Fungal interactions with the human host: exploring the spectrum of symbiosis

Abstract

Fungi are ubiquitous transient or persistent human colonisers, and form the mycobiome with shifts in niche specific mycobiomes (dysbiosis) being associated with various diseases. These complex interactions of fungal species with the human host can be viewed as a spectrum of symbiotic relationships (i.e. commensal, parasitic, mutualistic, amensalistic). The host relevant outcome of the relationship is the damage to benefit ratio, elegantly described in the damage response framework. This review focuses on Candida albicans, which is the most well studied human fungal symbiont clinically and experimentally, its transition from commensalism to parasitism within the human host, and the factors that influence this relationship.

Figure 1. Transition of C. albicans from commensal to parasite

During commensal growth C. albicans grows predominately as yeast and is tolerated by the host, and does not activate innate immune responses. Periods of dysbiosis and immune suppression permit C. albicans to proliferate, induce hyphal formation, and activate innate immune responses. At the vaginal mucosal, epithelial cells become sensitised resulting in hyperactivation of innate immune responses and neutrophil influx. This non-protective hyperactivation of neutrophils results in damage to the vaginal mucosa and symptomatic vaginal candidiasis.