Gram-negative bacteria decorate their outermost surface structure, lipopolysaccharide, with elaborate chemical moieties, which effectively disguises them from immune surveillance and protects them from the onslaught of host defences. Many of these changes occur on the lipid A moiety of lipopolysaccharide, a component that is crucial for host recognition of Gram-negative infection. In this Review, we describe the regulatory mechanisms controlling lipid A modification and discuss the impact of modifications on pathogenesis, bacterial physiology and bacterial interactions with the host immune system.

Amyloids are a class of protein with unique self-aggregation properties, and their aberrant accumulation can lead to cellular dysfunctions associated with neurodegenerative diseases. While genetic and environmental factors can influence amyloid formation, molecular triggers and/or facilitators are not well defined. Growing evidence suggests that non-identical amyloid proteins may accelerate reciprocal amyloid aggregation in a prion-like fashion. While humans encode ~30 amyloidogenic proteins, the gut microbiome also produces functional amyloids. For example, curli are cell surface amyloid proteins abundantly expressed by certain gut bacteria. In mice overexpressing the human amyloid α-synuclein (αSyn), we reveal that colonization with curli-producing Escherichia coli promotes αSyn pathology in the gut and the brain. Curli expression is required for E. coli to exacerbate αSyn-induced behavioral deficits, including intestinal and motor impairments. Purified curli subunits accelerate αSyn aggregation in biochemical assays, while oral treatment of mice with a gut-restricted amyloid inhibitor prevents curli-mediated acceleration of pathology and behavioral abnormalities. We propose that exposure to microbial amyloids in the gastrointestinal tract can accelerate αSyn aggregation and disease in the gut and the brain.

Despite its highly inflammatory nature, LPS is a molecule with remarkable therapeutic potential. Lipid A is a glycolipid that serves as the hydrophobic anchor of LPS and constitutes a potent ligand of the Toll-like receptor (TLR)4/myeloid differentiation factor 2 receptor of the innate immune system. A less toxic mixture of monophosphorylated lipid A species (MPL) recently became the first new Food and Drug Administration-approved adjuvant in over 70 y. Whereas wild-type Escherichia coli LPS provokes strong inflammatory MyD88 (myeloid differentiation primary response gene 88)-mediated TLR4 signaling, MPL preferentially induces less inflammatory TRIF (TIR-domain-containing adaptor-inducing IFN-β)-mediated responses. Here, we developed a system for combinatorial structural diversification of E. coli lipid A, yielding a spectrum of bioactive variants that display distinct TLR4 agonist activities and cytokine induction. Mice immunized with engineered lipid A/antigen emulsions exhibited robust IgG titers, indicating the efficacy of these molecules as adjuvants. This approach demonstrates how combinatorial engineering of lipid A can be exploited to generate a spectrum of immunostimulatory molecules for vaccine and therapeutics development.

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Integration of sensory and molecular inputs from the environment shapes animal behavior. A major site of exposure to environmental molecules is the gastrointestinal tract, where dietary components are chemically transformed by the microbiota 1 and gut-derived metabolites are disseminated to all organs, including the brain 2 . In mice, the gut microbiota impacts behavior 3 , modulates neurotransmitter production in the gut and brain 4,5 , and influences brain development and myelination patterns 6,7 . Mechanisms mediating gut-brain interactions remain poorly defined, though broadly involve humoral or neuronal connections. We previously reported that levels of the microbial metabolite 4-ethylphenyl sulfate (4EPS) were elevated in a mouse model of atypical neurodevelopment 8 . Herein, we identified biosynthetic genes from the gut microbiome that mediate conversion of dietary tyrosine to 4-ethylphenol (4EP), and bioengineered gut bacteria to selectively produce 4EPS in mice. 4EPS entered the brain and was associated with changes in region-specific activity and functional connectivity. Gene expression signatures revealed altered oligodendrocyte function in the brain, and 4EPS impaired oligodendrocyte maturation in mice as well as decreased oligodendrocyte-neuron interactions in ex vivo brain cultures. Mice colonized with 4EP-producing bacteria exhibited reduced myelination of neuronal axons. Altered myelination dynamics in the brain have been associated with behavioral outcomes 7,[9][10][11][12][13][14]13,14 . Accordingly, we observed that mice exposed to 4EPS displayed anxiety-like behaviors, and pharmacologic treatments that promote oligodendrocyte differentiation prevented the behavioral effects of 4EPS. These findings reveal that a gut-derived molecule influences complex behaviors in mice via effects on oligodendrocyte function and myelin patterning in the brain.