What is an exotoxin?

• A. Secreted by viruses during infection
• B. Stored in the cell wall
• C. Actively secreted by bacteria
• D. Produced during spore formation

Answer: (C)

Exotoxins are potent toxins made of proteins that bacteria secrete into their surroundings while they are growing. This active secretion means the toxin can reach host tissues and cause specific effects, such as neuron or tissue injury, even though the bacteria themselves are still alive. Classic examples include toxins that disrupt nerve signaling or protein synthesis, like those produced by Clostridium and Corynebacterium species.

This is different from toxins that are not secreted as part of the bacterial lifestyle. Viruses aren’t the source of exotoxins, so toxins secreted by viruses don’t fit. Also, exotoxins are not simply stored in the bacterial cell wall; that would describe other components like endotoxins, which are part of the outer membrane (LPS) and released mainly when the bacteria die. And while some toxins can be produced during sporulation in certain bacteria, the defining feature of an exotoxin is its active secretion as a soluble protein toxin by live bacteria.