Week 8

• Toxins are poisonous substances produced by certain microorganisms
• Effects of toxins

  • Can produce fever, cardiovascular problems, diarrhea, shock
  • Can block protein synthesis, destroy blood cells, blood vessels, disrupt the nervous system, disrupt the immune response
  • Toxins are the most powerful human poison known (e.g., botulinum, diphtheria, and tetanus toxins)
• 30 ng (0.000 000 030g) is sufficient to induce botulism in human adults by the oral route
• Toxin nomenclature

  • Named by activity, cell target (neurotoxin, enterotoxin, cytotoxins)
  • Named by producer (cholera toxin, tetanus toxin, diphtheria, etc.)
  • Named by order of discovery (a, b, d specific to species)
• Types of toxins

  • Endotoxins and cell wall components
  • Exotoxins: released from live bacteria, including Cytolysins, A-B toxins, and Toxins acting on host defences (superantigens)
• Endotoxins

  Released from growing bacteria, released from bacteria lysed by host defences, released upon antibiotic treatment
• Exotoxins

  Released from live bacteria, including Cytolysins with cell membrane targets, A-B toxins with intracellular targets, and Toxins acting on host defences (superantigens)
• LPS (Lipopolysaccharide)


  Gram-negative bacteria: lipid A is the toxic portion, activates complement and stimulates production of cytokines
• LPS Toxicity

  Lipid A from LPS bound by LPB and directed to CD14/TLR4, signal transduction to cytoplasm (NFkB + MAPK) and production of cytokines (IL-1, IL-6, TNFα), activation of complement cascade, histamine release, increased vascular permeability, vasodilation, activation of coagulation cascade leading to depletion of coagulation factors, platelets, internal bleeding, acute disseminated intravascular coagulation, inflammation, hypotension
• LPS net effect is induction of fever, inflammation, intravascular coagulation, which can lead to haemorrhage and septic shock
• Bacterial exotoxins can be classified into 3 classes according to their mechanisms of action: Toxins that damage membranes, Toxins that act as enzymes (A/B toxins, enterotoxins, cytotoxins, neurotoxins), Toxins that activate immune response (Superantigens)
• Toxins that damage membranes are inserted into the membrane and form trans-membrane pores, leading to cellular swelling and lysis with phagocytes as the main target
• Perfringolysin O from Clostridium perfringens and α-hemolysin (α-toxin, Hla) from Staphylococcus aureus are examples of toxins that damage membranes
• Large pore-forming toxins are proteins 50-60 kDa, cholesterol-dependent with conserved C-terminal motif, bind cholesterol on membrane, polymerise with pores from 3nm up to 35nm inducing membrane permea
• Cell membrane

  • Causes cell lysis
  • Alters signaling pathways (influx of Ca2+)
  • Contributes to disease
• Pore forming toxins (PFT)


  • Large pore-forming toxins: Proteins 50-60 kDa, cholesterol dependent with conserved C terminal motif
  • Bind cholesterol on membrane
  • Polymerisation with pores from 3nm up to 35nm inducing membrane permeability, lysis, and cell death
  • Examples: Streptolysine (Streptococcus pyogenes), Listeriolysine (Listeria monocytogenes)
• Small pore-forming toxins

  • Polymerisation 2 subunits, pores 1 to 1.5nm with selective permeability for small ions and nucleotides
  • Consequence: disruption of membrane permeability, activation of endonucleases, cytokines production, initiation of apoptosis
  • High concentration of toxins induce cell death
  • Ca2+ entry
  • Example: alpha-toxin (Staphylococcus aureus)
• Toxins that act as enzymes

  • A/B toxins: 2 subunits - A enzymatic activity, B binding to membrane receptor and translocation of A subunit
  • A/B: 1 polypeptide formed by 2 domains separated by proteolysis
  • A+B: 2 proteins which interact at cell host surface
  • A-(5)B: 2 subunits independently produced and non-covalently associated during secretion
• Attachment and entry AB toxins

  1. Binding of B subunit(s) to receptor on host target cell

  2. Endocytosis of A-B subunits

  3. H+, pH diminution = toxin separation, B subunit recycled by endosome, A subunit released in cytoplasm

  4. A subunit transported by retrograde transport to Golgi and ER before being released in cytoplasm (Shiga toxin)
• Three forms of anthrax disease: cutaneous, intestinal, and inhalation
• Inhalation anthrax usually fatal: severe breathing problems and shock
• Edema factor (EF): Calmodulin-dependent adenylate cyclase – produces cAMP – PKA signaling
• Lethal factor (LF): Zn-metalloprotease - degrades MAPK proteins – disable signal transduction
• Two different A-components lethal factor (LF) and edema factor (EF) share a common B-cell binding component called protective antigen (PA)
• Botulinum neurotoxin (BoNT) action

  Blocks release of acetylcholine causing muscle flaccid paralysis
• Tetanus neurotoxin (TeNT) action


  Blocks release of Glycine and GABA causing spastic paralysis
• Botulinum neurotoxins

  • Zn metalloproteases that cleave SNARE proteins and block exocytosis
• SNARE proteins

  • Necessary for vesicular transport and fusion between plasma membrane and vesicles
• Acute diarrheal illness with severe dehydration caused by intestinal infection with Vibrio cholera (gram-negative curved bacteria)
• Transmission of Vibrio cholerae by ingestion of contaminated food or water
• Severe Cholera symptoms: diarrhea, vomiting, dessication, hypotension
• Without therapy, severe cholera can advance to acute renal failure, coma, shock, and death (25-50%, typically within hours)
• Cholera toxin is an A-B toxin that ADP-ribosylates large G proteins regulating cyclic AMP (cAMP) via adenylate cyclase
• Cholera toxin stimulates chloride ions secretion via cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, leading to diarrhea
• In severe cases of Cholera, up to 12 liters of liquid can be lost per day
• Untreated cases of Cholera have a mortality rate of about 50%
• Treatment for Cholera includes fluids and electrolytes
• How does the cholera toxin induce diarrhea?

  ADP-ribosylation of PKA, leading to an increase in cAMP and stimulation of chloride ions secretion
• Toxins like Superantigens can induce Toxic Shock Syndrome by causing massive release of cytokines
• Superantigens cause widespread non-specific stimulation of T-cells, leading to excessive cytokine release and systemic effects