1. Microbial Mechanisms of Pathogenicity
Chapter 15
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. 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

7. Penetration into the Host Cell
Capsule
Components of cell wall
Enzymes: leukocidin, hemolysin
Penetration into the host cell cytoskeleton
Figure 15.2

8. 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

9. How Bacterial pathogens damage host cell
Direct damage
Toxins

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

11. By-products of growing cell
Exotoxin
Neurotoxin
Cytotoxin
Enterotoxin
Source
Mostly Gram +
Metabolic product
By-products of growing cell
Chemistry
Protein
Fever? No
Neutralized by antitoxin
Yes
LD50
Small

12. Exotoxins Superantigens or type I toxins
Cause an intense immune response due to release of cytokines from host cells
Fever, nausea, vomiting, diarrhea, shock, death

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

15. Exotoxins
A-B toxins or type III toxins
Figure 15.5

16. 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. Enterotoxin.

17. Exotoxins
Figure 15.4a

18. Endotoxin
Figure 15.4b

19. Endotoxins
Figure 15.6

20. Endotoxins Source Gram– Metabolic product
Present in LPS of outer membrane
Chemistry
Lipid
Fever?
Yes
Neutralized by antitoxin No
LD50
Relatively large

21. Cytopathic Effects of Viruses
Table 15.4

22. Pathogenic Properties of Fungi
Fungal waste products may cause symptoms
Chronic infections provoke an allergic response
Tichothecene: toxins inhibit protein synthesis
Headache, chills, sever nausea, vomiting, visual disturbance
Fusarium
Proteases
Candida albicans, Trichophyton
Capsule prevents phagocytosis
Cryptococcus neoformans
Ergot toxin: hallucination, gangrene of limbs
Claviceps purpurea

23. Pathogenic Properties of Fungi
Aflatoxin: mutagenic toxin
Aspergillus flavus
Mycotoxins
Neurotoxins: Phalloidin, amanitin
Amanita phalloides

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

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

26. Pathogenic Properties of Algae
Neurotoxins produced by dinoflagellates
Saxitoxin
Paralytic shellfish poisoning

27. Portals of Exit Respiratory tract Coughing, sneezing
Gastrointestinal tract
Feces, saliva
Genitourinary tract
Urine, vaginal secretions
Skin
Blood
Biting arthropods, needles/syringes

28. Mechanisms of Pathogenicity
Figure 15.9