Beyond Bacteria
When people say "microbiome," they almost always mean bacteria. Yet the gut harbours a diverse community of fungi — the mycobiome — comprising roughly 0.1 percent of the total microbial biomass but exerting biological influence far beyond its numbers. Fungal species interact with bacterial communities, modulate immune responses, and contribute to both health and disease in ways that are only beginning to be understood.
Key Fungal Residents
The most abundant gut fungal genera include Candida, Saccharomyces, Malassezia, Cladosporium, and Aspergillus. Candida albicans is the dominant species in most healthy adults — a commensal that becomes pathogenic only when immune defences are compromised or bacterial competitors are eliminated (typically by antibiotics). Saccharomyces cerevisiae (baker's yeast) and its close relative Saccharomyces boulardii are common dietary-derived fungi with immunomodulatory properties.
Fungal-Bacterial Cross-Talk
Fungi and bacteria do not exist independently in the gut — they engage in complex ecological interactions. Antibacterial antibiotics create ecological vacuums that permit Candida overgrowth. Conversely, Candida biofilm formation provides physical niches for bacterial colonisation. Bacterial SCFAs (particularly butyrate) inhibit Candida hyphal transition — the morphological switch from commensal yeast form to invasive filamentous form. This means that a healthy bacterial community actively suppresses fungal virulence.
Immune Interactions
Fungal cell wall components — particularly β-glucan, mannan, and chitin — are recognised by pattern recognition receptors (Dectin-1, Dectin-2, mannose receptor) on intestinal immune cells. This recognition activates Th17 responses — the same immune pathway implicated in IBD and other inflammatory conditions. In health, anti-fungal immunity is balanced by regulatory mechanisms. In IBD, disrupted fungal-immune interactions may contribute to disease: Crohn's disease patients show increased anti-Saccharomyces cerevisiae antibodies (ASCA) and altered mycobiome composition.
Candida Overgrowth: Separating Fact from Fiction
Wellness culture promotes "Candida overgrowth" as a widespread condition causing fatigue, brain fog, and digestive symptoms. While invasive candidiasis is a serious condition in immunocompromised patients, the concept of systemic Candida overgrowth in immunocompetent individuals lacks clinical validation. Candida is a normal gut resident; its mere detection on stool testing does not indicate disease. Symptoms attributed to "Candida overgrowth" in wellness narratives often overlap with IBS, SIBO, or food intolerance — conditions with established diagnostic criteria and evidence-based management.
The Phageome
Alongside fungi, bacteriophages (viruses that infect bacteria) represent another understudied component of the gut ecosystem. The gut phageome contains an estimated 10¹⁰ to 10¹¹ phage particles per gram of stool, and phage predation shapes bacterial community composition, drives horizontal gene transfer (including antibiotic resistance genes), and modulates immune responses through phage-immune cell interactions. Understanding the full gut ecosystem — bacteria, fungi, viruses, archaea — is essential for moving beyond the bacterial-centric view of microbiome science.