only in immunocompromised people.
• Many infections are asymptomatic or inapparent because our host
defenses have eliminated the microorganism before it could multiply to
sufficient numbers to cause the symptoms of disease.
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3. Introduction …..
• Virulence is a measure of a microbe’s ability to cause disease (i.e., a highly
virulent microbe requires fewer organisms to cause disease than a less virulent
one).
• The ID50 is the number of organisms required to cause disease in 50% of the
population.
• A low ID50 indicates a highly virulent organism.
• The virulence of a microbe is determined by virulence factors, such as capsules,
exotoxins, or endotoxins.
• Whether a person gets an infectious disease or not is determined by the balance
between the number and virulence of the microbes and the competency of that
person’s host defenses..
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4. Introduction …..
• The term infection has two meanings:
1. the presence of microbes in the body and
2. the symptoms of disease.
• The presence of microbes in the body does not always result in
symptoms of disease.
• Bacteria cause the symptoms of disease by two main mechanisms:
production of toxins (both exotoxins and endotoxins) and induction of
inflammation.
• Most bacterial infections are communicable (i.e., capable of spreading
from person to person), but some are not (e.g., botulism and Legionella
pneumonia).
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5. Determinantsof Bacterial Pathogenesis
• The modes of transmission of microbes include both human-to-human
and nonhuman-to-human processes.
• Bacteria, viruses, and other microbes can also be transmitted from
mother to offspring, a process called vertical transmission.
• Horizontal transmission, by contrast, is person-to-person transmission
that is not from mother to offspring
• The main “portals of entry” into the body are the respiratory tract,
gastrointestinal tract, skin, and genital tract.
• Human diseases for which animals are the reservoir are called
zoonoses.
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6. Adherence to Cell Surfaces
• Pili are the main mechanism by which bacteria adhere to human cells.
• They are fibers that extend from the surface of bacteria that mediate
attachment to specific receptors on cells.
• Glycocalyx is a polysaccharide “slime layer” secreted by some strains of
bacteria that mediates strong adherence to certain structures such as
heart valves, prosthetic implants, and catheters.
• The various molecules that mediate adherence to cell surfaces are called
adhesins.
• After the bacteria attach, they often form a protective matrix called a
biofilm consisting of various polysaccharides and proteins.
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7. Virulence Factors
• Bacterial features that allow evasion of host defenses
• Key examples to know are:
1. Protein A
2. IgA protease
3. M protein
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8. Protein A
• Key virulence factor of Staph Aureus
• Part of peptidoglycan cell wall
• Inhibits phagocytosis
• Binds Fc portion of IgG antibodies
• Prevents opsonization and phagocytosis by macrophages
• Prevents complement activation
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9. IgA Protease
• Enzymes that cleave IgA
• IgA key for mucosal immunity
• Protease allows colonization of mucosal surfaces
• S. pneumonia
• H. influenza
• Neisseria (gonorrhoeae and meningitidis)
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10. M Protein
• Surface molecule of group A strep (pyogenes)
o Strep throat, rheumatic fever
• M protein prevents phagocytosis
o Binds factor H
o Breaks down C3-convertase, prevent opsonization by C3b
• Shares properties with myosin
o May be the basis of rheumatic heart disease
• Post-strep complications
o Rheumatic heart disease
o Glomerulonephritis
o Different M protein subtypes associated each complication
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11. Invasion,Inflammation& Intracellular Survival
• The capsule surrounding bacteria is antiphagocytic (i.e., it retards the
phagocyte from ingesting the organism).
• The polysaccharide capsule prevents the phagocyte from adhering to the
bacteria; anticapsular antibodies allow more effective phagocytosis to occur
(a process called opsonization)
• Mutant strains of many pathogens that do not produce capsules are
nonpathogenic.
• Leukocidins are pore-forming toxins that degrade the cell membrane of
neutrophils and macrophages.
• The Panton Valentine leukocidin produced by S. aureus is a good example.
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12. Invasion,Inflammation& Intracellular Survival…..
• Inflammation is an important host defense induced by the presence of bacteria
in the body.
• There are two types of inflammation, pyogenic and granulomatous, and
bacteria typically elicit one type or the other.
• Pyogenic inflammation, the host defense against pyogenic (pus-producing)
bacteria such as S. pyogenes, consists of neutrophils (and antibody and
complement).
• Granulomatous inflammation, the host defense against intracellular,
granuloma-producing bacteria, such as M. tuberculosis, consists of macrophages
and CD4-positive T cells.
• The type of inflammatory lesion is an important diagnostic criterion.
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14. Invasion,Inflammation& Intracellular Survival…..
• Bacteria can evade our host defenses by a process called intracellular survival
(i.e., bacteria that can live within cells are protected from attack by macrophages
and neutrophils).
