MM 46-50, 400-439
1. To describe the dimorphic nature of the pathogenic fungi used in making a clinical diagnosis
2. To emphasize the eukaryotic nature of the fungi and its relationship to pathogenesis.
3. To establish familiarity with the scientific terminology peculiar to mycology.
4. To explore the nature of the pathogenesis of fungal infections.
5. To gain familiarity with the classification of medically-important fungi.
6. To develop an understanding of the nature and mode of action of anti-fungal agents.
The fungi are more evolutionarily advanced forms of microorganisms, as compared to the prokaryotes (prions, viruses, bacteria). They are classified as eukaryotes, i.e., they have a diploid number of chromosomes and a nuclear membrane and have sterols in their plasma membrane. Genetic complexity allows morphologic complexity and thus these organisms have complex structural features that are used in speciation.
Fungi can be divided into two basic morphological forms, yeasts and hyphae. Yeasts are unicellular fungi which reproduce asexually by blastoconidia formation (budding) or fission. Hyphae are multi-cellular fungi which reproduce asexually and/or sexually. Dimorphism is the condition where by a fungus can exhibit either the yeast form or the hyphal form, depending on growth conditions. Very few fungi exhibit dimorphism. Most fungi occur in the hyphae form as branching, threadlike tubular filaments. These filamentous structures either lack cross walls (coenocytic) or have cross walls (septate) depending on the species. In some cases septate hyphae develop clamp connections at the septa which connect the hyphal elements.
From Medical Microbiology, 1990, Murray, et al., p. 299, Fig. 28-1. Reproduced with permission.
A mass of hyphal elements is termed the mycelium (synonymous with mold). Aerial hyphae often produce asexual reproduction propagules termed conidia (synonymous with spores). Relatively large and complex conidia are termed macroconidia while the smaller and more simple conidia are termed microconidia. When the conidia are enclosed in a sac (the sporangium), they are called endospores. The presence/absence of conidia and their size, shape and location are major features used in the laboratory to identify the species of fungus in clinical specimens.
From Medical Microbiology, 1990, Murray, et al., p. 300, Fig. 28-2. Reproduced with permission.
Asexual reproduction, via conidia formation, does not involve genetic recombination between two sexual types whereas sexual reproduction does involve genetic recombination between two sexual types.
All fungi are free living, i.e., they are not obligate intracellular parasites. They do not contain chlorophyll and cannot synthesize macromolecules from carbon dioxide and energy derived from light rays. Therefore all fungi are heterotrophs, living on preformed organic matter. For medical purposes the important aspects of fungal metabolism are:
1. The synthesis of chitin, a polymer of N-acetyl glucosamine, and other compounds, for use in forming the cell wall. These induce immune hypersensitivity.
2. The synthesis of ergosterol for incorporation into the plasma membrane. This makes the plasma membrane sensitive to those antimicrobial agents which either block the synthesis of ergosterol or prevent its incorporation into the membrane or bind to it, e.g. amphotericin B.
3. The synthesis of toxins such as
a. Ergot alkaloids- these are produced by Claviceps purpurea and cause an alpha adrenergic blockade
b. Psychotropic agents - these include psilocybin, psilocin and lysergic acid diethylamide (LSD)
c. Aflatoxins - these are carcinogens produced by Aspergillus flavus when growing on grain. When these grains are eaten by humans or when they are fed to dairy cattle and they get into the milk supply, they affect humans.
4. The synthesis of proteins on ribosomes that are different from those found in bacteria. This makes the fungi immune to those antimicrobial agents that are directed against the bacterial ribosome, e.g., chloramphenicol.
5. The ability of certain metabolites to alter morphology of yeast and/or be assimilated by yeast with concomitant clinical identification affects.
A. Visualization of fungi in tissue preparations
1. Treatment with 10% potassium hydroxide
2. Positive stain with
3. Negative stain with India ink
B. Fluorescence of fungi under ultraviolet light
C. Culture of fungi on
1. Sabouraud's agar (favors fungal growth because of low pH)
2. Mycosel agar (selective for pathogenic fungi because of chloramphenicol and cycloheximide in medium)
D. Visualization of cultured fungi (25oC and 37oC)
1. Colonial morphology
2. Cellular morphology
E. Identification of yeast by
1. Biochemical tests
2. Behavior in broth and serum (germ tube formation)
3. Behavior on cornmeal agar (pseudohyphae formation)
In general, humans have a high level of innate immunity to fungi and most of the infections they cause are mild and self-limiting. This resistance is due to:
Fatty acid content of the skin pH of the skin, mucosal surfaces and body fluids Epithelial turnover Normal flora Transferrin Cilia of respiratory tract
When fungi do pass the resistance barriers of the human body and establish infections, the infections are classified according to the tissue levels initially colonized.
A. Superficial mycoses - infections limited to the outermost layers of the skin and hair. The superficial mycoses are:
B. Cutaneous mycoses - infections that extend deeper into the epidermis, as well as invasive hair and nail diseases.
These diseases are restricted to the keratinized layers of the skin, hair and nails. Unlike the superficial mycoses, various cellular immune responses may be evoked, causing pathologic changes in the host that may be expressed in the deeper layers of the skin. The agents causing these diseases are termed dermatophytes. The diseases are referred to as ringworm or tinea. All of the dermatophytic diseases are caused by members of three genera, Microsporum, Trichophyton and Epidermophyton, which comprise 41 species. The cutaneous mycoses are:
C. Subcutaneous mycoses - infections involving the dermis, subcutaneous tissues, muscle and fascia
These infections initially involve the deeper layers of the dermis, subcutaneous tissue or bone. Most infections have a chronic or insidious growth pattern, eventually extending into the epidermis and are expressed clinically as lesions on the skin surface. They are initiated by trauma to the skin and are difficult to treat and surgical intervention (excision or amputation) is frequently employed. The subcutaneous mycoses are:
D. Systemic mycoses - infections that originate primarily in the lung and may spread to many organ systems.
Unlike most other fungi, the five systemic mycotic agents are inherently virulent. Each species has biochemical and structural features that enable it to evade host defenses. The primary focus of infection is the lung but secondary infection may occur elsewhere in the body. The five etiological agents are identified by their morphology on agar plates (saprobic phase) and in tissue (parasitic phase):
Dimorphic with mold to yeast transition when infecting susceptible species. Yeast cells are relatively small. Saprobic phase shows tuberculate macroconidia.
