Fungi I -- Aspergillus and Mucor

Fungal cell membrane has ergosterol. Amphotericin B and nystatin bind to ergosterol; imidazoles inhibit synthesis of ergosterol.

Fungal cell walls are thick and formed by carbohydrate units of glucan, mannan, and chitin.

Asexual reproduction. Yeasts (i.e., Candida) divide by budding. Molds divide by elongation, and are fuzzy because aerial mycelia grow up into air (and eventually form spores). Molds do not form spores in tissue. Hence you cannot identify fungi by spore arrangement on tissue biopsy; furthermore, since spores are the infectious form, you can only get infected from the environment, not another human.

Aspergillus. A. fumigatus causes 90% of human disease due to molds; also, flavus is similar; A. niger is black. Acute branching septate hyphae. Ubiquitous in environment -- we all inhale hundreds of fumigatus spores (they are very aerodynamic) daily!

Invasive aspergillosis is typically seen only in immunocompromised (chemo, high dose steroids, CGD, AIDS) patients. Most commonly starts in lung, but can disseminate to other organs. The organism likes to invade blood vessel walls, causing infarction and hemoptysis. Definitive diagnosis difficult, since difficult to grow mold; serum antigen assay for galactomannan (specific to Aspergillus) is best: specific but not that sensitive. Can't look for antibody since most patients are immunocompromised! CT: "halo sign" -- air crescents around infiltrate. Treatment: Voriconazole.

Allergic aspergillosis. IgE-mediated, eosinophilia. Can look for precipitating antibody. Responds to corticosteroids like asthma.

Aspergilloma. Usually grows in previous cavity, like old TB cavitary lesion. Ball has minimal contact with tissue, so will move dependent on patient position. Surgery is an option.

Mucormycosis. Mucor, Rhizopus species. Non septate "empty looking" hyphae, 90 degree angle branching. Disease mostly in ketoacidotic diabetes (low pH: causes dissociation of iron from hemoglobin, interferes with PMN function, promtoes fungal growth) and leukemia patients (low PMNs). Also proliferate in blood vessel walls, causing infarction of distal tissue. Rhinocerebral, frontal lobe abscesses -- may require disfiguring surgery.