Abstract
Gut symbiont Bacteroides fragilis can produce α-galactosylceramides (BfaGCs), sphingolipids with immunomodulatory functions that regulate colonic natural killer T (NKT) cells. However, their synthesis pathway and whether other human gut bacteria can produce them are unclear. Here, using genetic and metabolomic approaches, we mapped the sphingolipid biosynthesis pathway of B. fragilis and determined that α-galactosyltransferase (agcT) is essential and sufficient for colonic NKT cell regulation in mice. The distribution of agcT is restricted to only a few species among Bacteroidales. However, structural homologues of AgcT, such as BgsB, are widely distributed in gut microbiota and produce α-glycosyldiacylglycerols (aGDGs), particularly in Enterococcus. Analysis of infant gut metagenomes revealed that B. fragilis predominantly accounts for agcT abundance regardless of the cohort, but bgsB-encoding bacteria were taxonomically diverse and showed dynamic changes with host age. In addition, aGDGs from bgsB-encoding species act as antagonistic ligands for BfaGC-mediated NKT cell activation in vitro and in vivo. Our findings highlight the distinct natures of immunoactive glycolipid-producing symbionts and their relevance in the human gut microbiome, particularly in early life.
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Data availability
Lipidomics datasets supporting this study are available via Harvard Dataverse (https://doi.org/10.7910/DVN/DYJJQJ). TEDDY microbiome data are available from dbGaP (phs001443.v1.p1) under dbGaP-controlled access. Curated human microbiome data are accessible via curatedMetagenomicData (https://doi.org/10.18129/B9.bioc.curatedMetagenomicData). The RNA-sequencing data are available in the NCBI database under BioProject accession number PRJNA1309292. Source data are provided with this paper.
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Acknowledgements
This study was supported by the National Institutes of Health (K01-DK102771, R01-AT010268, R01-AI165987: S.F.O.) and the National Research Foundation of Korea (2021R1A6A3A14044113: J-S.Y.; RS-2024-00411992: B.G.; 2021R1A6A3A14039202: D-J.J.; RS-2024-00348702: K.H.; RS-2023–00217123: J.I.S.; 2014R1A3A2030423 and 2012M3A9C4048780: S.B.P.). CD1d tetramers were provided by the NIH Tetramer Core Facility (contract number 75N93020D00005). We thank T. Yanostang for technical support and N. Surana (Duke University) for the helpful discussion and comments.
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S.F.O., D.L.K., S.B.P. and J-S.Y. conceived the idea and designed the outline of the research. J.-S.Y., K.H., J.I.S. and B.G. carried out the lipidomic profiling of gut symbionts. W.Z., C.C.L., J-S.Y., M.W. and S.F.O. carried out loss-of-function screening. K.H. and B.G. carried out gain-of-function screening. J.-S.Y. and N.G.-Z. carried out targeted bacterial knockout generation. D.-J.J., J.-S.Y. and S.F.O. carried out in vivo NKT analysis. J.-S.Y. carried out in silico structural analysis, in vitro NKT activation assays and metagenomic analysis. J.-S.Y. and S.F.O. wrote the paper, and all authors contributed to the relevant discussion.
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S.F.O. and D.L.K. filed a patent on the functions of BfaGCs and related structures (US patent 10,329,315). S.F.O., S.B.P. and D.L.K. filed a patent on the functions of BfaGCs and related structures (US patent application 17/427,756). The other authors declare no competing interests.
