Chapter 15: Microbial Mechanisms of Pathogenicity

1. Pathogenicity vs. Virulence

Pathogenicity

• Ability of a microorganism to cause disease.

• Qualitative (yes/no).

Virulence

• Degree of pathogenicity.

• Measures how severe a disease is.

• Quantitative.

Remember:

• Pathogenicity = Can it cause disease?

• Virulence = How severe is the disease?

2. Portals of Entry

A pathogen must enter through a portal of entry.

Common Portals

Mucous Membranes

• Respiratory tract

• Digestive tract

• Genitourinary tract

• Conjunctiva

Skin

• Usually blocks pathogens

• Entry through hair follicles and sweat glands

Parenteral Route

Direct deposition beneath skin or mucous membrane via:

• Punctures

• Injections

• Bites

• Cuts

Exam Tip

Blood is NOT a portal of entry.

3. Infectious Dose and Virulence

ID₅₀

• Infectious dose for 50% of hosts.

LD₅₀

• Lethal dose for 50% of hosts.

Interpretation

• Lower ID₅₀ = More infectious

• Lower LD₅₀ = More deadly

Example

Most Infectious?

✅ Legionella

Because it takes only 1 cell to infect.

4. Adherence (Attachment)

Pathogens attach using:

Adhesins

• Surface proteins

• Found on fimbriae

Host Receptors

Usually:

• Mannose-containing glycoproteins

Why Important?

No attachment = no infection.

Exam Tip

If a drug blocks mannose receptors:
→ Pathogenic E. coli cannot attach.

5. Capsules

Function

Prevent phagocytosis.

Why Important?

Phagocytes cannot easily engulf encapsulated bacteria.

Exam Question

Why are encapsulated bacteria virulent?

✅ Capsule resists phagocytosis.

6. Cell Wall Components

M Protein (Streptococcus pyogenes)

Function:

• Prevents phagocytosis

Drug binding M protein:

• Decreases virulence

7. Enzymes That Increase Virulence

Coagulase

Function

Creates fibrin clot around bacteria.

Benefit

• Hides bacteria from immune system.

Kinases

Function

Destroy blood clots.

Benefit

Allows spread through tissues.

Examples:

• Streptokinase

• Staphylokinase

Hyaluronidase

Function

Breaks down hyaluronic acid.

Benefit

Allows bacteria to move between cells.

Nickname:
"Spreading factor"

Collagenase

Function

Breaks down collagen.

Benefit

Invades connective tissue.

IgA Proteases

Function

Destroy IgA antibodies.

Benefit

Avoid immune defenses.

8. Siderophores

Function

Steal iron from host.

Iron is required for bacterial growth.

Exam Question

What happens if a drug binds iron in blood?

✅ Bacteria cannot obtain iron.
✅ Reduced pathogenicity.

9. Antigenic Variation

Definition

Changing surface antigens.

Benefit

Antibodies no longer recognize pathogen.

Examples:

• Neisseria gonorrhoeae

• Trypanosomes

10. Invasins

Function

Alter host cytoskeleton.

Benefit

Allows bacteria to enter cells.

11. Biofilms

Definition

Communities of microbes embedded in EPS.

Advantages

• Protection from antibiotics

• Protection from phagocytes

• Better adherence

12. Toxins

Exotoxins

Characteristics

Examples:

• Botulinum toxin

• Tetanus toxin

• Diphtheria toxin

Endotoxins

Characteristics

Example:

• Lipid A of LPS

Effects:

• Fever

• Shock

• Inflammation

13. A-B Toxins

Structure

A = Active component

• Causes damage

B = Binding component

• Attaches to host cell

Examples:

• Diphtheria toxin

• Cholera toxin

• Shiga toxin

14. Shiga Toxin (DRAW IT Question)

Steps

1. B subunit binds receptor on cell.

2. Cell engulfs toxin.

3. Toxin enters vesicle.

4. A subunit separates.

5. A subunit reaches ribosome.

6. Protein synthesis stops.

7. Cell dies.

Key Point

Shiga toxin inhibits protein synthesis.

15. Membrane-Disrupting Toxins

Hemolysins

Destroy red blood cells.

Example:

• Streptolysin

Leukocidins

Destroy white blood cells.

Benefit:

• Weakens immune defenses.

16. Superantigens

Cause massive cytokine release.

Symptoms:

• Fever

• Shock

• Nausea

Example:

• Toxic Shock Syndrome Toxin

17. Plasmids and Virulence

Plasmids often carry:

• Toxin genes

• Antibiotic resistance genes

Multiple Choice

Removal of plasmids reduces virulence in:

✅ E. coli

18. Viral Pathogenicity

Viruses avoid immunity by:

Growing Inside Cells

Antibodies cannot easily reach them.

Antigenic Variation

Change surface proteins.

Receptor Mimicry

Bind normal host receptors.

19. Fungi, Protozoa, and Helminths

Fungi

Disease caused by:

• Toxins

• Proteases

• Allergic reactions

Protozoa

Disease caused by:

• Tissue damage

• Waste products

• Antigenic variation

Helminths

Disease caused by:

• Tissue damage

• Immune reactions

• Nutrient theft

20. Important Clinical Cases

Tetanus Case

Puncture wound → jaw tightness → spasms

Cause:
✅ Tetanus toxin from Clostridium tetani

Type:
✅ Intoxication

Contagious?
❌ No

Food Infection vs Intoxication

Food Infection

Microbe grows inside body.

Usually:

• Fever

• Diarrhea

Example:

• Salmonella

Food Intoxication

Preformed toxin consumed.

Example:

• Botulism

Symptoms:

• Neurological problems

• Blurred vision

• Paralysis

MUST MEMORIZE TABLE

Highest-Yield Exam Facts

1. Capsules prevent phagocytosis.

2. Mannose receptors are common attachment sites.

3. Shiga toxin inhibits protein synthesis.

4. Exotoxins are proteins; endotoxins are Lipid A.

5. Lower ID₅₀ = greater infectivity.

6. Lower LD₅₀ = greater virulence.

7. Blood is not a portal of entry.

8. Tetanus symptoms are caused by toxin (intoxication).

9. Siderophores obtain iron from the host.

10. Biofilms protect microbes from antibiotics and phagocytes.

If your exam is multiple choice, those 10 facts alone will answer a large percentage of Chapter 15 questions.