• Note that many of these bacteria (e.g., M. tuberculosis) are not obligate
intracellular parasites (which can grow only within cells), but rather have the
ability to enter and survive inside cells.
• Two important diseases, diphtheria and pseudomembranous colitis, are
characterized by inflammatory lesions called pseudomembranes.
• Pseudomembranes are thick, adherent, grayish or yellowish exudates on the
mucosal surfaces of the throat in diphtheria and on the colon in
pseudomembranous colitis.
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16. Toxin Production
• The other major mechanism by which bacteria cause disease is the production of
toxins.
Main Features of Exotoxins and Endotoxins
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17. A. Exotoxin
• Exotoxins are polypeptides secreted by certain bacteria that alter specific cell
functions resulting in the symptoms of disease.
• They are produced by both gram-positive and gram-negative bacteria, whereas
endotoxin is found only in gram-negative bacteria.
• Exotoxins are antigenic and induce antibodies called antitoxins.
• Exotoxins can be modified to form toxoids, which are antigenic but not toxic.
• Toxoids, such as tetanus toxoid, are used to immunize against disease.
• Many exotoxins have an A–B subunit structure in which the A subunit is the
active (toxic) one and the B subunit is the one that binds to the cell membrane
and mediates the entry of the A subunit into the cell.
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19. Exotoxin
• Exotoxins have different mechanisms of action and different targets within the
cell and therefore cause a variety of diseases with characteristic symptoms
• Several exotoxins are enzymes that attach ADP-ribose to a cell component (ADP-
ribosylation).
• Some exotoxins act by proteolytic cleavage of a cell component, whereas others
act as superantigens, causing the overproduction of cytokines.
• Exotoxins are released from bacteria by specialized structures called secretion
systems.
• Some secretion systems transport the exotoxins into the extracellular space, but
others transport the exotoxins directly into the mammalian cell.
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20. Gram-PositiveBacteria MajorExotoxins
• Diphtheria toxin, produced by Corynebacterium diphtheriae, inhibits
protein synthesis by ADP-ribosylation of EF-2.
• The resulting death of the affected cells leads to two prominent
symptoms of diphtheria: pseudomembrane formation in the throat
and myocarditis.
1. Diphtheria toxin
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21. Modeof action of diphtheriatoxin
• The toxin binds to the cell surface via its binding subunit, and the active
subunit enters the cell.
• The active subunit is an enzyme that catalyzes the addition of ADP-ribose
(ADP-R) to elongation factor-2 (EF-2).
• This inactivates EF-2, and protein synthesis is inhibited
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22. • Tetanus toxin, produced by Clostridium tetani, is a neurotoxin that
prevents release of an inhibitory neurotransmitter involved in muscle
relaxation.
• When the inhibitory neurons are nonfunctional, the excitatory
neurons are unopposed, leading to muscle spasms and a spastic
paralysis.
• Inhibiting the release of the GABA and glycine leads to convulsive
contractions of the voluntary muscles, best exemplified by spasm of
the jaw and neck muscles (“lockjaw”).
2. Tetanus toxin
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23. • Botulinum toxin, produced by Clostridium botulinum, is a neurotoxin that
blocks the release of a different neurotransmitter, acetylcholine, at the
synapse of the neuromuscular junction, producing a flaccid paralysis.
• Approximately 1 μg is lethal for humans; it is one of the most toxic
compounds known.
• The toxin is composed of two polypeptide subunits held together by disulfide
bonds.
• One of the subunits binds to a receptor on the neuron; the other subunit is a
protease that degrades the protein(s) responsible for the release of
acetylcholine.
3. Botulinum toxin
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24. • Two exotoxins are produced by Clostridium difficile, both of which are
involved in the pathogenesis of pseudomembranous colitis.
• Exotoxin A is an enterotoxin that causes watery diarrhea.
• Exotoxin B is a cytotoxin that damages the colonic mucosa and causes
pseudomembranes to form.
• Exotoxins A and B are glucosyltransferases that modify target signal
transduction proteins (Rho GTPases), which interferes with their signal
transduction function.
• Glucosylation by exotoxin B causes disaggregation of actin filaments in the
cytoskeleton, leading to apoptosis and cell death.
4. Clostridium difficile
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25. • Multiple toxins are produced by Clostridium perfringens and other species of clostridia that
cause gas gangrene.
• A total of 7 lethal factors and 5 enzymes have been characterized, but no species of
Clostridium makes all 12 products.
• The best characterized is the alpha toxin, which is a lecithinase that hydrolyzes lecithin in
the cell membrane, resulting in destruction of the membrane and widespread cell death.
• The other four enzymes are collagenase, protease, hyaluronidase, and deoxyribonuclease
(DNase).
• The seven lethal toxins are a heterogeneous group with hemolytic and necrotizing activity.
• Certain strains of C. perfringens produce an enterotoxin that causes watery diarrhea.
