1. Microbial Mechanisms of Pathogenicity
Chapter 8
Microbial Mechanisms of Pathogenicity

2. Microbial Mechanisms of Pathogenicity
Pathogenicity: The ability to cause disease.
Virulence: The extent of pathogenicity.

3. Portals of Entry
Mucous membranes
Skin
Parenteral route

4. Numbers of Invading Microbes
ID50: Infectious dose for 50% of the test population.
LD50: Lethal dose (of a toxin) for 50% of the test population.

5. Bacillus Anthracis Portal of Entry ID50 Skin 10-50 endospores
Inhalation
10,000-20,000 endospores
Ingestion
250,000-1,000,000 endospores

6. Toxins Portal of Entry ID50 Botulinum 0.03 ng/kg Shiga toxin 250 ng/kg
Staphylococcal enterotoxin
1350 ng/kg

7. Adherence Adhesions/ligands bind to receptors on host cells
Glycocalyx: Streptococcus mutans
Fimbriae: Escherichia coli
M protein: Streptococcus pyogenes
Opa protein: Neisseria gonorrhoeae
Tapered end: Treponema pallidum

8. Adherence
Figure 15.1

9. Capsules Prevent phagocytosis Streptococcus pneumoniae
Haemophilus influenzae
Bacillus anthracis

10. Cell Wall Components M protein resists phagocytosis
Streptococcus pyogenes
Opa protein inhibits T helper cells
Neisseria gonorrhoeae
Mycolic acid (waxy lipid) resists digestion
Mycobacterium tuberculosis

11. Enzymes Coagulase: Coagulate blood Kinases: Digest fibrin clots
Hyaluronidase: Hydrolyses hyaluronic acid
Collagenase: Hydrolyzes collagen
IgA proteases: Destroy IgA antibodies
Siderophores: Take iron from host iron-binding proteins
Antigenic variation: Alter surface proteins

12. Penetration into the Host Cell
Figure 15.2

13. Toxins Toxin: Substances that contribute to pathogenicity.
Toxigenicity: Ability to produce a toxin.
Toxemia: Presence of toxin in the host's blood.
Toxoid: Inactivated toxin used in a vaccine.
Antitoxin: Antibodies against a specific toxin.

14. Present in LPS of outer membrane
Endotoxins
Source:
Gram –
Relation to microbe:
Present in LPS of outer membrane
Chemistry:
Lipid
Fever?
Yes
Neutralized by antitoxin? No
LD50:
Relatively large
Figure 15.4b

15. By-products of growing cell
Exotoxin
Source:
Mostly Gram +
Relation to microbe:
By-products of growing cell
Chemistry:
Protein
Fever? No
Neutralized by antitoxin?
Yes
LD50:
Small
Figure 15.4a

16. The Action of an A-B Exotoxin
Figure 15.5

17. Exotoxins Membrane-disrupting toxins Lyse host’s cells by
Making protein channels in the plasma membrane (e.g., leukocidins, hemolysins).
Disrupting phospholipid bilayer.

18. Exotoxins Superantigens
Cause an intense immune response due to release of cytokines from host cells.
Fever, nausea, vomiting, diarrhea, shock, and death.

19. Exotoxins Specific for a structure or function in host cell
Figure 15.4a

20. Exotoxins Exotoxin Lysogenic conversion Corynebacterium diphtheriae
A-B toxin. Inhibits protein synthesis. +
Streptococcus pyogenes
Membrane-disrupting. Erythrogenic.
Clostridium botulinum
A-B toxin. Neurotoxin
C. tetani
Vibrio cholerae
A-B toxin. Enterotoxin
Staphylococcus aureus
Superantigen.

21. Endotoxins
Figure 15.6

22. Cytopathic Effects of Viruses
Table 15.4

23. Pathogenic Properties of Fungi
Fungal waste products may cause symptoms.
Chronic infections provoke an allergic response.
Tichothecene toxins inhibit protein synthesis.
Fusarium
Proteases
Candida, Trichophyton
Capsule prevents phagocytosis.
Cryptococcus
Ergot toxin
Claviceps

24. Pathogenic Properties of Fungi
Aflatoxin
Aspergillus
Mycotoxins
Neurotoxins: Phalloidin, amanitin
Amanita

25. Pathogenic Properties of Protozoa
Presence of protozoa.
Protozoan waste products may cause symptoms.
Avoid host defenses by
Growing in phagocytes
Antigenic variation

26. Pathogenic Properties of Helminths
Use host tissue.
Presence of parasite interferes with host function.
Parasite's metabolic waste can cause symptoms.

27. Pathogenic Properties of Algae
Paralytic shellfish poisoning
Dinoflagellates
Domoic acid intoxication
Diatoms
Figure 27.15

28. Portals of Exit Respiratory tract Coughing and sneezing
Gastrointestinal tract
Feces and saliva
Genitourinary tract
Urine and vaginal secretions
Skin
Blood
Biting arthropods and needles or syringes

29. Mechanisms of Pathogenicity
Figure 15.9