Endotoxin is a complex lipopolysaccharide (LPS) and is a major component of the outer membrane of most gram-negative bacteria. A single Escherichia coli contains about 2 million LPS molecules per cell. Bacteria shed endotoxin into their culture environment when they are actively growing, and in larger amounts when they die. Endotoxins are around 10kDa in size, however their size could be up to 1000 kDa if largely aggregated.

Both in vitro and in vivo experiments can be affected by endotoxins and this can affect testing results. LPS has been shown to have a strong association with high levels of toxicity and immunogenicity due to its ability to stimulate leukocyte cultures to manifest tissue factors, induction of IL-6 in equine macrophages, and even to inhibit murine erythroid colony formation (1). As LPS is able to tolerate high temperatures, and it can be toxic even without the presence of bacterial cells, it is crucial to detect endotoxin to prevent contamination in culture. This can help save researchers time and effort before the their experiments are compromised.

Reference:

1. Ryan, J. Endotoxins and Cell Culture. Corning Technical Bulletin. (2008).