exotoxin

Exotoxins are toxic proteins secreted by certain bacteria into the surrounding environment or host tissues. They are produced by both Gram-positive and Gram-negative bacteria and can cause severe disease even when bacteria are present in small numbers. Exotoxins are typically potent and antigenic, and many are heat-labile, though some remain active after heating.

Most exotoxins act by specific mechanisms that disrupt normal cellular processes. Many are organized as A-B toxins, with a binding component (B) that attaches to target cells and a catalytic component (A) that enters the cell to interfere with essential functions. Examples include diphtheria toxin, which inhibits protein synthesis; botulinum and tetanus neurotoxins, which impair nerve signaling; and cholera toxin, which drives watery diarrhea by altering ion transport. Other exotoxins are enzymes that damage tissue or impair immune responses, while some form pores in cell membranes, leading to cell lysis. Superantigens, such as certain Staphylococcus aureus and Streptococcus pyogenes toxins, cause excessive immune activation and shock.

Exotoxins are typically released by bacteria during growth and can be encoded on various genetic elements, including plasmids and bacteriophages, or occur on the bacterial chromosome. Because of their specificity and high potency, exotoxins can be neutralized by antibodies. Vaccines using toxoids—inactivated toxin forms—provide immunity against diseases such as diphtheria and tetanus. Clinically, management of toxin-mediated disease combines antitoxin or antitoxin-containing therapies, antibiotics to eliminate the source of toxin production, and supportive care.

In contrast to exotoxins, endotoxins are lipopolysaccharide components of Gram-negative bacteria released mainly upon lysis and provoke different, often less specific, inflammatory responses.