Lipopolysaccharide Structure and the Phenomenon of Low Endotoxin Recovery

Lipopolysaccharide (LPS) is a cell-wall component of Gram-negative bacteria which contributes to bacterial toxicity. During processes such as cell division, shedding of outer membrane vesicles, or bacterial cell death, LPS is released into the surrounding media. If such contamination got into the bloodstream, it would induce pro-inflammatory immune responses which can result in sepsis and death. Therefore, detection of LPS is essential in the pharmaceutical and food industries to prevent patients being exposed to LPS. The Limulus Amebocyte Lysate (LAL) assay is the current major assay used by industry to detect and quantify LPS contamination. However, in recent years the phenomenon of Low Endotoxin Recovery (LER) has gained significant scientific attention. The phenomenon describes the inability of LAL assays, in some cases, to detect LPS due to a masking effect caused by interaction with formulation excipients. Although the mechanism of LER has not been fully determined, it is widely thought that the origin of the effect is associated with these interactions perturbing the supramolecular formation of LPS aggregates. Whilst the phenomenon of LER is highly complex and remains to be entirely understood, herein we aim to provide a state-of-the-art review of the ongoing and, at times, controversial topics of LER research. We overview the current understanding of the relationship between LPS structure and toxicity, conditions in which the supramolecular arrangement of LPS can be altered, the hypothesised mechanisms of LER, and discuss the possible risk of masked LPS remaining biologically toxic upon administration to patients.