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Extended data
Extended Data Fig. 1 Characterization of screened genes and generation of knockout and transformant strains.
a) Graphical scheme of transposon insertion (loss-of-function) and heterologous expression (gain-of-function) library generation, followed by targeted metabolomic analysis to characterize α-galactosyltransferase, was presented. b) Loss-of-function screening identified a transposon insertion mutant (Tn8A7) with depleted BfaGC production. The relative abundance was quantified by UHPLC-MS/MS based on peak area. c) BfaGC production was abrogated in the B. fragilis Δ3069 mutant (n = 2) compared to B. fragilis WT (n = 3). The abundance of BfaGC (C34:0) was quantified by LC-MS/MS based on peak area. d) RNA-seq volcano plot comparing B. fragilis Δ3069 to wild type revealed multiple genes downregulated in Δ3069 mutant. e) The expression of agcT is reduced in B. fragilis Δ3069 (n = 3) compared to B. fragilis WT (n = 3). Transcript levels of agcT, normalized to leuB, were measured by qRT-PCR. Significance was determined by an unpaired two-tailed t-test. f) Gain-of-function screening identified P. vulgatus transformant producing aGCs. The relative abundance of aGC/Cer was quantified by UHPLC-MS/MS based on peak area. g) agcT-transformed P. vulgatus produced molecules identical to BfaGC. XICs of aGCs from B. fragilis, P. vulgatus wild type and P. vulgatus expressing agcT were displayed. Chromosome-integrated expressing system pNBU2-tetR was used to expressing agcT in P. vulgatus. h) MS/MS spectra mirror plot demonstrated a match between aGCs from P. vulgatus expressing agcT and those from B. fragilis. i) B. thetaiotaomicron and P. vulgatus heterologously expressing agcT produced aGCs. Chromosome-integrated expressing system pNBU2-tetR was used to expressing agcT in B. thetaiotaomicron and P. vulgatus (n = 2 per group). j) Relative abundance of KDS, ketoCer, Cer, and BfaGC in B. fragilis wild type (n = 4), Δspt (n = 4), ΔcerS (n = 4), ΔcerR (n = 4), and ΔagcT (n = 3) were shown. The abundance was quantified by UHPLC-MS/MS based on peak area.
Extended Data Fig. 2 Tandem mass spectra analysis determined sphingolipid intermediate structures.
To assign the structures unambiguously, we chose chain length variants of major sphingolipid species, whose synthetic version was available. a) MS/MS spectra of B. fragilis KDS (C18:0) matched those of the synthetic standard. b) MS/MS spectra of synthetic C35:1 ceramide (m/z 568.53) was displayed. c) The parent ion with an m/z value of 570.53 from B. fragilis was structurally assigned as C35:0 dihydroceramide. d) The parent ion of m/z 568.53 from B. fragilis ΔcerR was assigned as C35:0 ketoCer. Comparison with isobaric C35:1 ceramide (Panel b) exhibits a distinct fragmentation pattern, as shown by the presence and absence of m/z 264 (300-2H2O). e-f) MS/MS spectra of C34:0 ketoCer and dhCer from B. fragilis were shown.
Extended Data Fig. 3 In vitro and in vivo assays for NKT cell modulation by BfaGCs.
a) Schematic process illustrates the in vitro antigen presentation assay. Bacterial lipid extracts were loaded onto biotinylated CD1d molecules, which were subsequently immobilized on streptavidin-coated plates. After washing, NKT cell hybridomas (24.7) were added and incubated overnight. IL-2 secretion was measured by ELISA to assess NKT cell activation. b) Representative gating strategies for colonic NKT cell analysis by flow cytometry are shown. NKT cell refers to the CD1d tetramer⁺ cells within the CD3⁺CD45⁺TCRβ⁺ T cell population. c) Representative flow cytometry plots of unloaded CD1d tetramer controls are shown.
Extended Data Fig. 4 B. fragilis and B. salyersiae produce aGCs and induce NKT cell activation.
a) A MS/MS mirror plot of B. fragilis and B. salyersiae C34:0 aGC species showed an exact match. b) All B. fragilis and B. salyersiae strains, including type strains and clinical isolates, synthesize aGC, in contrast, no tested B. nordi, P. gordornii, and P. copri strains produce aGCs. c) Only aGC-producing bacterial lipid extract can elicit NKT cell activation. NKT cell hybridomas (24.7) were incubated with CD1d-lipid complex and IL-2 secretion was measured by ELISA. Each dot indicates a biologically independent replicate (n = 4 per group). Bars and error bars depict the mean ± s.e.m. Data are representative of three independent experiments showing consistent trends. Statistical analysis was performed using one-way ANOVA. d) Symbiotic Bacteroidales species did not produce aGlcACers, whereas Sphingomonas species served as positive controls and produced aGlcACers.