Dimorphic with mold to yeast transition when infecting susceptible species. Yeast cells are medium size with thick walls.
Dimorphic with mold to yeast transition when infecting susceptible species. Yeast cells have multiple buds.
Dimorphic with mold to spherule transition when infecting susceptible species. Spherules are multinucleate.
Monomorphic with yeast phase only. This is the only pathogenic yeast with a capsule. The capsule is extremely large.
From Medical Microbiology, 1990 Murray, et al., p. 324, Fig. 30-1. Reproduced with permission. E. Opportunistic mycoses - infections in patients with immune deficiencies who would otherwise not be infected.
E. Opportunistic mycoses are seen in those people with impaired host defenses such as occurs in
The major opportunistic mycoses include:
From Medical Microbiology, 1990, Murray, et al., p. 345-346, Figures 31-6 and 31-7. Reproduced with permission.
Amphotericin B - binds to ergosterol moiety in the plasma membrane causing derangement of the membrane integrity and leakage of cytoplasmic contents. Administered sys-temically.
Nystatin - binds to ergosterol and disrupts plasma membrane. Highly insoluble and toxic and therefore used topically only.
Block ergosterol synthesis at one or more sites with the accumulation of 14 -methyl sterol (which replaces ergosterol in the plasma membrane causing selective leakage and increased osmotic sensitivity). They also disrupt chitin synthesis. All effects are due to the binding to cytochrome P-450.
Naftifine - Binds to and inhibits squalene epoxidase which blocks ergosterol synthesis.
Terbinafine - Binds to and inhibits squalene epoxidase which blocks ergosterol synthesis.
Flucytosine - It is deaminated to fluracil where it is either (1) incorporated into RNA in place of uracil where it inhibits protein synthesis or (2) metabolized to 5-fluorodeoxy-uridylic acid where it inhibits thymidylate synthetase, thus blocking DNA synthesis.
Griseofulvin - causes disruption of the mitotic spindle by interacting with polymerized microtubules through binding to microtubule protein. Administered systemically for dermatophytic infections.
Haloprogin - a halogenated phenolic ether administered topically for dermatophytic infections.
Ciclopirox olamine - a topical for the treatment of dermatophytic infections and Candida albicans.
Tolnaftate - a thiocarbonate used to treat dermatophytic infections.
Potassium iodide (KI) - Given orally for sporotrichosis
1. Morphologically fungi are unicellular (yeasts) or multicellular (hyphae). Some fungi can alternate between the two forms (dimorphic fungi).
2. Hyphae are branching, threadlike, tubular filaments that either lack cross walls (coenocytic) or have cross walls (septate).
3. Hyphae reproduce asexually via the formation of spores termed microconidia or macroconidia.
4. Fungi produce toxins such as ergot alkaloids, psychotropic agents and aflatoxins.
5. Visualization of fungi in human tissue is accomplished by treatment with 10% KOH and staining with lactophenol cotton blue, Grocott silver stain, hematoxylin or eosin.
6. India ink may be used as a negative stain to emphasize the capsule of yeast.
7. The physician can use an ultraviolet lamp (Wood's lamp) to detect fluorescent compounds produced by fungi growing in or on human tissue. 8. Fungal diseases are classified according to their depth of penetration of human tissue. Thus, there are the superficial mycoses, cutaneous mycoses, subcutaneous mycoses and systemic mycoses.
9. The superficial mycoses include pityriasis versicolor, tinea nigra, black piedra and white piedra.
10. The cutaneous mycoses (tineas or ringworms) are restricted to growth in the keratinized layers of the skin, hair and nails.
11. The subcutaneous mycoses penetrate the dermis, subcutaneous tissue, muscle and fascia. These include sporotrichosis, chromoblastomycosis and mycetoma.
12. Systemic mycoses originate in the lung and then may spread to many organ systems. These include histoplasmosis, North American blastomycosis, South American blastomycosis, coccidioidomycosis and fungal meningitis.
13. Fungi are most commonly cultured on Sabouraud's agar or Mycosel agar.
14. Opportunistic mycoses are often secondary to other diseases that compromise host immunity such as AIDS, diabetes, and malignancy.
15. Antifungal agents are classified according to their chemical structure as macrolides, azoles, allylamines, pyrimidine analogs and miscellaneous.
16. The polyene antifungals are amphotericin B and nystatin which bind to ergosterol in the plasma membrane, thus disrupting it. 17. The azole antifungals include fluconazole and ketoconazole plus numerous others. They all block ergosterol synthesis by binding to cytochrome P-450.
18. The allylamines include naftifine and terbinafine which inhibit squalene epoxidase, thus blocking ergosterol synthesis.
19. The pyrimidine analogs such as flucytosine incorporate into RNA and/or DNA thus blocking protein synthesis or DNA synthesis.
20. The miscellaneous antifungals include griseofulvin, haloprogin, ciclopirox olamine, tolnaftate and potassium iodide.
21. The three genera of dermatophytes are Microsporum, Trichophyton and Epidermophyton.
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