L18 - Toxins & Transmission

Damage to the Host Can be:

• Direct

• Indirect

Types of Direct Damage to the host:

• Toxins

  • Exotoxins

  • Endotoxins

Exotoxin

Secreted toxin, which is usually a protein often utilised in the production of vaccines

Types of exotoxins

• A-B toxins

• Membrane-disrupting toxins

• Superantigens

Types of A-B toxins

Simple (AB) and Compound (AB₂)

What is the A subunit?

The active component of A-B toxins, responsible for the toxic effect, often enzymatic in nature.

What is the B subunit?

The binding component of A-B toxins that facilitates the entry of the A subunit into the target cell.

Examples of simple A-B exotoxins

• Diphtheria

• Neurotoxins

What does the A-B toxins in diphtheria do to cells?

Destroys cardiac, kidney, and nervous cells by inhibiting protein synthesis

Botox

Neurotoxin that binds to Synaptotagamin II receptors on neuron cells, preventing contraction of muscles, leading to flaccid paralysis

Examples of Compound A-B exotoxin

Cholera

Difference between simple and compound A-B toxins

Simple A-B toxins consist of a single polypeptide chain, while compound A-B toxins have multiple chains.

How Compound exotoxins are involved in Cholera

Is produced in intestine, where B subunits bind to cells, and induces cells to over-secrete water

Cholera compound A-B toxin results in:

• Increased cAMP

• Massive loss of water & electrolytes

• Production of ‘rice water stools’

Membrane-disrupting toxin

A type of toxin that disrupts the integrity of cellular membranes, leading to cell lysis and tissue damage.

Example of pathogen that uses pore-forming toxin

Staphylococcus aureus

Pore-forming toxin in Staphylococcus aureus

alpha-haemolysin

Examples of enzymatic lysis toxin

alpha toxin (released by Clostridium perfringens)

What does alpha toxin do?

Hydrolyses phosphatidylcholine in target host cell membrane, leading to tissue necrosis

Examples of pathogens that produce superantigens

• S. aureus

• S. pyogenes

How do superantigens work?

Link the MHC-II and TCRs non-specifically, causing massive over-activation of the immune system (namely excess cytokines), leading to organ shock & failure

Endotoxin

Structural component of Gram-negative cell envelope, which is NOT secreted

Example of endotoxin

Lipid A, which is found in Lipopolysaccharide (LPS)

Effects of LPS

• Fever

• Inflammatory response

• Release of IL-1 and TNF

How can direct transmission be disseminated

• Direct Contact

• From mother to child

• AS STIs

Examples of diseases transmitted by Horizontal contact

Tinea, Gonorrhoea, Syphilis, and Leprosy

How syphilis is horizontally transmitted

Via STIs (Venereal syphilis) or in utero (Congenital syphilis)

Vertical transmission

When the unborn child acquires a pathogen from an infected mother

Fomite

Common objects or materials capable of carrying pathogens

Types of direct transmission

• Horizontal contact

• Airborne droplets

• Vertical contact

• Vector

Types of indirect transmission

• Contact with fomites

• Food, Water, Biological products

• Airborne