The fungal component of the human microbiome, comprising ~0.1-1% of the total gut microbial community by abundance but disproportionately active in immune signaling and cross-kingdom interactions. While bacterial communities dominate microbiome research, emerging evidence implicates the mycobiome in IBD, colorectal cancer, cardiovascular disease, MS, and metabolic syndrome — often through mechanisms distinct from bacterial dysbiosis.
Key Fungal Genera
Candida
- Most abundant and best-studied gut fungus; C. albicans is the dominant species.
- Enriched in obesity (contributes to elevated intestinal free fatty acids), T2DM, coronary artery disease, and heart failure [1].
- In colon cancer, Candida-dominant tumors show reduced survival via IL-22, TP53, and CD44 pathways [2].
- Capable of yeast-to-hyphal transition, forming biofilms and invading epithelium when immune surveillance is compromised.
Saccharomyces
- S. cerevisiae is commensal; S. boulardii is used as a probiotic.
- Enriched in some cardiometabolic diseases; S. boulardii supplementation failed to improve cardiac function in the GutHeart trial.
- Saccharomycetes-dominant GI cancers show distinct patterns from Candida-dominant ones.
Malassezia
- Lipophilic yeast; primarily skin-associated but detected in gut.
- Significantly enriched in hypertension cohorts, positively correlated with immunoglobulin light chains [3].
- M. restricta increased in obesity; Malassezia spp. enriched in pancreatic ductal adenocarcinoma (PDAC), where antifungal therapy shows therapeutic promise.
Aspergillus
- Environmental mold; A. fumigatus is a major opportunistic pathogen.
- Produces siderophores (TAFC, ferricrocin) for iron acquisition — detectable in urine as infection biomarkers [4].
- A. dublinensis cell wall components induce islet-resident macrophage infiltration in diabetes models.
Cross-Kingdom Interactions
The mycobiome does not exist in isolation. Fungi and bacteria interact through:
- Competition for nutrients: Bacteria and fungi compete for iron, carbon sources, and mucosal adhesion sites.
- Mutual inhibition: Bacterial SCFAs lower pH, suppressing fungal overgrowth; antibiotic-induced bacterial depletion triggers Candida bloom.
- Immune co-stimulation: Fungal beta-glucan (Dectin-1 ligand) and bacterial LPS (TLR4 ligand) synergistically activate innate immunity.
- Biofilm cooperation: Mixed bacterial-fungal biofilms are more resistant to antimicrobials than single-kingdom biofilms.
Disease Associations
- IBD: Increased Candida, decreased Saccharomyces; anti-S. cerevisiae antibodies (ASCA) are a diagnostic marker for Crohn's disease.
- CRC: Intratumoral mycobiome detectable; oral fungi can reach the colon within 30 minutes via the sphincter of Oddi [2].
- CVD/Hypertension: Malassezia and Candida enrichment; Mortierella appears protective in normotensive populations [1].
- MS: Altered fungal diversity; cross-kingdom shifts under dietary intervention.
Metal Connections
- Candida biosorption: C. albicans can biosorb heavy metals (Cd, Pb, Cu), potentially sequestering metals in the gut lumen but also shifting competitive dynamics with metal-sensitive bacteria.
- Aspergillus siderophores: Iron-chelating metallophores (TAFC, ferricrocin, coprogen) are virulence factors that compete with host nutritional immunity for iron.
- Metal-driven fungal bloom: Heavy metal-induced bacterial dysbiosis (loss of SCFA producers and pH control) creates conditions permissive for fungal overgrowth, paralleling antibiotic-induced candidiasis.
See Also
- gut microbiome — the broader microbial community
- dysbiosis — microbial community disruption
- siderophores metallophores — fungal iron acquisition
- biofilm — mixed-kingdom biofilm formation