Endotoxin: the contaminant that produces a plausible result

Bacterial endotoxin is lipopolysaccharide from the outer membrane of gram-negative bacteria. It enters a peptide preparation through water, glassware, handling or the fill environment, and it survives conditions that kill the organisms that produced it — autoclaving does not reliably remove it.

Why it is different from other contamination

A heavy-metal carry-over or a truncated sequence tends to degrade an assay: signal falls, variance rises, and the experiment looks wrong in a way that prompts a second look. Endotoxin does the opposite. At picogram-per-millilitre concentrations it is a potent activator of TLR4 signalling, and in any system with an immune or inflammatory readout it produces cytokine release, NF-κB activation and downstream effects that are real, reproducible and entirely attributable to the contaminant.

The result looks like a finding. It replicates, because the same contaminated lot is used across the replicates. It survives review, because nothing about the data is anomalous. It fails only when someone else, using different material, cannot reproduce it.

Where it matters most

• Any primary immune cell work — macrophages, monocytes, dendritic cells, PBMCs.
• Cytokine and chemokine readouts of any kind.
• Endothelial and epithelial barrier models, where LPS alone changes permeability.
• Any in-vitro system that includes serum, which modulates but does not remove the effect.

It matters less for a cell-free binding assay or an analytical measurement, which is exactly why some suppliers omit the test — for a portion of buyers it will never bite.

Reading the number

The LAL assay reports endotoxin units per milligram of peptide. USP <85> is the standard method. A specification of < 1.0 EU/mg is a reasonable floor for research material; a result an order of magnitude below the specification is the outcome you want, and a certificate that reports the result as "< specification" without a figure has given you the least information it could while remaining technically accurate.

Every peptide lot we sell is tested by LAL under USP <85> against a < 1.0 EU/mg specification, and the measured result appears on the certificate rather than a pass mark.

If the certificate has no endotoxin line

It is not evidence of contamination. It is the absence of evidence about the variable most able to manufacture a convincing false positive in the systems most peptide research uses. Ask for it. A supplier who tests will send it; a supplier who does not will explain why it is not necessary, which is itself an answer.