Steps in Bacterial Pathogenesis

Overview of Pathogenesis

Bacterial pathogenesis involves a series of steps by which bacteria infect a host and cause disease. Understanding these steps is crucial for comprehending how infections develop and how they can be prevented or treated.

• Reservoirs: Natural habitats where pathogens live and multiply.

• Transmission: The process by which pathogens are spread to a new host.

• Entry to the Host's Body: Pathogens gain access through portals such as the respiratory tract, gastrointestinal tract, or skin.

• Adhesion to the Host's Tissue: Bacteria use adhesins to attach to host cells.

• Invasion, Colonization, and Growth within the Host's Tissue:

  • Internalization into non-phagocytic host cells

  • Production of spreading factors

  • Evading the immune system

• Production of Toxins: Bacteria produce toxins that damage host tissues and contribute to disease.

• Tissue Damage and Disease: The culmination of the above steps leads to clinical symptoms.

Damage Done to the Host

Mechanisms of Host Damage

Bacterial infections can harm the host through several mechanisms, either directly or indirectly.

• Direct Action of the Pathogen:

  • Intracellular bacteria (e.g., Mycobacterium, Legionella)

  • Destruction of host cells by metabolic products (e.g., acids)

  • Production of spreading factors

• By Host Defenses:

  • Immune-mediated damage (e.g., rheumatic fever, acute glomerulonephritis)

• By Bacterial Toxins:

  • Exotoxins

  • Endotoxins

Toxicity of Some Naturally Occurring Substances

Comparative Toxicity Table

This table compares the toxicity of various substances, highlighting the potency of bacterial toxins.

Bacterial Toxins

Definition and Importance

Bacterial toxins are proteins or cell wall components that, when isolated and purified, can cause intoxication in the host. They are major virulence factors responsible for many symptoms of bacterial diseases.

• Exotoxins: Proteins, usually secreted by the pathogen.

• Endotoxins: Lipopolysaccharides (LPS) of Gram-negative bacteria; teichoic and lipoteichoic acids of Gram-positive bacteria.

• Enterotoxins: Exotoxins produced by enterobacteria, often affecting the intestines.

Classes and Classification of Toxins

Major Classes

• Exotoxins:

  • A-B toxins

  • Cytolytic toxins

  • Superantigens

  • Heat stable toxins

• Endotoxins

Naming of Toxins

Toxins are named based on various criteria, but the system is often inconsistent.

• Bacterial species producing the toxin/disease (e.g., cholera toxin, tetanus toxin)

• Affected organ/tissue (e.g., neurotoxin, enterotoxin)

• Enzymatic activity (e.g., ADP-ribosylase toxin)

• Symptoms/effects (e.g., edema-producing toxin)

• Capital letter (e.g., toxin A)

• Some toxins have even two names (e.g., shiga-like toxin = verotoxin)

Note: Naming of toxins is confusing and lacks a real system.

A-B Toxins

Structure and Mechanism

A-B toxins are a major class of exotoxins consisting of two components:

• A (Active) Subunit: Possesses enzymatic activity and is responsible for the toxic effect.

• B (Binding) Subunit: Binds to specific receptors on the host cell surface and mediates uptake of the A subunit into the host cell.

The A and B subunits can be separate proteins or part of a single polypeptide chain.

Uptake Mechanisms

• B subunit binds to the eukaryotic receptor on the target cell.

• Both subunits are taken up into the cell, often via endocytosis.

• Subunit A is translocated into the cytoplasm, where it exerts its toxic effect.

Types of A-B Toxins (by Enzymatic Function)

• ADP-Ribosyltransferases

• Glycosyltransferases

• N-Glycosidases

• Metalloendoproteases

• Adenylyl cyclases

ADP-Ribosyltransferases

Mechanism and Examples

• Examples: Cholera toxin, pertussis toxin, diphtheria toxin

• Use NAD as ADP-ribose donor

• ADP-ribose becomes attached to target protein via an N- or S-glycosidic bond

• Target proteins:

  • Small GTP-binding proteins (e.g., cholera toxin, pertussis toxin)

  • Elongation factor 2 (diphtheria toxin)

Mode of Action of Cholera Toxin

• Cholera toxin ADP-ribosylates the Gs protein, leading to permanent activation of adenylate cyclase.

• Increased cAMP levels cause excessive secretion of chloride ions and water into the intestinal lumen, resulting in diarrhea.

Equation:

Mode of Action of Diphtheria Toxin

• Diphtheria toxin ADP-ribosylates elongation factor 2 (EF-2), inhibiting protein synthesis.

• This leads to cell death due to the shutdown of protein synthesis.

Glycosyltransferases

Mechanism and Examples

• Examples: Clostridioides difficile toxins TcdA, TcdB (also called Large Clostridial Toxins, LCTs)

• AB-toxin taken up via endocytosis; drop in pH mediates transport of A-subunit into cytoplasm

• Use UDP-glucose as glucose donor

• Glucose becomes attached to a hydroxy group of threonine in target proteins

• Target proteins: small GTP-binding proteins (Rho, Rac, Ras, Cdc42)

• Glycosylation inactivates host proteins, leading to loss of cell signaling, cell death, and diseases such as pseudomembranous colitis

N-Glycosidases

Mechanism and Examples

• Examples: Shiga toxin (Shigella dysenteriae), Shiga-like toxin (enterohemorrhagic E. coli)

• Inhibit protein translation by hydrolyzing the N-glycosidic bond in 28S rRNA

• Consequences: death of epithelial cells, reduced water resorption, diarrhea

Metalloendoproteases

Mechanism and Examples

• Examples: Tetanus toxin (Clostridium tetani), botulinum toxin (C. botulinum)

• These are neurotoxins that cleave proteins involved in neurotransmitter release, leading to paralysis or muscle spasms

Summary Table: Major Bacterial Toxins and Their Actions

Applications and Clinical Relevance

• Vaccination: Many vaccines target bacterial toxins (e.g., diphtheria, tetanus toxoids) to prevent disease.

• Diagnosis: Detection of toxins in clinical samples is crucial for diagnosing certain infections (e.g., Clostridioides difficile colitis).

• Treatment: Understanding toxin mechanisms guides the development of antitoxins and supportive therapies.