Extended Data Fig. 5 Homologs of AgcT share conserved structures.
a) The histogram displays the distribution of e-values from the search for proteins containing cd03817 domain. The red dashed vertical line indicates the cutoff threshold used for identifying cd03817 family hits by HMMER search. b) An AgcT homolog from B. salyersiae and a cd03817 family protein from E. faecalis belong to the same protein family and have comparable structures.
Extended Data Fig. 6 aGDG production by cd03817-encoding bacteria.
a) Mirror plot of MS/MS spectra of E. faecalis aGDGs and a commercially available standard (Avanti #840522p) showed a match. b) The retention time of aGDGs of E. faecalis matches that of standard molecule (Avanti #840522p). c) aGDGs from cd03817-domain-containing gut bacterial species exhibit chain-length variation. XICs of aGDGs from Streptococcus mitis, Lacticaseibacillus rhamnosus, and E. faecalis were shown. d) Various Enterococcus strains produce aGDGs. XICs of C34:1 aGDGs were displayed. e) E. faecalis WT and bgsB mutant exhibit comparable colonization abilities in 4-week-old male C57BL/6 mice (n = 3 per group). f) aGDG levels in stool samples from 5-week-old male/female C57BL/6 SPF (n = 6), conventionalized (n = 6), and GF (n = 7) mice were shown. “Conventionalized” indicates GF mice conventionalized via cohousing with SPF mice.
Extended Data Fig. 7 Metagenomic profilies of agcT and bgsB in multiple cohorts.
a) Skimming process to search species with homologs of AgcT or BgsB was shown. b) Histograms show sequence identity distributions from the search for homologs of AgcT or BgsB, with vertical dashed lines indicating identity cutoffs. c) In the DIABIMMUNE cohort, B. fragilis consistently dominates agcT abundance, while bgsB abundance shows dynamic changes and is contributed by a taxonomically diverse set of species. Pie charts show the proportion of species abundance in the dataset. d-e) Across multiple infant and adult cohorts, agcT abundance is consistently driven by B. fragilis, whereas bgsB is distributed among a broader array of species.
Extended Data Fig. 8 aGDGs species inhibit the effects of BfaGCs on NKT cells.
a) Bacterial lipid extracts from various Lactobacillales bgsB-encoding species exhibit dose-dependent antagonism (n = 4 per group) to SB2217-induced (n = 2) NKT cell activation. b) aGDGs competed with BfaGCs for CD1d binding in a dose-dependent manner. After loading the lipids onto CD1d, CD1d-lipids complexes were purified, and the bound lipids were extracted and analyzed by LC-MS (n = 4 per group). c) Representative flow cytometry plots show the gating strategy used for analysis of splenic dendritic cells.
Extended Data Fig. 9 Overview of screening and exploration of related metabolites.
Unannotated metabolites were linked to their biosynthetic gene agcT through forward genetics-based metabolomic screening. Protein structural homologs were identified in gut symbionts, revealing structurally related glycolipids with contrasting immunological activities. Species-level contributions of agcT and its homologs bgsB were profiled in human gut metagenomes. Figure created with BioRender.com.
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Yoo, JS., Jung, DJ., Goh, B. et al. Human gut bacteria produce structurally related monoglycolipids with contrasting immune functions. Nat Microbiol 10, 2797–2807 (2025). https://doi.org/10.1038/s41564-025-02141-1
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DOI: https://doi.org/10.1038/s41564-025-02141-1


