Endotoxin

Endotoxin is bacterial lipopolysaccharide associated with the bacterial outer membrane of Gram-negative bacteria. Its ability to provoke a host response is distinct from the ability of a living bacterium to multiply. Detecting an endotoxin-related response and detecting a replicating infection therefore answer different questions.^[ref]

An illuminating genetic experiment connected this response to a host recognition pathway. In 1998, A. Poltorak and colleagues examined two mouse strains with weak lipopolysaccharide signaling. One carried a missense mutation in the gene encoding Toll-like receptor 4; the other carried a null mutation of that gene. Independent disruptions converging on the same component supplied evidence for its role in the response.^[ref]

The reported animals were relatively resistant to endotoxin effects but susceptible to infection with Gram-negative bacteria. A reduced response to a bacterial component did not amount to improved defence against the living organism. This distinction connects molecular recognition to the protective function of innate immunity, rather than treating every inflammatory response as an independent measure of pathogen abundance.^[ref]

The genetic result establishes a requirement in a signaling pathway under the studied conditions. By itself it does not reveal every physical contact made by the receptor or show that a measured host response depends only on how much lipopolysaccharide is present. Ligand identity, host genotype and the measured endpoint are separate parts of the explanation.