Endotoxin is a contaminant associated with which type of microorganism?

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

Answer: (A)

Endotoxin comes from the outer membrane of Gram-negative bacteria, specifically the lipopolysaccharide (LPS) that anchors that membrane. It is released when the bacteria die or lyse and acts as a powerful trigger of the immune system, causing fever and inflammation. Gram-positive bacteria don’t have this LPS outer membrane, so they’re not endotoxin sources in the same way. Fungi and viruses don’t carry this bacterial outer-membrane component, so they aren’t the source of endotoxin. In indoor environments, endotoxin exposure can come from dust and surfaces contaminated with Gram-negative bacteria, and it’s often measured in research and safety contexts with the Limulus amebocyte lysate assay.