Fungi 101

Fungi are eukaryotes (not prokaryotes like bacteria). However, unlike human cells, fungi have both a cell membrane AND a cell wall. What you need to know about this is that the predominant sterol in the fungal cell membrane is ergosterol (rather than cholesterol, which is found in mammals). This is high-yield because many antifungal agents target ergosterol or its metabolism (e.g., azoles, amphotericin B, terbinafine). In contrast, the fungal cell wall is composed of complex polysaccharides (rather than peptidoglycan, lipopolysaccharide [LPS], or teichoic acids, which are found in bacteria). Key cell wall components include mannans, glucans and chitin, which play important roles in both activating and modulating the immune response. These cell wall components are worth remembering because some of them (galactomannan, beta-D-glucan) are important diagnostic markers for fungal infections.

Most fungi live ubiquitous in the environment, although some exist as commensal organisms within the normal human microbiota. For example, Candida species commonly colonize the skin, respiratory, genital and gastrointestinal tract without causing disease. You must know that fungi may be either environmental organisms or harmless colonizers because their presence, growth, or detection alone does not necessarily mean infection. Clinical context is sooooooo important! 

Don’t miss this! Because fungi live mostly either as commensals or as saprophytes in the environment (e.g., soil), most of them do not cause human disease under normal circumstances. The one exception is in mucocutaneous candidiasis or dermatophytosis, which occur in otherwise healthy individuals. Otherwise, any invasive fungal disease usually occurs when host immune defenses are impaired, whether through disruption of natural barriers (e.g., skin breakdown, invasive catheters), dysbiosis (e.g., prior antibiotic exposure) or subtle or overt immunosuppression (e.g., transplant, HIV/AIDS). 

Figure. Fungal cell diagram and its drugs targets