Which exotoxin-related statement is correct?

• A. They are secreted proteins produced by bacteria and can be highly toxic.
• B. They are lipopolysaccharide components of Gram-negative outer membranes.
• C. They are structural components of Gram-positive cell walls.
• D. They are non-toxic by-products of metabolism.

Answer: (A)

Exotoxins are secreted proteins produced by bacteria that can be highly toxic. They are released into the surrounding environment or host tissues as the bacteria grow, and their effects are often very specific—damaging nerve, digestive, or immune system functions with tiny amounts. This sets them apart from endotoxins, which are not secreted proteins but components of the bacterial outer membrane, such as lipopolysaccharide in Gram-negative bacteria, released mainly when the bacteria lyse. They aren’t structural parts of Gram-positive cell walls, whose main components are peptidoglycan and teichoic acids rather than toxins. They also aren’t merely non-toxic by-products of metabolism; wastes from metabolism aren’t toxins produced for virulence.