This slide presentation describes conception of LD50, ID50, Pathogenicity Island, virulence factors and their mechanism. Immunopathogenesis.

1. PATHOGENESIS I
Bacterial virulence factors
MBBS Program
Dr. Tarek Mahbub Khan
MBBS, M.Phil (virology)
Assistant Professor
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2. TLO
• Definition of some terminologies
• Koch’s postulates
• Stages of Bacterial pathogenesis
• Definition of bacterial virulence
• Explanation f bacterial virulence
• Description of bacterial virulence factors
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3. DEFINITION
• PATHOGEN:
– A microorganism capable of causing disease.
• NON-PATHOGEN:
– A microorganism that does not cause disease.
• OPORTUNISTIC PATHOGEN:
– An organism capable of causing disease when host
resistance is impaired.
• INFECTION:
– Multiplication of infectious (pathogenic) agent within the
body.

4. • COMUNICABLE DISEASE:
– Diseases that are directly transmitted from host to host.
– Not all infectious diseases are communicable.
• ENDEMIC:
– Infections that are constantly present at low level in a define
population. (e.g., Malaria is endemic in hill-tracts)
• EPIDEMIC:
– Disease that occurs in more frequent pattern. (e.g., Cholera is
endemic in India)
• PANDEMIC:
– Infections that are distributed worldwide. (e.g., Swine-flu was
pandemic in the world in 2009)
DEFINITION

5. • In 1884, Robert Koch proposed a
series of postulates that have been
applied broadly to link many specific
bacterial species with particular
diseases. These are Koch’s postulates.
• Microbial genes are related to their
virulence factors. Molecular diagnosis
established the genetic relation ship
of infectious diseases through
Molecular Koch’s Postulates
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KOCH’S POSTULATES

6. 1. Microorganism should be found in all cases of the disease in
question, and its distribution in the body should be in
accordance with lesions observed.
2. The microorganism should be grown in pure culture for
several generations.
3. When such a pure culture is inoculated into susceptible
animal species, the typical disease must result.
4. The microorganism must again be isolated from the lesions
of such experimentally produced disease
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KOCH’S POSTULATES

7. • Phenotype or property under investigation should be related
with a pathogenic strain of a species and not with non-
pathogenic strains.
• Inactivation of the gene/genes related to virulence factors
should lead to measurable decrease in pathogenicity.
• Reversion or replacement of mutated gene with wild type
gene should lead to restoration of pathogenicity or virulence.
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MOLECULAR KOCH’S POSTULATES

8. • Some microorganisms do not grow in pure culture however
they have animal model to grow. Example: Treponema
pallidum, Mycobacterium leprae.
• Some microorganism do not have animal model to grow
though can readily be cultured in vitro. Example: Neisseria
gonorrhoeae.
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LIMITATION OF KOCH’S POSTULATES

9. • Detection of rise of specific antibody titer.
• Molecular detection of microbial gene by PCR,
hybridization or other related molecular tests.
• Demonstration of pathogenicity in in-vitro model.
Example: Escherichia coli induced diarrhea in in-vitro
cell culture.
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ADJUNCT OF KOCH’S POSTULATES

10. • Analysis of infection and diseases.
• Classification of bacteria into pathogens, non-pathogens
or opportunistic pathogens.
• Laboratory application in infectious disease diagnosis
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APPLICATION OF KOCH’S POSTULATES

11. 1. Transmission from the source of infection into the portal of
entry.
2. Evasion of primary host defense.
3. Adherence to mucous membrane.
4. Colonization by growth of the bacteria at the site of
adherence.
5. Disease symptoms caused by bacterial toxin or invasion.
6. Host immune response during steps 3,4,5
7. Progression or resolution of the disease.
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STAGES OF BACTERIAL PATHOGENESIS

12. SOURCE OF INFECTION
Soil Tetanus
Water Legionnaire’s disease
Animal Anthrax, lyme disease, rabies
Fomites Staphylococcal skin infection
Foods Diarrhoea, food poisoning

13. ZOONOTIC DISEASES
DISEASES BACTERIA SOURCES
Anthrax Bacillus anthracis Domestic animals
Diarrhea Campylobacter jejuni Domestic animals
Salmonella enteritidis Poultry, egg, cattle
Hemorrhagic
colitis
E.coli O157:H7 Cattle
Intestinal
tuberculosis
Mycobacterium bovis Unpasteurized milk
Leptospirosis Leptospira interrogans Urine of rats and
dogs

14. MODES OF TRANSMISSION
(Human to human)
Direct contact Gonorrhea
No direct contact
(flies, flea, fomites, food)
Dysentery
Vertical Congenital syphilis
Blood born Syphilis, hepatitis B, AIDS

15. VERTICAL TRANSMISSION
(Infected mother to the offsprings)
Transplacental Treponema pallidum
Listeria monocytogenes
Toxoplasma gondii
Cytomegalovirus
Within birth canal Streptococcus agalactiae
Escherichia coli
Neisseria gonorrhoeae
Herpes simplex
Breast milk Staphylococcus aureus
Cytomegalovirus

16. POTALS OF ENTRY
RESPIRATORY ROUTE Streptococcus pneumoniae
Neisseria meningitidis
Haemophilus influenzae
Mycobacterium tuberculosis
GASTROINTESTINAL ROUTE Shigella dysenteriae
Salmonella, Vibrio cholerae
SKIN Clostridium tetani
GENITAL ROUTE Neisseria gonorrhoeae
Treponema pallidum

