INTRODUCTION• Eukaryotes• Don’t possess chlorophyll• Unicellular/multicellular• Mostly soil saprophytes• Role in degradation of organic compounds• About 250000 fungal species are identified.• Out of them only about 150-200 are known to cause human infections.• Fungi cause infections in debilitated patients, eg. Immunocompromised patients like AIDS patients.• Useful fungi: edible mushrooms, yeasts used in fermentation, fungi producing antibiotics (eg penicillium).
STRUCTURE• Fungal cells are eukaryotic cells containing cell wall, cell membrane, true nuclei, nuclear membrane, mitochondria, vaculoes, reticular endothelium, ribosomes etc like other eukaryotic cells.• Cell division – sexual/asexual• Cell wall – made up of chitin, glucans, mannans and complex polysaccharides.• Cell membrane contains ergosterol in contrast to mammalian cells which contain cholesterol.
Fungal cell wall• Consists of chitin not peptidoglycan like bacteria.• Thus fungi are insensitive to antibiotics as penicillins.• Chitin is a polysaccharide composed of long chain of n-acetyleglucasamine.• Also the fungal cell wall contain other polysaccharide, B-glucan, which is the site of action of some antifungal drugs.
Fungal cell membrane• Consist of ergosterol rather than cholesterol like bacterial cell membrane.• Ergosterol is the site of action of antifungal drugs, amphtericin B & azole group
Atmospheric & carbon source requirements• Most fungi are obligatory aerobes, some are facultative anaerobes, but none are obligatory anaerobes.• All fungi require a performed organic source of carbon –association with decaying matter.• Natural habitat• The environment.
Opportunistic FungiIn addition to those species which aregenerally recognized as pathogenic toman it is firmly established that underunusual circumstances of abnormalsusceptibility of patient, or thetraumatic implantation of thefungus, other fungi are capable ofcausing lesions. Those are called(Opportunistic Fungi.)
These circumstances may be :1. A debilitating condition of the host, as Diabetes.2. A concurrent disease such as leukaemia.3. Prolonged treatment with corticosteroids.4.Immunosuppressive drugs or an antibiotic for long duration.
5. Deuteromycota - (Imperfect Fungi)-Regarded as imperfect because no sexual stage has been observedin their life cycle-Members are not closely related and are not necessarily similarin structure or appearance; do not share a common ancestry
Morphological classification:1. Yeasts2. Yeast like fungi3. Moulds/ filamentous fungi/ mycelial fungi4. Dimorphic fungi Description: Yeasts are round to oval unicellular fungi which reproduce by budding or binary fission eg cryptococcus. Yeasts like fungi – some yeasts grow partly as yeasts and partly as chains of elongated budding cells joined end to end forming pseudohyphae and pseudomycelium eg candida.
Filamentous fungi/moulds:• They grow as multicellular branching filaments.• Hypha = each filament is called hypha.• Mycelia = tangled masses of hyphae are known as mycelia.• Thallus = body of fungus.• Septa = transverse walls dividing hyphae at irregular intervals.• The septate hyphae are morphologically coenocytic because septae have holes through which free flow of nuclei and other cytoplasmic material can occur.• Aerial mycelium = the part of mycelium projecting above the culture medium.• Vegetative mycelium = the part of mycelium growing in the culture medium.
• There is abundant formation of spores on aerial mycelium that affects their airborne transmission.• Eg dermatophytes, aspergillus, zygomycetes, penicillium. Dimorphic fungi:• They exist as both yeast form and filamentous form depending on conditions of growth.• Yeast form (parasitic phase) occurs in host tissues and in cultures at 37 C.• Filamentous form (saprophytic phase) occurs in soil and in cultures at 22-25 C.• Eg fungi responsible for systemic mycoses like histoplasma, blastomyces, coccidiodes, paracoccidiodes .
Yeasts1. These occur in the form of round or oval bodies which reproduce by the formation of buds known as blastospores.2. Yeasts colonies resemble bacterial colonies in appearance and in consistency.3. The only pathogenic yeast in medical mycology is Cryptococcus neoformans.
Yeast-Like1.These are fungi which occur in the form of budding yeast-like cells and as chains of elongated unbranched filamentous cells which present the appearance of broad septate hyphae. these hyphae intertwine to form a pseudomycelium.2. The yeast like fungi are grouped together in the genus Candida.
