A Gram-negative, obligate anaerobic genus within the Firmicutes phylum (family Acidaminococcaceae, class Negativicutes). The primary species P. faecium is a specialized propionate producer that utilizes succinate as its main carbon source, occupying a unique metabolic niche in the cross-feeding network of the human colon. Named after its original isolation from koala feces, it has emerged as a consistently protective commensal in autoimmune and metabolic disease contexts.
Role in Gut Ecosystem
- Specialized succinate utilizer: converts succinate (produced by bacteroides fragilis, parabacteroides, and other primary fermenters) into propionate via the succinate pathway.
- This cross-feeding relationship is ecologically important -- by consuming succinate, Phascolarctobacterium prevents succinate accumulation, which at high levels can promote inflammation and pathogen expansion.
- Propionate production supports gut barrier integrity via GPR43 signaling, promotes Treg differentiation, and contributes to metabolic homeostasis through hepatic gluconeogenesis regulation.
- Enriched by Mediterranean diet patterns, which provide the complex carbohydrate substrates that fuel the Bacteroidetes-succinate-Phascolarctobacterium cross-feeding chain.
Disease Associations
Graves' Disease -- Protective
- Identified as a protective taxon in Graves' disease through Mendelian randomization evidence, representing one of the stronger causal associations between a specific gut microbe and thyroid autoimmunity.
- Propionate production by Phascolarctobacterium promotes Treg differentiation via GPR43 signaling, counteracting the Th17/Treg imbalance that drives GD pathogenesis [jiang 2023 gut microbiome metabolites graves].
- Depleted in GD patients alongside other SCFA-producing commensals [chen 2021 gut microbiota thyroid graves].
Hypertension
- MR evidence supports associations between Phascolarctobacterium and blood pressure regulation, consistent with propionate's known effects on renin-angiotensin system modulation [li 2023 gut microbiome hypertension bidirectional mr].
Autism Spectrum Disorder
- Altered abundance in ASD, with some studies showing depletion in constipation-predominant ASD children, consistent with the broader loss of SCFA producers in autism spectrum disorder [he 2023 altered gut microbiota scfa constipated asd chinese].
Endometriosis
- Identified among differentially abundant taxa in endometriosis stool microbiome signatures [hicks 2025 oral vaginal stool microbial signatures endometriosis].
Key Metabolites
- Propionate -- primary product; anti-inflammatory, promotes Treg differentiation, regulates hepatic gluconeogenesis and lipid metabolism
- Acetate -- minor product contributing to overall SCFA pool
Dietary Modulation
- Mediterranean diet enriches Phascolarctobacterium through increased availability of complex carbohydrates that promote succinate-producing Bacteroidetes, which in turn feed Phascolarctobacterium.
- High-fiber diets generally support the succinate cross-feeding network that sustains this genus.
- Western diet patterns (high fat, low fiber) deplete the upstream succinate producers, indirectly starving Phascolarctobacterium.
Connections
- Graves' disease -- protective via Mendelian randomization; propionate supports Treg differentiation
- bacteroides fragilis -- upstream cross-feeding partner; provides succinate substrate
- parabacteroides -- another succinate source in the cross-feeding network
- short chain fatty acids -- specialized propionate producer via succinate pathway
- autism spectrum disorder -- altered in ASD; part of broader SCFA producer depletion
- cardiovascular disease -- propionate effects on blood pressure regulation
- inflammation -- anti-inflammatory via propionate-GPR43-Treg axis
- faecalibacterium prausnitzii -- fellow SCFA producer; complementary butyrate production
- dysbiosis -- depletion indicates disrupted cross-feeding networks