Introduction to Pathogenic Toxins: Videos & Practice Problems

Exotoxins and endotoxins are two major types of toxins produced by pathogens. Exotoxins are soluble proteins released by bacteria during infection or after lysis. They are highly potent and can be categorized into three groups: AB toxins, membrane-damaging toxins, and superantigens. In contrast, endotoxins are lipopolysaccharides (LPS) found in the outer membrane of gram-negative bacteria. Unlike exotoxins, endotoxins are not secreted but are released when the bacterial cell disintegrates. Exotoxins generally have specific targets and mechanisms, while endotoxins trigger a broad immune response, often leading to inflammation and fever.

AB toxins are a type of exotoxin produced by pathogenic bacteria. They consist of two components: the A (active) subunit and the B (binding) subunit. The B subunit binds to specific receptors on the surface of host cells, facilitating the entry of the A subunit into the cell. Once inside, the A subunit exerts its toxic effect, often by disrupting cellular processes such as protein synthesis or signal transduction. This dual-component structure allows AB toxins to be highly effective in targeting and damaging host cells.

Superantigens are a category of exotoxins that can cause an intense immune response by non-specifically activating a large number of T-cells. Unlike typical antigens, which activate a small, specific subset of T-cells, superantigens bind directly to MHC class II molecules and T-cell receptors, bypassing normal antigen processing. This leads to the massive release of cytokines, resulting in a cytokine storm. The excessive immune response can cause severe inflammation, tissue damage, and potentially life-threatening conditions such as toxic shock syndrome.

Lipopolysaccharides (LPS) are crucial components of the outer membrane of gram-negative bacteria. They serve as endotoxins and play a significant role in the pathogenicity of these bacteria. LPS consists of three parts: lipid A, core polysaccharide, and O antigen. Lipid A is responsible for the toxic effects, triggering strong immune responses such as fever and inflammation. The core polysaccharide and O antigen contribute to the structural integrity and protection of the bacterial cell. LPS can cause septic shock if large amounts are released into the bloodstream during bacterial infections.

During an infection, the host immune system aims to eliminate the invading pathogens. However, an overstimulated immune response can inadvertently cause damage to host tissues. This can occur through the excessive release of cytokines, leading to a cytokine storm, which causes widespread inflammation and tissue damage. Additionally, immune cells such as neutrophils and macrophages can release reactive oxygen species and enzymes that, while targeting pathogens, also harm surrounding host cells. This unintended damage can exacerbate the severity of the infection and contribute to conditions like sepsis and organ failure.