Discussions about microorganisms in the human body often center on bacteria, overlooking the coexisting kingdom of fungi. This community of fungal species is known as the mycobiota. Similar to how the bacteriome refers to bacteria, the mycobiota denotes the yeasts and molds adapted to life in a human host as a normal component of our microbial ecosystem.
Composition and Location of the Mycobiota
The fungal population is much smaller than the bacterial population, making up less than 0.1% of gut microorganisms, but it is a diverse community. The mycobiota’s composition varies by location. These fungal communities are found in several primary areas:
- The gastrointestinal tract
- The oral cavity
- The skin
- The respiratory system
In the gut, common fungal residents include genera like Candida, Saccharomyces, and Cladosporium. The oral cavity also hosts a distinct set of fungi. On the skin, the environment dictates which fungi thrive, and the genus Malassezia is dominant, accounting for a significant percentage of the fungal population in these areas.
An individual’s mycobiota begins to form at birth, influenced by factors like delivery method and initial feeding. Throughout life, this composition remains dynamic. Unlike the more stable bacterial microbiome, fungal communities experience considerable variation over time, reflecting their interaction with the host and environment.
The Role of Fungi in the Body
Resident fungi actively participate in and influence host physiology. A balanced mycobiota contributes to health by interacting with other microorganisms and the host’s systems. One primary function is modulating the immune system, helping it differentiate between harmless microbes and pathogenic invaders.
Fungi also have a protective role through competition. By occupying niches and consuming nutrients, resident fungal species can prevent harmful fungi or bacteria from establishing a foothold. For instance, in the gut, established communities of Saccharomyces and Candida make it difficult for foreign microbes to colonize.
The mycobiota contributes to metabolic processes. Fungi have enzymes that break down complex carbohydrates and dietary fibers that human enzymes cannot process. This digestive assistance helps unlock nutrients from food, contributing to overall nutrition.
Fungal Imbalance and Its Consequences
A disruption in the mycobiota’s balance, known as fungal dysbiosis, can lead to negative health outcomes. This imbalance can manifest as an overgrowth of one species, a change in the ratio of different fungi, or a loss of diversity. These disturbances compromise the mycobiota’s protective functions and can contribute to disease.
A well-documented example of fungal dysbiosis is the overgrowth of Candida albicans. While a normal inhabitant of the gut and mucosal surfaces, its proliferation can lead to candidiasis, manifesting as oral thrush or vaginal yeast infections. This occurs when the local microbial environment is altered, allowing Candida to outcompete its neighbors.
Beyond localized infections, fungal dysbiosis is associated with complex systemic conditions. Links have been identified between specific gut fungal profiles and chronic inflammatory disorders like Inflammatory Bowel Disease (IBD). For instance, an altered ratio of Basidiomycota to Ascomycota phyla is observed in individuals with IBD. Specific changes in the skin’s mycobiota are also connected to certain dermatological conditions.
Factors That Shape the Mycobiota
The mycobiota is continually shaped by internal and external factors. Diet is a major driver of fungal composition in the gut. Diets high in carbohydrates and sugars can favor the growth of yeasts like Candida, while different dietary fats and plant-based foods can encourage other fungal species.
Medications can impact fungal communities. Antibiotics target bacteria but can disrupt the microbial balance, sometimes leading to fungal overgrowth as fungi are no longer checked by their bacterial competitors. Antifungal medications also alter the composition and diversity of the resident mycobiota.
Host-specific factors help determine the mycobiota’s makeup. An individual’s genetic background can influence which fungal species thrive. The host’s immune system is another determinant, as a compromised immune system may be less able to manage fungal populations, leading to dysbiosis and infection.