Candida albicans• A germ tube is an outgrowth produced by spores of spore-releasing fungi during germination.• The germ tube differentiates, grows, and develops by mitosis to create somatic hyphae.• A germ tube test is a diagnostic test in which a sample of fungal spores are suspended in serum and examined by microscopy for the detection of any germ tubes. It is particularly indicated for colonies of white or cream color on fungal culture, where a positive germ tube test is strongly indicative of Candida albicans.
Thermally Dimorphic FungiThese are fungi which exhibit afilamentous mycelial morphology(saprophytic phase) when grown atroom temperature 27oC, but have atypical yeast morphology (parasiticphase) inside the body and whengrown at 37oC in the laboratory (e.g.Histoplasmosis).
REPRODUCTION• Reproduction in fungi can be of two types:• Asexual• Sexual Asexual spores:• Sporangiospores = spores formed in sporangium by formation of cleavage planes. Seen in zygomycetes eg rhizopus, mucor.• Conidiospores = borne externally on sides eg aspergillus, penicillium, dermatophytes.• Microconidia = small single spores.• Macroconidia = large single or multicellular spores.
Sporangiospore, conidiospore, blastospore and chlamydospore, arthrospore
• Sexual spores: four types have been identified• Oospores• Zygospores• Ascospores• Basidiospores• Other vegetative spores:• Blastospores – a fungal spore that arises by budding.• Arthrospores - one of a number of spores of various fungi and certain blue-green algae, united in the form of a string of beads, formed by fission.• Chlamydospores - a thick-walled intercalary or terminal asexual fungal spore formed by the rounding-up of a cell; it is not shed. Formed by differentiation of hyphae; seen in Candida and Histoplasma spp.• Phialospores - A type of conidium found, for example, in many of the Eurotiales and Hypocreales. Phialospores develop at the tips of specialized finger-like cells termed phialides.
FUNGI IMPERFECTI (DEUTEROMYCETES)• All those fungi whose sexual or perfect state is not known.• They form septate hyphae and asexual spores or no spores.• Most fungi causing human infections belong to this group.
INFECTION Fungal infections are of 4 types based on target tissue:• Superficial mycoses = surface infections limited to outermost layers of skin and hair.• Cutaneous mycoses = fungal infections extending deeper into the epidermis and its integuments.• Subcutaneous mycoses = infections involving dermis, subcutaneous tissue, muscles and fascia.• Systemic mycoses = infections originating primarily in the lungs ( acquired by inhalation) and spreading to other organs.• Opportunistic mycoses = besides the above four mentioned types, this category includes infections in which fungi of no significance or low virulence infect humans with compromised immune system.
LABORATORY DIAGNOSIS (main points) Specimens:• Skin scrapings, nail clippings, hairs• Scrapings from mucous membrane• Scrapings, crusts, aspirated pus, tissue biopsy.• Blood, CSF etc in systemic mycoses. Microscopy:• KOH mount – KOH dissolves keratin and cellular material but does not affect fungi. Specimen is placed on a slide, a drop of 10-20% KOH is added and covered with a coverslip, left for 20 min in incubator at 37°C to digest keratin. Then examined microscopically.
LACTOPHENOL COTTON BLUE• The lactophenol cotton blue (LPCB) wet mount preparation is the most widely used method of staining and observing fungi and is simple to prepare. The preparation has three components: phenol, which will kill any live organisms; lactic acid which preserves fungal structures, and cotton blue which stains the chitin in the fungal cell walls.
Culture:• Sabouraud’s dextrose agar is commonly used for fungal culture.• pH =5.6 does not allow bacterial growth.• Drugs like chloramphenicol, cyclohexamide and other antibiotics are added to prevent bacterial or saprophytic fungal infection.• Cultures are incubated at two temperatures:• One tube at 25°C (room temperature)• One tube at 37°C (incubator).• This helps reveal fungal dimorphism.
• Cultures are incubated for atleast 2-3 weeks and in some cases upto 6 weeks.• Cultures are examined macroscopically for colony morphology, and microscopically for fungal morphology.• Czapek-Dox agar• Cornmeal agar