Exotoxina

Exotoxina, or exotoxin, refers to a potent, soluble protein toxin secreted by certain bacteria into the surrounding environment. Exotoxins can cause disease at very low concentrations and are often heat-labile. They are produced by many Gram-positive bacteria and, in some cases, by Gram-negative species as well. Genes encoding exotoxins are frequently carried on plasmids, bacteriophages, or pathogenicity islands, enabling transfer between strains.

Most exotoxins are released actively by live bacteria and act on specific targets in the host. A common architecture is an A-B toxin, where the B subunit binds to the target cell and the A subunit carries an enzymatic activity that disrupts cellular function. Mechanisms include enzymatic modification of host proteins (for example, diphtheria toxin ADP-ribosylates elongation factor-2 to halt protein synthesis), pore formation in membranes, and superantigen activity that triggers widespread, non-specific T cell activation.

Prominent examples include Corynebacterium diphtheriae toxin, Clostridium botulinum neurotoxin, Clostridium tetani toxin, Vibrio cholerae toxin, and Shiga toxins produced by Shigella and certain Escherichia coli strains. Enterotoxins stimulate intestinal secretion (as with cholera toxin); neurotoxins disrupt neurotransmitter release or signaling; cytotoxins damage a variety of cell types.

Clinical and public health relevance includes vaccines and toxoids for diphtheria and tetanus, antitoxins that neutralize circulating toxins, and diagnostic methods that detect or neutralize toxin activity. Exotoxins are distinct from endotoxins, which are lipopolysaccharide components of Gram-negative bacteria that provoke inflammation in a less targeted manner.