1. Chapter 25 Lecture Outline
Microbial Pathogenesis

2. Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.
Overview
25.1 Host Pathogen Interactions
25.3 Virulence factors: Microbial attachment
25.4 Toxins: A way to subvert host cell function
25.5 Type III protein secretion and pathogenesis
25.6 Finding virulence genes with signature tagged mutagenesis
25.7 Surviving within the host
25.8 Viral pathogenesis
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

3. Pathogens and Parasites
Pathogens: bacterial, viral, and fungal agents of disease
Parasites: protozoans and worms causing disease
Definitions are typically used in microbial pathogenesis
Fungus Trichophyton rubrum causing athletes’s foot.
Filaria Wucheria bancrofti causing elephantiasis.
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

4. Host-Pathogen Interactions: Definitions (1)
Colonization
Presence of microbes on body surfaces
Infection
Entry and growth of pathogen or parasite
Infection does not always cause disease
Most infections removed by immune system
Primary pathogens
Have ability to penetrate host defenses and cause disease in a healthy host
Opportunistic pathogens
Cause disease only in compromised hosts
Immune system defective
Break in tissue barrier allows organism access to new site
Loss of normal microbiota allows organism to bloom
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

5. Host-Pathogen Interactions: Definitions (2)
Virulence
Measure of degree or severity of disease
Rate of lethal infections
Lethal dose = LD50
Number of organisms to kill 50% of hosts
Infectious dose (for organisms that do not cause death) = ID50
Number of organisms to colonize 50% of hosts
Pathogenicity of organism
Measure of ability to cause disease
A function of infectivity and virulence
Determined by genetic makeup of organism
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

6. Measurement of Virulence
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

7. Infection Cycles Transmission Entry Dissemination Exit Colonization
Invasion
Dissemination
Exit
Adherence factors
Toxins
Immune evasion
Pathogenicity factors
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

8. Transmission of Infections (1)
Spread via direct contact
Example: common cold
Indirect contact
Contact with fomites
Inanimate objects
Example: S. aureus infection
Via vectors
Example: Mosquitoes (malaria)
Reservoir for disease organism
May not show disease symptoms
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

9. Transmission of Infections (2)
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

10. Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.
Port of Entries
Skin
Hard to penetrate (keratin)
Requires usually lesion or injection
Mucosal surfaces
Respiratory tract
Intestinal tract
Urogenital tract
Exit site is usually same as entry site
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

11. Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.
Adherence Factors
Serve microbial attachment
Human body expels invaders
Mucosa, dead skin constantly expelled
Liquid expelled from bladder
Coughing, cilia in lungs
Expulsion of intestinal contents
Bacteria must adhere to host tissue
Pili (fimbriae)
Hollow fibrils with tips to bind to host cells
Adhesins
surface proteins that bind to host cells
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

12. Microbial Attachment via Pili
Type I pili adhere to mannose
Grows from outer membrane
New subunits added at base
E.g. uropathogenic E.coli
Type IV pili
Grow from inner membrane
Can grow and retract
Twitching motility
E.g. Neisseria gonorrhoeae
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

13. Microbial Attachment via Adhesins
Streptococcus pyogenes uses M protein to attach to respiratory cells
B. pertussis uses pertactin and filamentous hemagglutinin to bind to respiratory ciliated cells
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

14. Toxins Subvert Host Function
Exotoxins
Secreted
Typically proteins
Specific targets and effects
Endotoxin
Same as lipopolysaccharide
Part of the outer membrane
Activates host defense resulting in acute inflammatory response including fever
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

15. Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.
Exotoxins
Five target of protein exotoxins
Cell membrane
Leakage
Protein synthesis
Inhibition
2nd messenger pathway
Disruption
Immune system
Overstimulation (superantigens)
Proteases
Degrade host proteins
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

16. Mode of Action of Selected Exotoxins
Pore forming toxins
E.G: S. aureus alpha toxin
Transmembrane, oligomeric, beta barrel pore
AB toxin
B: mediates binding to host
A: subunit with toxic activity cleaving 28S rRNA in eukaryotic ribosomes
E. coli enterotoxigenic heat stable toxin
Stimulates guanylate cyclase
Increases cGMP levels
Affects electrolyte flux, increases water efflux.
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

