58 terms

MDD Fungi

Fungal cellular organization and structure
Eukaryotic - biochemistry and molecular biology resemble those of mammalian cells. Many components of the cellular machinery required for protein synthesis and secretion, transcription, DNA replication, and control of cell division are functionally interchangeable in human and fungal cells
Structure of fungal cell envelope
Cell membrane: bilayered cell membrane is the innermost layer around the fungal cytoplasm. Ergosterol is the essential sterol that helps to make the cell membrane in fungi.
Cell wall: composed mostly of carbohydrate with some protein. Fungal cell walls are potent antigens to the human immune system. They also, however, limit access of drugs to the cytoplasm.
Capsule: some fungi have a polysaccharide coating that surrounds the cell wall - antiphagocytic virulence factor
Why development of antifungal drugs is difficult
Many cellular components between human and fungal cells are interchangeable - and fungi have a thick rigid cell wall which limits access of drugs to the cytoplasm
Selectivity of antifungals is not as high as that of antibacterial drugs and most are to some extent toxic.
Mode of action of 5-fluorocytosine
A structural analogue of cytosine. Fungi convert it to 5-fluorodeoxyuridine monophosphate, an analogue of dTMP. This inhibits synthesis of thymine and blocks fungal DNA synthesis. Because genetic resistance regularly emerges, 5FC is used in combination with amphotericin.
Modes of action of polyenes and azoles; basis of their selective toxicity
Take advantage of ergosterol, which is a component of fungi cell membranes but not mammalian membranes.
Polyenes: binds ergosterol in the membrane, increasing membrane permeability and causing loss of small molecules from the cell - amphotericin (used for system of deep seated infections) and nystatin (topical use)
Azoles: interferes with synthesis of ergosterol - names end in -azole
Sexual and asexual modes of cell division
Sexual: diploid fungi undergo meiosis making haploid sexual spores which can fuse to produce a diploid cell
Asexual: growing fungi divide by mitosis making thick walled asexual spores
Imperfect state
Mitotic mode of growth which produces thick walled asexual spores
Perfect state
The ability to go through meiosis and produce sexual spores
Asexual spores
(Conidia) Spores specialized for dispersal and resistance to unfavorable environments. Come from growing (vegetative) fungi which divid by mitosis. When these conidia encounter favorable nutrition, they germinate and resume growth
Asexual spores specialized for dispersal and resistance to unfavorable environments
an asexual reproductive stage (morph), often mold-like
the sexual reproductive stage (morph), typically a fruiting body
Sexual spores
Haploid products of meiosis that serve as resting germinate to produce vegetative haploid cells when they encounter favorable conditions
How fungi are classified and the characters that define the four major groups
Major groups of fungi are defined by the structures in which sexual spores are produced
Zygomyces: sexual spores produced in a large sack like zygospore
Ascomyces: single cells undergo meiosis and the resulting four haploid spores are contained in a sac (ascus)
Basidomyces: diploid nuclei in a club-like structure (basidium) undergo meiosis and haploid spores bud in chains from the tip of the basidium
Deuteromyces: (fungi imperfecti) no known sexual stage
Why the same species of fungus may have two different taxonomic names and which one is used medicine
Sometimes a long-known deuteromycete is discovered to be a previously-named perfect fungus. This requires a change in the organism's scientific name. Because perfect stages are just about never found in clinical material and because medical usage is conservative, in medicine the old name continues to be used.
How fungi are cultured in the lab
Fungi are non fastidious - can be cultured on media such as Saboraud's medium which contains protein hydrolysate and glucose. Add antibiotics to suppress growth of bacteria. A few pathogenic fungi require blood agar for culture. Incubate at 30C, since many grow better below body temperature. Fungi grow slowly - incubate for 30 days before reporting as negative.
Why microscopy is useful in identification and four important staining methods
Useful because identification depends heavily on morphology of asexual forms
Observation of a sample in a drop of KOH
Calcofluor white staining
Methenamine silver
PAS stains
Why a KOH mount is especially useful for rapid diagnosis of skin infections
Mammalian cells are destroyed but the thick walls of fungi resist alkali so they become easier to see in tissue
Also known as Mycelia - multicellular colonies composed of clumps of intertwined branching hyphae. Grow by longitudinal extension and produce spores.
Filamentous fungus
A mold
Threadlike, branching, cylindrical tubules composed of fungal cells attached end to end. These grow by extending in length from the tips of the tubules.
Unicellular growth form of fungi. These cells can appear spherical to ellipsoidal. Yeast reproduce by budding. When buds do not separate, they can form long chains of yeast cells, which are called pseudohyphae. Yeast reproduce at a slower rate than bacteria.
When progeny cells of yeast remain persistently attached, they form pseudohyphae
A thick-walled asexual spore
Small conidia that contain only a single cell
Large conidia that contain multiple cells
Form by fragmentation of hyphae at septal planes
Form by differentiation of the terminal cell of a hypha. Large, round, and thick-walled
Conidia produced by budding
Conidia produced by budding
Mother cell
A cell that divides to form daughter cells
An outgrowth from an organism (e.g., a yeast cell) that separates to form a new individual without sexual reproduction taking place
Contains haploid spores formed by meiosis, created by fusion of hyphal tips of two 'mating types'
A large sack like structure in zygomyces where many sexual spores are produced, formed at the position at which two haploid cells have fused
A sac structure in ascomyces where four haploid spores are contained, derived from the wall of the original diploid cell
A club-like structure that house diploid nuclei which undergo meiosis and from which haploid spores bud in chains
Four classes of fungal infections and how they are defined
Superficial mycoses: involve the outermost cornified layers of skin, hair, and nails without invasion of deeper tissue
Cutaneous mycoses: involve only the skin without invasion of subcutaneous tissue
Subcutaneous mycoses: localized infections of deeper tissue without spread to distant sites
Systemic mycoses: start as a focal infection, often of the lung and spread elsewhere
Difference between 'true pathogens' and 'opportunists' - 2 important fungal pathogens that straddle this division
True pathogens are able to infect a healthy host - opportunists infect immunocompromised individuals
Candida albicans
Skin infections which may develop hyper or hypo-pigmentation
A syndrome where fungi grow not on skin but in or on hair shafts
Cutaneous mycoses - surface infections of the skin
Fungi which cause cutaneous mycoses
Dermatophytes found in soil
Dermatophytes that come from animals
Dermatophytes that come from other humans!
A subcutaneous mass of fungi surrounded by granulomatous inflammation
Three important genera of dermatophytes
Epidermophyton (macroconidia), Microsporum, and Trichophyton (distinguished by the presence or absence of microconidia and appearance of macroconidia)
Typical appearance of a dermatophytic lesion and the location from which a sample should be taken for diagnosis
Lesions expand, with a rim of active infection and inflammation and a central area of healing.
Scrapings for diagnosis should be taken from the rim, not the center.
Sporothrix schenkii
Typical infection: common cause of subcutaneous infections
Route of transmission: entry of the infecting fungus through the skin via a minor injury (as in penetration by a rose thorn carrying the fungus) and may affect an otherwise healthy individual
Appearance on staining: elongated yeast: pseudohyphae
Four classic systemic fungal pathogens and the characteristics they share
Blastomyces dermatitidis
Histoplasma capsulatum
Coccidiodes immitis
Paracoccidiodes braziliensis

