Phascolarctobacterium

Classification and Ecology#

The Negativicutes class within Firmicutes is distinctive: these organisms stain Gram-negative despite being phylogenetically Gram-positive. Phascolarctobacterium shares this class with Veillonella, Selenomonas, and Dialister—all succinate utilizers or SCFA producers occupying specialized metabolic niches.

P. faecium is the dominant species in human adults; P. succinatutens was described in 2012 and is present in subset of healthy individuals.

Role in Gut Ecosystem—The Succinate Cross-Feeding Niche#

The defining ecological feature of Phascolarctobacterium is its role as a second-tier cross-feeder in the colonic fermentation network:

1. Primary fermenters (Bacteroides fragilis, Parabacteroides, and other Bacteroidetes) ferment complex carbohydrates and produce succinate as an intermediate metabolic product. 2. Phascolarctobacterium converts succinate to propionate via the methylmalonyl-CoA decarboxylase pathway—a sodium-gradient-coupled mechanism distinct from the acrylate pathway used by other propionate producers. 3. By consuming succinate, it prevents succinate accumulation, which at high concentrations: - Acts as a pro-inflammatory signal via SUCNR1 (GPR91) receptor activation - Promotes HIF-1α stabilization and macrophage activation in gut tissue - Creates a metabolic environment that favors pathogen expansion 4. The resulting propionate: - Signals through GPR43 (free fatty acid receptor 2) to promote Treg differentiation - Contributes to hepatic gluconeogenesis regulation and lipid metabolism suppression - Reduces histone deacetylase activity, exerting epigenetic anti-inflammatory effects

This cross-feeding function means Phascolarctobacterium abundance is indirectly controlled by dietary patterns that regulate upstream Bacteroidetes activity—it is a second-order fiber responder.

Metal Dependencies#

No essential metal dependencies specific to the succinate-to-propionate pathway have been confirmed for Phascolarctobacterium. The methylmalonyl-CoA decarboxylase enzyme uses biotin as a cofactor. This absence of heavy metal dependency is notable—unlike iron-dependent fermenters, Phascolarctobacterium may be relatively resilient to the metal-driven Dysbiosis that disrupts iron-sulfur-cluster-dependent organisms.

Disease Associations#

Hypertension—Depleted by Disease#

Hypertension causally decreases Phascolarctobacterium abundance in reverse Mendelian randomization—listed alongside Alistipes, Bilophila, and Butyricomonas as taxa reduced by hypertensive disease state.^{[4]Causality of gut microbiome and hypertension: A bidirectional mendelian randomization studyYihui Li, Ru Fu, Ruixuan Li et al. · 2023Open reference 4 ↓} This is consistent with propionate's established role in renin-angiotensin system modulation and blood pressure regulation.

Autism Spectrum Disorder—Context-Dependent#

Depletion in ASD children with constipation, consistent with the broader loss of SCFA producers in constipation-predominant ASD; lower propionate may contribute to reduced Treg activity and gut-brain axis disruption.^{[5]He 2023 — Altered Gut Microbiota and Short-Chain Fatty Acids in Chinese Children with Constipated Autism Spectrum DisorderJianquan He, Xiuhua Gong, Bing Hu et al. · 2023Open reference 5 ↓}

Endometriosis—Differential Abundance#

Identified among differentially abundant taxa in Endometriosis stool microbiome signatures, with significant difference noted alongside Blautia, Dorea, Bifidobacterium, Streptococcus, Bacteroides, and Subdoligranulum.^{[7]Gut microbiota imbalance and its correlations with hormone and inflammatory factors in patients with stage 3/4 endometriosisShan J, Ni Z, Cheng W et al. · 2021Open reference 7 ↓[8]Hicks et al. 2025 — Oral, Vaginal, and Stool Microbial Signatures in Patients With Endometriosis as Potential Diagnostic Non-Invasive BiomarkersChloe Hicks, Mathew Leonardi, Xin-Yi Chua et al. · 2025Open reference 8 ↓}

The direction of change in endometriosis is context-dependent; reduced propionate production in endometriosis may contribute to the inflammatory milieu and estrogen dysregulation characteristic of the condition.

What Wikipedia Doesn't Cover#

Wikipedia does not have a Phascolarctobacterium page. This page provides the first structured characterization of its succinate-to-propionate cross-feeding mechanism with clinical disease associations, including the methimazole treatment recovery marker data, the paradoxical schizophrenia enrichment, and the distinction between its protective (P. faecium) and potentially pathological (P. succinatutens) species.

Cross-References#

• Graves' Disease—protective via MR evidence; increases after methimazole treatment
• Bacteroides fragilis—upstream cross-feeding partner; provides succinate substrate
• Parabacteroides—another upstream succinate source in the cross-feeding network
• Short-Chain Fatty Acids (SCFAs)—specialized propionate producer via succinate pathway
• Autism Spectrum Disorder—depleted in constipation-predominant ASD
• Schizophrenia—P. succinatutens paradoxically enriched in SCZ patients
• Endometriosis—differentially abundant in endometriosis stool signatures
• Cardiovascular Disease—propionate effects on blood pressure; depleted by hypertension
• Metal-Driven Inflammation—anti-inflammatory via propionate-GPR43-Treg axis; prevents succinate-SUCNR1 signaling
• Faecalibacterium prausnitzii—fellow SCFA producer; complementary butyrate production
• dysbiosis—depletion indicates disrupted cross-feeding networks; restoration parallels immune recovery