17. ADP-Ribosylating Toxins
Often AB toxins
Active toxin is an ADP-ribosyltransferase
Found among toxins that inhibit protein synthesis (e.g. diphteria toxin) and among toxins that disrupt 2nd messenger pathways (e.g. cholera toxin)
ADP-ribosylated proteins may be inhibited or activated
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

18. Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.
ADP-Ribosylating Toxins
V. cholerae
Cholera toxin
Ribosylates to overactivate adenylate cyclase
cAMP activates ion transport, water follows
Uncontrollable diarrhea
Diphtheria toxin
Ribosylates elongation factor 2
Blocks ribosome function, cell dies
Forms pseudomembrane over trachea
C. diphtheriae
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

19. ADP-Ribosylating Toxins
Animation: Cholera Toxin Mode of Action
Click box to launch animation
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

20. Type III Protein Secretion
Injects proteins directly into host cell
“Hypodermic needle” similar to base of flagellum!
Genes on pathogenicity island
Injected proteins cause host to engulf bacterium
Salmonella injects over 13 toxins
Alters fusion of vesicles in cell
Causes diarrhea in host
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

21. The Needle Complex of the Type III Secretion System
Proteins are directly injected into host cytoplasma
Apparatus is related to flaggela assembly proteins
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

22. Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.
Immune Evasion
Prevent uptake
Allow uptake but prevent degradation by host
Facultative intracellular organisms
Obligate intracellular organisms
Change surface
Inhibit immune defenses
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

23. Surviving within the Host
Outside host cell, how to avoid death?
Complement, antibodies bind pathogen and enhance uptake by phagocytes (opsonization)
Some pathogens secrete thick capsule
Streptococcus pneumoniae, Neisseria meningitidis
Some pathogens make proteins to bind antibodies
Staphylococcus aureus cell wall Protein A
Antibodies attach “upside down”
Prevents opsonization
Some pathogens cause apoptosis (suicide) of phagocytes
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

24. Surviving within the Host
Once inside host cell, how to avoid death?
Cell ingests pathogens in phagosome
Some pathogens use hemolysin to break out
Shigella dysenteriae, Listeria monocytogenes
Phagosome fuses with acidic lysosome
Some pathogens secrete proteins to prevent fusion
Salmonella, Chlamydia, Mycobacterium
Some pathogens mature in acidic environment
Coxiella burnetii—Q fever
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

25. How do Bacteria Recognize Host Environment?
Two-component signal transduction
Detect magnesium concentration, pH
Both low in host cell vacuole
Quorum sensing
Detect exotoxins made by other cells
Delays toxin synthesis until many bacteria present
Possible pathway for preventing pathogen growth?
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

26. Selected Viral Pathogenesis: Immune Evasion
Change of capsid alteration allows virus to re-infect
Creates novel epitopes
Influenza virus antigenic drift
Subtle changes in surface structure
RNA Polymerase does not correct replication errors
Frequent mutations in haemagglutinin gene
If allows virus to avoid immune system, virus spreads
New flu vaccine needed every year
Influenza virus antigenic shift
Major changes in surface proteins
Infection of the immune system
HIV infects T helper cells
Central regulators of our adaptive immune system
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

27. Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.
Concept Quiz
Bacteria prevent being flushed from the intestine by means of
The Type II secretion system
Inhibition of immune system cells
Type IV pili and adhesins
Answer: C
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

28. Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.
Concept Quiz
Cholera toxin A subunit causes ADP-ribosylation of its target, and is an example of
A neurotoxin, which inhibits neural function
An enterotoxin, which disrupts function of the intestine
A cytotoxin, which kills infected host cells
Answer: B
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

29. Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.
Concept Quiz
Most bacteria avoid the lysosome. One bacterium that welcomes lysosome acidity is
Shigella flexneri
Legionella pneumophila
Coxiella burnetii
Answer: C
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

30. Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.
Concept Quiz
New vaccines to influenza virus must be taken each year, because this virus undergoes
Antigenic drift
Binding to multiple receptors
Inhibition of p53 to allow unchecked growth
Answer: A
Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.