LPS Structure and Pathogen Recognition
Lipopolysaccharide (LPS) is the major outer-membrane component of gram-negative bacteria, consisting of lipid A (an endotoxin), core polysaccharide, and O-antigen. Lipid A's highly conserved structure is recognized by toll-like receptor 4 (TLR4), in complex with MD-2 and CD14 cofactors. This recognition triggers MyD88-dependent and TRIF-dependent intracellular signaling, activating NF-κB and MAPK pathways, and producing pro-inflammatory cytokines (TNF-α, IL-1, IL-6) and chemokines.
Metabolic Endotoxaemia Concept
Cani et al. (2007) published a landmark Cell Metabolism paper demonstrating that high-fat, low-fiber diets increase gram-negative bacteria and circulating LPS levels (2-3-fold elevation). Despite no infection, low-grade LPS elevation triggered chronic inflammation, increased TLR4 signaling, impaired insulin signaling, and weight gain. They termed this "metabolic endotoxaemia"—chronic elevation of LPS without active infection. This discovery linked microbiota composition to systemic inflammation and metabolic disease.
Leaky Gut Mechanism
Mechanisms linking dysbiosis to metabolic endotoxaemia include "leaky gut" (increased intestinal permeability). Dysbiosis-associated reduction in butyrate producers (Faecalibacterium) decreases HDAC inhibition, reducing tight-junction protein expression. Dysbiosis also reduces expression of antimicrobial peptides (lysozyme, RegIII-γ), permitting gram-negative overgrowth. Increased permeability allows LPS translocation across the epithelium into the lamina propria and draining lymph vessels. While low levels of LPS translocation are normal and may be immune-priming, sustained elevation becomes pathologic.
Circulating Biomarkers
LPS-binding protein (LBP) and soluble CD14 (sCD14) are circulating biomarkers of LPS exposure. Elevated LBP and sCD14 correlate with dysbiosis, obesity, and metabolic disease. They're not mechanistically causal but indicate degree of LPS translocation. Clinical use is limited; LBP and sCD14 aren't standard tests. Research settings use them to stratify patients by endotoxaemia burden.
Connections to Insulin Resistance and NAFLD
Metabolic endotoxaemia impairs insulin signaling via TLR4 → NF-κB → IKK-β → JNK activation, disrupting insulin receptor substrate (IRS) phosphorylation. Additionally, LPS-triggered hepatic inflammation promotes hepatic steatosis (fatty infiltration) and non-alcoholic fatty liver disease (NAFLD). LPS also impairs intestinal barrier integrity through zonula occludens-1 (ZO-1) disruption, creating a feed-forward loop. Dietary intervention (fiber, polyphenols) restoring butyrate producers can reduce LPS translocation and improve metabolic parameters, demonstrating the microbiota's role in metabolic disease causation.