• This enterotoxin acts as a superantigen similar to the enterotoxin of S. aureus
5. Clostridium perfringens
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26. • Three exotoxins are produced by Bacillus anthracis, the agent of anthrax: edema factor,
lethal factor, and protective antigen.
• The three exotoxins associate with each other, but each component has a distinct function.
• Edema factor is an adenylate cyclase that raises the cyclic AMP concentration within the
cell, resulting in loss of chloride ions and water and consequent edema formation in the
tissue.
• Lethal factor is a protease that cleaves a phosphokinase required for the signal
transduction pathway that controls cell growth.
• Loss of the phosphokinase results in the failure of cell growth and consequent cell death.
• Protective antigen binds to a cell surface receptor and forms pores in the human cell
membrane that allow edema factor and lethal factor to enter the cell.
6. Bacillus anthracis
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27. • TSST is a superantigen produced primarily by certain strains of S. aureus but also
by certain strains of S. pyogenes.
• TSST binds directly to class II major histocompatibility (MHC) proteins on the
surface of antigen-presenting cells (macrophages) without intracellular
processing.
• This complex interacts with the T-cell receptor of many helper T cells, resulting in
activation of these T cells
• This causes the release of large amounts of interleukins, especially interleukin-1,
interleukin-2, and TNF.
• These cytokines produce many of the signs and symptoms of toxic shock.
7. Toxic shock syndrome toxin-1 (TSST)
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28. • Staphylococcal enterotoxin is also a superantigen, but because it is
ingested, it acts locally on the lymphoid cells lining the small intestine.
• The enterotoxin is produced by S. aureus in the contaminated food and
causes food poisoning, usually within 1 to 6 hours after ingestion.
• The main symptoms are vomiting and watery diarrhea.
• The prominent vomiting seen in food poisoning is caused by cytokines
released from the lymphoid cells stimulating the enteric nervous
system, which activates the vomiting center in the brain.
8. Staphylococcal enterotoxin
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29. • Exfoliatin is a protease produced by S. aureus that causes scalded skin
syndrome.
• Exfoliatin cleaves desmoglein, a protein in the desmosomes of the skin,
resulting in the detachment of the superficial layers of the skin. Exfoliatin is
also called epidermolytic toxin.
9. Exfoliatin
• Panton-Valentine (PV) leukocidin is a pore-forming exotoxin produced by
methicillin-resistant strains of S. aureus (MRSA).
• It destroys white blood cells, skin, and subcutaneous tissue.
• The two subunits of the toxin assemble in the cell membrane to form a pore
through which cell contents exit into the extracellular space.
10. Panton-Valentine (PV) leukocidin
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30. • Erythrogenic toxin, produced by S. pyogenes, causes the rash
characteristic of scarlet fever.
• Its mechanism of action is similar to that of TSST (i.e., it acts as a
superantigen).
• The DNA that codes for the toxin resides on a temperate bacteriophage.
• Nonlysogenic bacteria do not cause scarlet fever, although they can
cause pharyngitis.
11. Erythrogenic toxin
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31. Gram-NegativeBacteriaMajorExotoxins
• The heat-labile enterotoxin produced by E. coli causes watery,
nonbloody diarrhea by stimulating adenylate cyclase activity in cells
in the small intestine.
• The resulting increase in the concentration of cyclic AMP causes
excretion of the chloride ion, inhibition of sodium ion absorption, and
significant fluid and electrolyte loss into the lumen of the gut.
1. The heat-labile enterotoxin
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32. ModeofactionofE.coliandV.choleraeenterotoxins
• The enterotoxin (e.g., cholera toxin) binds to the surface of the enterocyte
via its binding subunit.
• The active subunit then enters the enterocyte.
• The active subunit is an enzyme that catalyzes the addition of ADP-ribose
(ADP-R) to the GS regulatory protein.
• This activates adenylate cyclase to overproduce cyclic adenosine
monophosphate (AMP).
• As a consequence, cyclic AMP–dependent protein kinase activity increases,
and water and electrolytes leave the enterocyte, causing watery diarrhea.
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34. • Heat-stable toxin, which is a polypeptide that is not inactivated by
boiling for 30 minutes.
• The heat-stable toxin affects cyclic guanosine monophosphate (GMP)
rather than cyclic AMP.
• It stimulates guanylate cyclase and thus increases the concentration
of cyclic GMP, which inhibits the reabsorption of sodium ions and
causes diarrhea.
2. Heat-stable toxin
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35. • Shiga toxin is an exotoxin produced primarily by strains of Shigella dysenteriae
and E. coli with the O157:H7 serotype, though in E. coli it’s called shiga-like toxin
• Shiga toxin cytotoxic activity:
a) Damage to epithelial cells of the intestine
• Inhibit protein synthesis, causing cell death
• Vascular damage to the intestines, and hemorrhage (blood and fecal leukocytes in
stool.
b) Damage to glomerular endothelial cells, leading to hemolytic uremic
syndrome (HUS)
• Shiga toxin binds to receptors on the kidney and on the endothelium of small
blood vessels.
