Medical Mycology Fundamentals: Fungi, Antifungal Drugs &...

-- MEDICAL VIROLOGY & MYCOLOGY MOUNT KENYA UNIVERSITY — MBCHB YEAR 3 UNIT CODE: MBMM 3300 / MBMM 3333 TARGETED EXAM NOTES — SECTION 2 OF 3 --- MYCOLOGY FUNDAMENTALS General characteristics of fungi Eukaryotes Heterotrophic organisms Cell walls made of chitin Can be unicellular or multicellular Exist as yeasts, molds, or mushrooms Reproduce by both sexual and asexual spores NOT photosynthetic — this is the false statement MCQ trick Hyphae and mycelium Hyphae are filamentous structures making up the mycelium A fungal mycelium may contain: septate hyphae, coenocytic cytoplasm, chitinous cell walls, mitochondria — all of the above Separating wall in hyphae is called a septum, not hyphae — MCQ trick Cell wall contains chitin — a polysaccharide Cell membrane Major component of fungal cell membrane: ergosterol NOT cholesterol, NOT peptidoglycan — these are trick options Ergosterol is the target of most antifungal drugs pH for fungal growth Best growth range: pH 4-6 Staining Fungal cell wall with periodic acid Schiff stain: appears red --- FUNGAL CLASSIFICATION Classes based on reproduction Zygomycota: sexual reproduction by zygospores Ascomycota: sexual reproduction by ascospores Basidiomycota: sexual reproduction by basidiospores Chytridiomycota: sexual reproduction by zoospores Mitosporic fungi / Deuteromycetes / Fungi Imperfecti: NO known sexual stage Deuteromycetes — Fungi Imperfecti Sexual reproduction method not clearly known They are imperfect fungi Also called Deuteromycetes Answer is B and C together Dimorphic fungi Exist as mold at environmental temperature and yeast at body temperature Most dimorphic pathogens are filamentous at 23°C and yeast at 37°C Examples: Histoplasma, Blastomyces, Coccidioides, Paracoccidioides, Sporothrix --- FUNGAL REPRODUCTION Asexual spore formation Arthrospores: single celled spores from fragments of hyphae Conidia: asexual spores Sporangiospores: asexual spores Yeast budding: asexual Basidiospores are NOT asexual — they are sexual, found in club-shaped structure called basidium Sexual spores Ascospores: produced in a sac Basidiospores: single celled, found in club-shaped structure Zygospores: Zygomycota Asexual methods in fungi Fragmentation, budding, fission NOT spermatization — that is sexual --- ANTIFUNGAL DRUGS Essay and MCQ — modes of action heavily tested. Polyenes Nystatin and Amphotericin B Target: ergosterol in cell membrane, disrupts membrane integrity Nystatin belongs to polyene class Amphotericin B: broad spectrum, used for blastomycosis, histoplasmosis, aspergillosis, valley fever NOT used for ergotism Azoles Fluconazole, Miconazole, Ketoconazole, Itraconazole, Voriconazole, Posaconazole Target: ergosterol biosynthesis — inhibit proper formation of plasma membrane Miconazole inhibits plasma membrane formation Fluconazole: drug of choice for most candida infections Posaconazole: alternative for fluconazole-resistant Candida glabrata Itraconazole: drug of choice for sporotrichosis Echinocandins Micafungin, Caspofungin Target: glucan synthesis in cell wall — NOT ergosterol Antifungal drug class that targets glucans Allylamines Terbinafine Target: ergosterol biosynthesis Griseofulvin Target: DNA — inhibits microtubule formation, disrupts mitosis Flucytosine Target: DNA synthesis NOT a microtubule — trick MCQ option 5-Flucytosine and Fluconazole similarity Both are antifungals used for systemic infections Drug that does NOT target ergosterol biosynthesis Micafungin — targets cell wall glucans --- SUPERFICIAL MYCOSES These affect only the outermost layers of skin and hair. No inflammation. Tinea nigra Caused by Hortaea werneckii Pigmented macules on palm and soles Non-inflammatory, non-scaly NOT difficult to isolate by microscopy — trick false statement Usually related to poor hygiene White piedra Caused by Trichosporon species Superficial infection of hair Species causing scalp hair white piedra: Trichosporon ovoides — repeated MCQ Other species: T. inkin, T. asahii, T. mucoides Black piedra Caused by Piedraia hortae Hard black nodules on hair Tinea versicolor / Pityriasis versicolor Caused by Malassezia furfur Lesion characteristics: hypopigmented and hyperpigmented irregular macules on the skin NOT round and smooth — that is the trick option Mild itching, scaling, non-contagious Appears on back, chest, upper arms, underarms Least frequently associated with immunocompromised persons Superficial mycoses list Tinea nigra, Tinea versicolor, White piedra, Black piedra Tinea imbricate is NOT superficial — it is cutaneous --- CUTANEOUS MYCOSES — DERMATOPHYTOSES Essay question in multiple papers — high priority. General characteristics Affect hair, skin, and nails — keratinized tissues Caused by dermatophytes: Trichophyton, Microsporum, Epidermophyton Commonly called ringworm or tinea Three genera of dermatophytes Trichophyton: infects hair, skin, nails — has microconidia Microsporum: infects hair and skin — has macroconidia Epidermophyton: infects skin and nails — does NO