What are endotoxins?
Endotoxins are lipopolysaccharide (LPS) components of the outer membrane of Gram-negative bacteria. They are released when cells die or divide, and they are heat-stable: autoclaving and many filtration steps do not reliably remove them. Because immune cells respond to picogram-per-milliliter quantities of LPS, endotoxin in a research reagent can produce confounding signals in cell-based assays even when no live bacteria are present.
Why it matters in research
- Cytokine and inflammatory-signaling readouts in cell culture can be activated by trace LPS, mimicking or masking the effect under study.
- Primary cells and immune-lineage cell lines are especially sensitive.
- Endotoxin contamination is a common, under-recognized cause of irreproducible results.
The LAL assay
The Limulus Amebocyte Lysate (LAL) assay uses a lysate derived from horseshoe-crab blood cells that clots, or generates a measurable signal, in the presence of endotoxin. Common formats are:
| Format | Principle |
|---|---|
| Gel-clot | Qualitative or semi-quantitative; clot forms at or above a labeled sensitivity. |
| Turbidimetric | Measures rise in turbidity over time. |
| Chromogenic | Measures color development from a cleaved substrate; quantitative over a range. |
Recombinant factor C assays offer an animal-free alternative that detects endotoxin through the same first step of the cascade.
Units and limits
Results are expressed in endotoxin units (EU), normally per milliliter of solution or per milligram of material. A COA should state the limit applied and the measured result. Samples must be validated for the assay, since some peptides can inhibit or enhance the reaction, so a good report notes a positive-product-control recovery.
Reading the result
A reported value below the stated limit means the lot met that specification under the stated method. It does not mean endotoxin is absent. Where your own assay is sensitive to LPS, confirm the specification fits your application.