Which statement correctly pairs exotoxin and endotoxin with their heat stability?

• A. Exotoxin is heat-labile; Endotoxin is heat-stable.
• B. Exotoxin is heat-stable; Endotoxin is heat-labile.
• C. Both are heat-labile.
• D. Both are heat-stable.

Answer: (A)

Exotoxins and endotoxins differ in what they’re made of and how heat affects them. Exotoxins are protein toxins secreted by bacteria, so heating tends to denature their three-dimensional structure and destroy their activity—these toxins are heat-labile. Endotoxins are lipopolysaccharide components of the Gram-negative outer membrane, not proteins, and they resist heat more effectively, so they remain toxic after moderate heating, making them heat-stable relative to exotoxins. This is why sterilization and decontamination efforts must account for endotoxins more stringently than for many exotoxins. For example, many exotoxins lose activity with heat (unlike endotoxins, which persist), which is the essence of the pairing: exotoxins are heat-labile, endotoxins are heat-stable.