Endotoxin is a contaminant found in the cell walls of which type of microorganism?

• A. Gram-positive bacteria
• B. Gram-negative bacteria
• C. Viruses
• D. Fungi

Answer: (B)

Endotoxin is a contaminant tied to the outer membrane of Gram-negative bacteria. This membrane contains lipopolysaccharide (LPS), which is the endotoxin molecule. It’s released when the bacterial cells die or lyse and can trigger strong inflammatory responses in humans. Gram-positive bacteria lack this outer membrane with LPS, so endotoxin isn’t a feature of them. Viruses don’t have cell walls at all, and fungi have a different cell wall composition, so endotoxin isn’t associated with them either. In indoor environments, endotoxin often signals the presence of Gram-negative bacteria in dust or water systems, making Gram-negative bacteria the source to recognize.