Learning Objective

By the end of this section, students should be able to differentiate endotoxins and exotoxins, describe their mechanisms of action, clinical effects, and the role of major bacterial toxins in disease, including structural toxins, protein toxins, and cytolysins.

Toxins – Overview

Bacterial toxins can:

Aid in invasiveness
Damage host cells
Inhibit cellular processes
Trigger immune-mediated tissue damage

Structural Toxins

Toxin	Source	Mechanism / Features	Clinical Significance
Lipopolysaccharide (LPS, endotoxin)	Gram-negative outer membrane	Lipid A is released on cell death (exception: N. meningitidis overproduces fragments); heat-stable and not strongly immunogenic	Activates macrophages → TNF-α, IL-1, IL-6 → fever, vasodilation, DIC; septic shock
Peptidoglycan, Teichoic acids	Gram-positive cell wall	Structural components are released upon cell death	Stimulate immune response, neutrophil chemotaxis, tissue damage

Key Points:

Endotoxins are heat-stable and cannot form a toxoid.
The main mediator of Gram-negative septic shock.

Exotoxins

Features:

Secreted protein toxins (Gram-positive or Gram-negative)
Highly toxic; some can be chemically or thermally inactivated to form toxoids for vaccines
Classified as:
- A-B toxins: B binds host receptor; A is the active component (often an enzyme, e.g., ADP ribosyltransferase)
- Cytolysins: lyse host cells by disrupting membranes
- Neurotoxins, enterotoxins, superantigens, cAMP inducers

Activity

Major Exotoxins Table

Organism (Gram)	Toxin	Mechanism / Target	Role in Disease
Corynebacterium diphtheriae (+)	Diphtheria toxin	ADP ribosyltransferase; inactivates eEF-2	Inhibits protein synthesis; heart, nerve, epithelium
Pseudomonas aeruginosa (−)	Exotoxin A	ADP ribosyltransferase; inactivates eEF-2	Inhibits protein synthesis; liver
Shigella dysenteriae (−)	Shiga toxin	60S ribosomal subunit	Cytotoxic, enterotoxic, neurotoxic; inhibits protein synthesis
EHEC (−)	Verotoxin (Shiga-like)	60S ribosomal subunit	Enterohemorrhagic; inhibits protein synthesis
Clostridium tetani (+)	Tetanus toxin	Blocks glycine & GABA release	Inhibits inhibitory synapses → spastic paralysis
Clostridium botulinum (+)	Botulinum toxin	Blocks acetylcholine release	Flaccid paralysis
Staphylococcus aureus (+)	TSST-1	Superantigen	Fever, shock, rash, capillary leakage
Streptococcus pyogenes (+)	Exotoxin A (erythrogenic)	Superantigen	Fever, rash, shock, cardiotoxicity
ETEC (−)	Heat-labile toxin (LT)	ADP ribosylates G protein → ↑cAMP	Secretory diarrhea
Vibrio cholerae (−)	Cholera toxin	ADP ribosylates G protein → ↑cAMP	Profuse watery diarrhea
Bacillus anthracis (+)	Anthrax toxin	EF = adenylate cyclase; LF = lethal factor; PA = protective antigen	Decreases phagocytosis, causes edema, kills cells
Bordetella pertussis (−)	Pertussis toxin	ADP ribosylates Gi → ↑cAMP	Lymphocytosis, histamine sensitization, pertussis symptoms

Activity

Cytolysins / Membrane-Damaging Toxins

Organism	Toxin	Mechanism	Effect
Clostridium perfringens (+)	Alpha toxin	Lecithinase	Damages membranes → myonecrosis (gas gangrene)
Staphylococcus aureus (+)	Alpha toxin	Inserts into the membrane, forming pores	Membrane becomes leaky → cell lysis