• Inhibition of protein synthesis results in death of those cells, leading to renal
failure and microangiopathic hemolytic anemia.
3. Shiga Toxin
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36. • Pertussis toxin, produced by Bordetella pertussis , the cause of whooping cough, is an
exotoxin that catalyzes the transfer of ADP-ribose from NAD to an inhibitory G protein.
• Inactivation of this inhibitory regulator has two effects: one is to stimulate adenylate
cyclase activity, leading to an increase in cyclic AMP concentration within the
affected cells
• This results in edema and other changes in the respiratory tract, leading to the cough
of whooping cough.
• It also inhibits the signal transduction pathway used by chemokine receptors.
• This causes the marked lymphocytosis seen in patients with pertussis.
• The toxin inhibits signal transduction by all chemokine receptors, resulting in an
inability of lymphocytes to migrate to and enter lymphoid tissue (spleen, lymph
nodes).
• Because they do not enter tissue, there is an increase in their number in the blood
4. Pertussis toxin
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37. B. Endotoxins
• Endotoxins are lipopolysaccharides (LPS) located in the outer membrane
only of gram-negative bacteria. They are not secreted by bacteria.
• Lipid A is the toxic component of LPS. It induces the overproduction of
cytokines, such as tumor necrosis factor, interleukin-1, and nitric oxide, from
macrophages, which causes the symptoms of septic shock, such as fever and
hypotension.
• In addition, LPS activates the complement cascade (alternate pathway),
resulting in increased vascular permeability, and the coagulation cascade,
resulting in increased vascular permeability and disseminated
intravascular coagulation.
• Endotoxins are poorly antigenic, do not induce antitoxins, and do not form
toxoids.
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38. EndotoxinsModeof Action
• Endotoxin is the most important cause of septic shock, which is
characterized primarily by fever, hypotension, and disseminated
intravascular coagulation (DIC).
• Endotoxin causes these effects by activating three critical processes:
1. Activating macrophages to produce interleukin-1 (IL-1), tumor
necrosis factor (TNF), and nitric oxide;
2. Activating complement to produce C3a and C5a; and
3. Activating tissue factor, an early component of the coagulation
cascade.
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42. The biologic effects of endotoxin
1. Fever due to the release by macrophages of IL-1 (endogenous
pyrogen) and IL-6, which act on the hypothalamic temperature-
regulatory center.
2. Hypotension, shock, and impaired perfusion of essential organs due
to nitric oxide–induced vasodilation, TNF-induced increased capillary
permeability, bradykinin-induced vasodilation, and increased capillary
permeability.
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43. The biologic effects of endotoxin …..
3. Disseminated intravascular coagulation (DIC)
• This is due to activation of the coagulation cascade, resulting in thrombosis,
a petechial or purpuric rash, and tissue ischemia, leading to failure of vital
organs.
• The coagulation cascade is activated when tissue factor is released from the
surface of endothelial cells damaged by infection.
• Tissue factor interacts with circulating coagulation factors to cause
widespread clotting within capillaries.
• A positive d-dimer test provides laboratory evidence for a diagnosis of DIC
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44. The biologic effects of endotoxin ….
4. Activation of the alternative pathway of the complement cascade, resulting in
inflammation and tissue damage. C5a is potent chemokine that attracts
neutrophils to the site of infection.
5. Activation of macrophages, increasing their phagocytic ability, and activation
of many clones of B lymphocytes, increasing antibody production. (Endotoxin is
a polyclonal activator of B cells, but not T cells.)
• The end result of the above five processes is called the systemic inflammatory
response syndrome, or SIRS.
• The most common clinical signs of SIRS are fever, hypotension, tachycardia,
tachypnea, and leukocytosis.
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45. Endotoxinsand Septic shock
• The endotoxins of gram-negative bacteria are the best-established causes of
septic shock, but surface molecules of gram-positive bacteria (which do not
have endotoxins) can also cause septic shock.
IMPORTANT NOTE
1. Septic shock is different from toxic shock.
• In septic shock, the bacteria are in the bloodstream, whereas in toxic
shock, it is the toxin that is circulating in the blood.
• The clinical importance of this observation is that in septic shock, blood
cultures are usually positive, whereas in toxic shock, they are usually
negative
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46. Endotoxinsand Septic shock
2. Septic shock can cause the death of a patient.
• This can happen even though antibiotics have killed the bacteria in
the patient’s blood (i.e., the blood cultures have become negative).
• This occurs because septic shock is mediated by cytokines, such as
TNF and IL-1, which continue to act even though the bacteria that
induced the cytokines are no longer present.
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