Host-microbe interactions: commensal fungi in the gut

Abstract

Fungi are ubiquitous microbes that are common in diverse environments including as commensal organisms on the human body. In addition to its obvious role as a digestive organ, the intestines have been further appreciated as important for the development, maintenance, and instruction of the immune system. The gut harbors many types of microorganisms including bacteria, archaea, fungi, and viruses, and many studies over the past couple of decades have documented an important role for intestinal bacteria in immunological function. Recent studies are now suggesting that intestinal fungi (the gut 'mycobiome') may similarly play important roles in host immunity and inflammation. This review will discuss recent studies that will influence our growing understanding of the role(s) of intestinal fungi in health and disease.

Figure 1. Fecal fungi

Fungi are easily detectable in stool samples from humans and specific pathogen-free (SPF) mice, but not in germ-free (GF) mice. Stool was collected and stained fresh with calcofluor white, a fluorescent dye that binds to chitin in fungal cell walls and is commonly used to detect pathogenic fungi in histology sections. Fungi are readily recognizable by their size and are distinct from plant material, which can also stain with calcofluor white but are not visible in the images shown.

Figure 2. Diverse effects of commensal fungi

The diagram depicts many of the effects of commensal fungi discussed in the text. Bacterial microbiota and the intestinal epithelium influence the ability of fungi to colonize the gut. Commensal fungi produce metabolites and products that influence immunity and inflammation at local and distal sites. Immune cells in the gut respond to commensal fungi to influence inflammation and immune homeostasis.