Endotoxins are degradation products from dying gram-negative bacteria and complex aggregates of acidic lipopolysaccharides (LPS). Each is composed of lipophilic lipids and hydrophilic polysaccharides. In humans, endotoxins can cause immune responses such as fever (pyrogenic threshold is approximately 0.1 ng/kg body weight). Unlike bacteria themselves, endotoxins are extremely heat and pH stable and therefore withstand sterilization methods.

During protein purification, the reduction of endotoxins is one of the most important and difficult steps. It often includes complex purification strategies (e.g., chromatography steps) with more or less satisfactory results.

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