17. BACTERIAL VIRULENCE
• Define as a quantitative measurement of bacterial
pathogenicity
• It is measured by amount of bacteria required to
cause disease
• Virulence of an organism is determined by its ability
to produce various virulence factors
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18. • LD50 (50% lethal dose): Referred as number of
bacteria required to kill half (50%) of the host
• ID50 (50% infectious dose): Referred as number of
bacteria required to cause infection in half (50%) of
the host
• Infectious dose varies among the pathogenic bacteria
BACTERIAL VIRULENCE
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20. VIRULENCE FACTORS
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21. • Adherence
• Invasion of host cells and tissues
• Toxin production:
– Exotoxin
– Endotoxin(lipopolysaccharide)
• Peptidoglycan of gram positive bacteria
• Intracellular survival
• Antigenic heterogeneity
VIRULENCE FACTORS
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22. ADHERENCE
• The first step in bacterial pathogenicity
• Occurs by an interaction between bacterial surface
structures (ligand) and host cell receptor
• ADHERENCE FACTORS:
– Pili or fibriae
– Capsule or glycocalyx
– Teichoic acid of gram positive bacteria
– Adhesin
• Antibody against these adherence factors will
prevent bacterial entry into the host cell
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23. ADHERENCE
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24. • Invasive FACTORS are:
• Enzymes
– Coagulase
– Collagenase and hyaluronidase
– Immunoglobulin A protease: degrades IgA
• Antiphagocytic factors
– Leukocidin :destroy neutrophils, macrophages
– Capsule: prevents adherence of phagocytic cells
– M protein of Group A streptococci: Antiphagocytic
– Protein A of Staphylococcus: Binds IgG thereby block
complement activation
INVASION
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25. • EXOTOXIN
– Are polypeptides secreted from gram positive bacteria
– Secreted by a ‘secretion system’ present in the bacteria
– Some are secreted extracellularly, some in between cells
– Among SIX secretion system, type III is more in virulence
• ENDOTOXIN
– Lipopolysaccharide
– Cell wall of gram negative bacteria
– It is not secreted, but released after death of bacteria
TOXINS
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26. EXOTOXIN ENDOTOXIN
Polypeptide Lipopolysaccharide
From gram positive bacteria From gram negative bacteria
Gene located in the plasmid or
bacteriophage
Gene located in the bacterial
chromosome
Induce antibody (antitoxin) Poorly antigenic
Toxoid (inactive toxin) used as
vaccine
No vaccine production is
possible
Usually destroyed at 600C Stable at 1000C for an hour
DIFFERENCES BETWEEN EXOTOXIN
AND ENDOTOXIN
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27. • Structurally all exotoxins have two sub-units:
– Sub-unit A: A sub-unit acts as enzyme that catalyzes
different activities
– Sub-unit B: This sub-unit binds with membrane receptor of
the target cell
• Sub-unit A causes ADP-ribosylation: adds ADP-ribose
to a target protein and exerts different function
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MECHANISM OF ACTIONS OF
EXOTOXIN

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MECHANISM OF ACTIONS OF
EXOTOXIN

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MECHANISM OF ACTIONS OF
ENDOTOXIN

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VASCULAR EVENTS OF ENDOTOXIN
Initial arteriolar and venular constriction
Peripheral vasodilatation and increase vascular permeability
Decrease venous return, low cardiac output and stagnant microcirculation
Peripheral vasoconstriction, shock and impaired organ perfusion

31. • Gene clusters in a region of chromosome.
• These genes are not capable of self replication.
• Encode different virulence factors.
• Determines different pathogenic strains in a
particular species.
• EXAMPLES OF THE BACTERIA:
– Escherichia coli, Salmonella, Shigella
– Streptococcus pneumoniae
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PATHOGENECITY ISLANDS

32. • More thick in the gram positive bacteria
• Can produce shock due to vascular changes
• There effect is similar as lipopolysaccharide in gram
negative bacteria:
– Peripheral vasodilatation
– Decrease venous return
– Lower cardiac output
– Shock and impaired organ perfusion
PEPTIDOGLYCAN
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33. • Some bacteria grow inside PMN, macrophages or in
monocytes
• They survive by several mechanism:
– Avoid entry into phagolysosome
– Prevent phagososme-lysososme fusion
– Acquire resistance to lysosomal activity
• EXAMPLE: Mycobacterium tuberculosis, Brucella
species, Listeria species
INTRACELLULAR SURVIVAL
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34. • Bacterial surface proteins are antigenic
• One Species of bacteria may have different antigenic
type (serotypes)
– EXAMPLES: 2000 different types of Salmonella
• Microorganism has the ability to make frequent shift
of their antigens
– EXAMPLE: Borrelia recurrentis, Neisseria gonorrhoeae
ANTIGENIC HETEROGENEITY
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35. BACTERIA AND VIRULENCE FACTORS
Corynebacteria diphtheriae
• –May only have 1 virulence mechanism
• –diphtheria toxin
Staphylococcus aureus
• –Express many virulence factors
• –Adhesins, degradative enzymes, toxins, catalase, coagulase
• –Produce spectrum of disease
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36. Mycobacterium tuberculosis
• –able to inhibit phagolysosome fusion and promote the
development of a granuloma
• –viable bacteria may reside inside the infected person
Neisseria gonorrhoeae
• –Vary the structure of surface antigen
• –Produce protease to degrade IgA
BACTERIA AND VIRULENCE FACTORS
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37. • Warren Levinson. Review of Medical Microbiology
and Immunology, 11th edition (2010). Appleton and
Lange.
• Geo. F. Brooks, Karen C. Carroll, Janet S. Butel,
Stephen A. Morse, Timothy A. Mietzner. Medical
Microbiology, 25th edition (2010). Appleton & Lange.
REFERENCES
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