General mycology

Mycology is pretty straightforward for USMLE. The commonly confused concept is systemic mycoses, so spend some time revising that. Also, as usual, all antifungals, with their mechanism of action, adverse effects, and resistance mechanisms, are high yield; we will cover them under Pharmacology.

Morphology

Fungi have an outer cell wall and an inner cell membrane. The cell wall is unique in its composition, made of beta 1,3 glucan, chitin, and mannosylated glycoproteins in some species. The cell membrane has ergosterol. The inner walls of many fungal spores and so-called black yeasts contain melanin which protects them from oxidants and exoenzymes. Cryptococcus has a cell wall that is enveloped by a gelatinous capsule composed of polysaccharides glucuronoxylomannan and galactoxylomannan (remember mannans). Some fungi, like Candida, produce biofilms composed of glucans, chitin, nucleic acids, and other polymers.

Classification

Fungi can be classified by morphology as follows:

1. Yeasts: Unicellular fungi that reproduce by budding, e.g., Cryptococci.
2. Yeast like fungi: They reproduce by budding and form pseudohyphae e.g. Candida albicans.
3. Moulds or filamentous fungi: They show hyphae and mycelia and reproduce through spores, e.g., Aspergillus, Rhizopus, etc. Remember moulds on bread.
4. Dimorphic fungi: They are yeasts at body temperature and moulds at lower temperatures or in the environment. e.g.Histoplasma.

Reproduction

Fungi reproduce by sexual and /or asexual means. Fungi reproduce asexually by fragmentation, budding, or spore formation, e.g., budding in yeasts and sporangiophore formation in Mucor. Sexual reproduction occurs by plasmogamy and karyogamy, followed by meiosis and spore formation.

Toxins

Fungal toxins are called mycotoxins. They mainly result from eating contaminated foods. Skin contact with mold-infested substrates and inhalation of spore-borne toxins are also important sources of exposure. Mycotoxins can be rarely used as chemical warfare agents. Following are the common mycotoxins:

1. Amanitin and Phalloidin: They cause poisoning after eating Amanita mushrooms. The toxins inhibit human RNA polymerase, thus interfering with mRNA synthesis and transcription. They are hepatotoxic.
2. Aflatoxins: They are widely prevalent in food, especially peanuts, stored grains, etc. They are produced by Aspergillus flavus. It is hepatotoxic and can cause hepatocellular carcinoma. Aflatoxin B1 induces a mutation in tumor suppressor gene p53 leading to cancer.
3. Fumonisin B1: It is produced by Fusarium spp. And is found in corn. It has been associated with esophageal cancer.
4. Ochratoxin A: It was discovered as a metabolite of Aspergillus and is present in corn, barley, oats, wheat, and wine. It is nephrotoxic, hepatotoxic, and oncogenic. It inhibits mitochondrial ATP production and stimulates lipid peroxidation.
5. Sick building syndrome: Fungal spore exposure in people living in humid environments with insufficient ventilation causes a syndrome presenting with irritation of the eyes and respiratory tract, asthma-like symptoms, headache and fatigue, skin irritation, nonspecific hypersensitivity reactions, and peculiar odor and taste sensations. Molds of Aspergillus, Cladosporium, etc., have been implicated.