Microbial Mechanisms of Pathogenicity: Study Guide

Microbial Mechanisms of Pathogenicity

Principal Portals of Entry

Microorganisms must enter the host through specific routes known as portals of entry to initiate infection. These portals are anatomical sites where pathogens gain access to host tissues.

• Mucous membranes: Includes respiratory tract (e.g., inhalation), gastrointestinal tract (e.g., ingestion), genitourinary tract, and conjunctiva.

• Skin: Usually through breaks, cuts, or bites.

• Parenteral route: Direct deposition into tissues via punctures, injections, bites, or wounds.

• Example: Streptococcus pneumoniae enters via the respiratory tract.

ID50 and LD50

These are quantitative measures of pathogenicity and virulence.

• ID50 (Infectious Dose 50): The number of microbes required to cause infection in 50% of a test population.

• LD50 (Lethal Dose 50): The amount of toxin or number of microbes required to kill 50% of a test population.

• Formula:

• $\text{ID}_{50} = \text{Dose causing infection in 50\% of subjects}$

• $\text{LD}_{50} = \text{Dose causing death in 50\% of subjects}$

Microbial Adherence to Host Cells

Adherence is a critical step in pathogenesis, allowing microbes to attach to host tissues.

• Adhesins: Surface molecules on pathogens that bind to specific host cell receptors.

• Examples: Fimbriae of Escherichia coli, glycocalyx of Streptococcus mutans.

• Biofilm formation: Microbes may aggregate and adhere in biofilms, increasing resistance.

Capsules and Cell Wall Components in Pathogenicity

Structural features of microbes can enhance their ability to cause disease.

• Capsules: Polysaccharide layers that inhibit phagocytosis (e.g., Streptococcus pneumoniae).

• Cell wall components: Include M protein (Streptococcus pyogenes), mycolic acid (Mycobacterium tuberculosis), and others that resist immune responses.

Effects of Coagulases, Kinases, Hyaluronidase, and Collagenase

These enzymes modify host tissues to facilitate infection.

• Coagulases: Cause blood clotting, protecting bacteria from phagocytosis.

• Kinases: Dissolve clots, allowing spread (e.g., streptokinase).

• Hyaluronidase: Breaks down hyaluronic acid in connective tissue, aiding spread.

• Collagenase: Breaks down collagen, facilitating tissue invasion.

Antigenic Variation

Pathogens alter their surface antigens to evade immune detection.

• Definition: The process by which microbes change their surface proteins.

• Example: Neisseria gonorrhoeae changes its pili proteins.

Bacterial Use of Host Cell Cytoskeleton

Bacteria manipulate the host cell cytoskeleton to facilitate entry and movement.

• Mechanism: Induction of actin rearrangement to engulf bacteria (e.g., Salmonella).

• Example: Shigella uses actin to move within and between cells.

Mechanisms of Avoiding Destruction by Phagocytosis

Pathogens employ various strategies to resist phagocytic killing.

• 1. Capsule formation

• 2. Inhibition of phagosome-lysosome fusion

• 3. Survival within phagolysosome

• 4. Escape from phagosome

• 5. Killing phagocytes (e.g., leukocidins)

• 6. Suppression of immune response

Siderophores

Siderophores are molecules secreted by bacteria to acquire iron from the host.

• Function: Bind iron tightly and transport it into bacterial cells.

• Importance: Iron is essential for microbial growth; host limits iron availability as a defense.

Direct Damage vs. Toxin Production

Pathogens can cause disease by directly damaging host cells or by producing toxins.

• Direct damage: Physical destruction of host cells during invasion and multiplication.

• Toxin production: Release of toxic substances that disrupt host cell function.

• Example: Clostridium perfringens produces toxins causing tissue necrosis.

Exotoxins vs. Endotoxins

These are two major classes of bacterial toxins with distinct properties.

• Exotoxins: Proteins secreted by bacteria; highly specific and potent; mostly from Gram-positive bacteria.

• Endotoxins: Lipopolysaccharide (LPS) component of Gram-negative bacterial cell wall; released upon cell death.

• Effects: Exotoxins cause specific symptoms; endotoxins cause generalized inflammation and fever.

Mechanisms of Action of Toxins

Bacterial toxins act via several mechanisms:

• A-B toxins: Two-part toxins; A (active) and B (binding) components. Example: Diphtheria toxin.

• Membrane-disrupting toxins: Cause cell lysis by disrupting plasma membranes.

• Superantigens: Trigger excessive immune response, leading to shock.

• Genotoxins: Damage host DNA, potentially leading to cancer.

LAL Assay

The Limulus Amebocyte Lysate (LAL) assay is used to detect endotoxins.

• Importance: Ensures pharmaceutical products are free from endotoxin contamination.

• Mechanism: Uses horseshoe crab blood, which clots in the presence of endotoxin.

Roles of Plasmids and Lysogeny in Pathogenicity

Genetic elements contribute to microbial virulence.

• Plasmids: Carry genes for toxins, antibiotic resistance, and other virulence factors.

• Lysogeny: Integration of bacteriophage DNA can confer new traits, such as toxin production.

• Example: Corynebacterium diphtheriae produces diphtheria toxin only when lysogenized.

Cytopathic Effects of Viral Infections

Viruses cause distinct changes in host cells, known as cytopathic effects (CPE).

• 1. Cell death

• 2. Inclusion bodies

• 3. Syncytium formation

• 4. Cell transformation

• 5. Loss of contact inhibition

• 6. Cell fusion

• 7. Chromosomal changes

• 8. Disruption of host cell function

• 9. Altered cell membrane permeability

• 10. Production of interferons

• 11. Antigenic changes

Causes of Symptoms in Fungal, Protozoan, Helminthic, and Algal Diseases

Different types of pathogens cause symptoms via unique mechanisms.

• Fungi: Produce toxins (mycotoxins), provoke allergic responses, or invade tissues.

• Protozoa: Damage host cells directly or via immune response.

• Helminths: Cause mechanical damage, provoke inflammation, or secrete toxic products.

• Algae: Produce toxins (e.g., saxitoxin) that affect humans via contaminated food or water.

Portal of Entry vs. Portal of Exit

Pathogens enter and leave the host via specific routes.

• Portal of entry: Site where pathogen enters the host (e.g., respiratory tract).

• Portal of exit: Site where pathogen leaves the host (e.g., respiratory secretions, feces).

• Importance: Understanding these routes is crucial for disease prevention and control.