Endemic to specific areas, true pathogens (infect immunocompetent people), infection occurs initially in the lung and can spread, exposure often occupational, little or no human to human transmission, most infections are asymptomatic (like TB), dimorphic (yeast at 37C, hyphae in standard culture at 30C)
Blastomyces dermatitidis
Microscopic appearance in clinical samples and in culture: Large yeast with broad-based buds - view on KOH mount
Area of endemism: Midwest and Northern United States and Canada
Natural habitat: The natural reservoir of this organism in the environment is not clearly defined, but it seems to be associated with rivers and lakes
Histoplasma capsulatum
Microscopic appearance in clinical samples and in culture: tuberculate conidia in culture, tiny intracellular yeast in clinical material
Area of endemism: Ohio and Mississippi river valleys
Natural habitat: Soil contaminated with bird droppings or excrements of bats
Coccidiodes immitis
Microscopic appearance in clinical samples and in culture: large spherule in lung
Area of endemism: desert regions of the southwestern United States, and in Central and South America
Natural habitat: soil particularly at warm and dry areas with low rain fall, high summer temperatures, and low altitude
Paracoccidiodes braziliensis
Microscopic appearance in clinical samples and in culture: yeast with multiple buds - 'ship's wheel'
Area of endemism: countries in Central America and South America, most notably Brazil, Argentina, Colombia, and Venezuela, in regions classified as subtropical mountain forests
Natural habitat: unknown
Candida albicans
Natural habitat: normal flora
Microscopic appearance in clinical samples and in culture: hyphae in tissue (silver stain), yeast convert to hyphae (calcofluor white stain), long dark streaks on gram stain, buds on wet mount of vaginal smear
Major types of infections produced: vaginal infections, GI and skin infections, oral thrush (sign of compromised cell-mediated immunity)
Cryptococcus neoformans
Microscopic appearance in clinical samples and in culture: yeast forms suspended in India ink to visualize capsules
Major virulence factor: heavily encapsulated
Major type of infections produced: lung to brain or meninges
How hyphae of zygomycetes can be distinguished from those of other types of fungi on the basis of thickness, presence/absence of septa, and angle of branching
Broad ribbon like hyphae that lack septa
Pneumocystis carinii
Typical infection: fever, non-productive cough (because sputum is too viscous to become productive), shortness of breath (especially on exertion), weight loss and night sweats, also pneumothorax
Groups at high risk of infection: immunocompromised individuals
Drugs used for treatment: Bactrim with steroids, pentamidine
Prophylaxis: pentamidine, Bactrim