Gut mycobiota: The fungal community in the gut

The human gut is home to a vast microbial ecosystem consisting of a wide variety of species, including bacteria, archaea, viruses, protists, and fungi. The fungal component is known as the “mycobiome,” and is important for your gut health.

Fungal communities in the intestine

Fungi are a minor component of the gut, representing only 0.1% of the total microbes. Yes, that little! So this may explain why they are still less understood, despite their importance,

In the past few years, science has attempted to define the “healthy” microbiome in humans, but has shed little light on the function of the mycobiome.

Compared to bacteria, the human gut mycobiome is very low in diversity, with less than 20 species identified.

Studies suggest that an individual's mycobiome is neither similar to itself over time nor similar to another person's. This means that both inter- and intraindividual variation is high, and is the opposite of that for the bacterial community.

Despite this, studies suggest that there may be a core gut mycobiota . This includes a high abundance of ten identified genera, which more deeply characterize the fungal microbiome.

However, the list of species is likely to evolve as analytical techniques improve and the amount of data increases.

Which fungal genera are they dominated by?

According to most studies, yeasts from the genera Saccharomyces , Malassezia and Candida been the dominant fungi found in stool samples.

Saccharomyces spp. are widely used in the food and beverage industry. They are commonly found in foods such as wine, bread and beer. Malassezia spp., on the other hand, is a well-known resident of the skin.

However, there are still many questions about whether Saccharomyces and Malassezia should really be considered true residents of the gut. However, species of the genus Candida are certainly considered true gut symbiotic fungi.

Fungal-bacterial interactions

Commensal bacteria (those that live in symbiosis with the host and other species) are incredibly beneficial to our health for many reasons. They influence the way we absorb nutrients, help us fight colonization by pathogenic bacteria, help maintain epithelial barrier integrity, and are also involved in our immune response.

But a stable equilibrium between all our microbes, seems to be the key to optimizing health and ensuring that all of these functions are performed properly.

However, it is a competitive relationship between bacterial and fungal microorganisms in the gut. Therefore, bacterial-fungal interactions are crucial to prevent the overgrowth of specific microbes and the development of disease.

Candida albicans: The major fungal species in the human gut

Candida albicans is a type of fungus that is often found in the stool of healthy people and generally causes problems.

Why is it always portrayed in a bad light?

Certain conditions, such as an imbalance in the microbiome, a weakened immune system, and a compromised gut barrier, can lead to C.albicans infections.

We can detect the overgrowth of this benign organism through testing and thus prevent future health complications .

The fungus can colonize multiple parts of the body in addition to the gut. Disseminated candida infections typically occur in the vagina, mouth, and throat in people with compromised immune systems. Although they negatively affect quality of life, they resolve with proper diagnosis and treatment.

More rarely, it can also spread from the intestine to the bloodstream and invade internal organs, causing invasive, life-threatening infections.

Fungi, intestinal immunity and inflammatory bowel disease

It is widely accepted that fungal communities are not only altered in disease, but also play a role in intestinal homeostasis and influence systemic immunity .

Bacterial-fungal associations are crucial for maintaining this balance and protecting us from getting sick.

Interestingly, intestinal fungi can influence immune response by reducing or promoting local inflammatory reactions.

Apparently, C. albicans is a central modulator of human T-helper 17 (Th17) responses in health and during intestinal inflammation. Dysregulated Th17 responses contribute to local inflammatory disorders, such as inflammatory bowel disease.

The intestines are a very complex ecosystem, where disruptions in some of the components can greatly affect our health.

Key points:

• Mycobiome research is a new component of the gut microbiome, but this field is still in its infancy.
• There may exist a  core gut mycobiome , and it probably consists of ten genera found in most human gastrointestinal tracts.
• The diversity of fungi in the intestine is  low compared to the bacterial community and further studies are required to answer whether there are true fungal gut colonizers or whether fungi are only transiting.
• Intestinal fungi forms  metabolic and immune reactions  in different ways.
• A promising future : Understanding normal versus dysfunctional host/fungal interactions will benefit our ability to improve health and personalized treatments.

---

Author: Cecilia Clausen (Clinical Dietitian)

References:

Chin, VK, Yong, VC, Chong, PP, Amin Nordin, S., Basir, R., & Abdullah, M. (2020). Mycobiome in the Gut: A Multiperspective Review. Mediators of inflammation, 2020, 9560684. https://doi.org/10.1155/2020/9560684

Forbes, JD, Bernstein, CN, Tremlett, H., Van Domselaar, G., & Knox, NC (2019). A Fungal World: Could the Gut Mycobiome Be Involved in Neurological Disease?. Frontiers in microbiology, 9, 3249. https://doi.org/10.3389/fmicb.2018.03249

Kumamoto, CA, Gresnigt, MS, & Hube, B. (2020). The gut, the bad, and the harmless: Candida albicans as a commensal and opportunistic pathogen in the intestine. Current opinion in microbiology, 56, 7–15. https://doi.org/10.1016/j.mib.2020.05.006

Martins, C., Brandão, T., Almeida, A., & Rocha, SM (2017). Metabolomics strategy for the mapping of volatile exometabolome from Saccharomyces spp. widely used in the food industry based on comprehensive two-dimensional gas chromatography. Journal of separation science, 40(10), 2228–2237. https://doi.org/10.1002/jssc.201601296

Perez JC, Fungi of the human gut microbiota: Roles and significance, International Journal of Medical Microbiology (2021), DOI: https://doi.org/10.1016/j.ijmm.2021.151490

Richard, ML, Sokol, H. The gut mycobiota: insights into analysis, environmental interactions, and role in gastrointestinal diseases. Nat Rev Gastroenterol Hepatol 16, 331–345 (2019). https://doi.org/10.1038/s41575-019-0121-2