Mycology is usually manageable for the USMLE. A common area of confusion is systemic mycoses, so make sure you review those carefully. Also, all antifungals (mechanism of action, adverse effects, and resistance mechanisms) are high yield; these are covered under Pharmacology.
Morphology
Fungi have an outer cell wall and an inner cell membrane.
- The cell wall has a unique composition. It contains beta 1,3 glucan, chitin, and (in some species) mannosylated glycoproteins.
- The cell membrane contains 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 the 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:
- Yeasts: Unicellular fungi that reproduce by budding, e.g., Cryptococci.
- Yeast like fungi: They reproduce by budding and form pseudohyphae e.g. Candida albicans.
- Moulds or filamentous fungi: They show hyphae and mycelia and reproduce through spores, e.g., Aspergillus, Rhizopus, etc. Remember moulds on bread.
- 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.
- Asexual reproduction occurs 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:
- 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.
- 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.
- Fumonisin B1: It is produced by Fusarium spp. And is found in corn. It has been associated with esophageal cancer.
- 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.
- 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.
