MYCOLOGY

• Mycology


  The branch of biology that deals with the study of fungi, including their genetic and biochemical properties, their taxonomy as well as pathogenesis and toxicity

  
• Fungi


  Eukaryotic microorganisms that occur ubiquitously in nature in the domain Eucarya. The kingdom fungi (Mycota) has over 50,000 different species but only 200 have been identified as human pathogens

  
• Yeast

  A single cell that reproduce through budding
• Molds


  • Grow as long filaments (hyphae) and form a mat (mycelium)
  • Some hyphae for Transverse walls (septate hyphae); others do not (nonseptate hyphae)
• Taxonomy of fungi

  • Based on their morphology

  
• Classifications of fungi


  • Dermatophytes: causes infection of the keratinized tissues
  • Yeast: single cells
  • Molds: grow in filamentous structures
  • Dimorphic fungi: exist as molds in the environment at ambient temperature and as yeasts (or other structures) in human tissues at body temperature
• Hypha


  The basic element of filamentous fungi with a branched, tubular structure, 2–10 μm in width

  
• Mycelium


  • The web or matlike structure of hyphae
  • Substrate mycelia (specialized for nutrition) penetrate into the nutrient substrate
  • Aerial mycelia (for asexual propagation) develop above the nutrient medium
• Fungal thallus


  The entirety of the mycelia and is also called the fungal body or colony

  
• Yeast


  The basic element of the unicellular fungi. It is round to oval and 3–10 micrometer in diameter. Several elongated yeast cells chained together and resembling true hyphae are called pseudohyphae

  
• Dimorphism


  Some fungal species can develop either the yeast or the mycelium form depending on the environmental conditions, a property called dimorphism. Dimorphic pathogenic fungi take the form of yeast cells in the parasitic stage and appear as mycelia in the saprophytic stage
• Metabolism of fungi


  • All fungi are carbon heterotrophs: Dependent on exogenous nutrient substrates as sources of organic carbon, and with a few exceptions, fungi are obligate aerobes. None are obligate anaerobes
  • Known Metabolic Types: Thermophilic, Psychrophilic, Acidophilic, Halophilic
• Reproduction of fungi


  • Sexual reproduction: Zygospores, Ascospores, Basidiospores
  • Asexual reproduction: Arthrospore, Chlamydospore, Blastospores, Sporangiospores

  
• Arthrospore


  Arises by fragmentation of the ends of the hyphae (mode of transmission of Coccidiodes immitis)
• Chlamydospore


  Rounded, thick-walled and quite resistant
• Blastospores


  Formed by budding process of yeast. Some yeast (C. albicans) can form multiple buds that do not detach, thus producing sausage-like chains called pseudohyphae
• Sporangiospores


  Formed within a sac (sporangium) on a stalk by molds such as Rhizopus and Mucor

  
• Medical Mycoses according to site of infection

  • Superficial mycoses
  • Cutaneous mycoses
  • Subcutaneous mycoses
  • Deep Mycoses
  • Systemic mycoses
  • Opportunistic mycoses
• Superficial mycoses

  Limited to the stratum corneum and essentially elicit no inflammation
• Dermatophytoses

  Caused by fungi (dermatophytes) that infect only superficial keratinized structures (skin, hair, and nails), not deeper tissues
• Dermatophytoses


  • Tinea pedis
  • Tinea unguium
  • Tinea corporis
  • Tinea cruris
  • Tinea manuum
  • Tinea capitis
  • Tinea barbae
  • Tinea faciei

  
• Dermatomycoses


  Cutaneous infections due to other fungi, the most common of which are Candida spp

  
• Subcutaneous mycoses


  Characterized by infection of the subcutaneous tissues usually at the point of traumatic inoculation, with an inflammatory response developing in the subcutaneous tissue frequently with extension into the epidermis
• Types of subcutaneous mycoses

  • Chromoblastomycosis
  • Mycetoma
  • Sporothricosis
• Chromoblastomycosis


  Subcutaneous mycosis characterized by verrucoid lesions of the skin (usually of the lower extremities); Histological examination reveals muriform cells (with perpendicular septations) or so-called "copper pennies" that are characteristic of this infection

  
• Mycetoma


  A suppurative and granulomatous subcutaneous mycosis, which is destructive of contiguous bone, tendon, and skeletal muscle, characterized by the presence of draining sinus tracts from which small but grossly visible pigmented grains or granules are extruded

  
• Causes of mycetoma


  • Eumycotic mycetoma: Pseudallescheria boydii
  • Actinomycotic mycetoma: Nocardia brasiliensis
• Dematiaceous (melanized) fungi


  Pigmented brown to black, with the melanin pigment deposited in the cell walls. These fungi may produce a range of infections from superficial to subcutaneous to deep (visceral) infection characterized by the presence of dematiaceous hyphal and/or yeast-like cells in tissue. Such deep infections due to dematiaceous fungi are termed phaeohyphomycosis
• Sporotrichosis


  Infection due to Sporothrix schenckii that involves the subcutaneous tissue at the point of traumatic inoculation, usually spreading along cutaneous lymphatic channels of the extremity involved
• Primary pathogenic fungi


  Able to establish infection in a normal host, usually gaining access via the respiratory tract. Include Coccidioides immitis, Histoplasma capsulatum, Blastomyces dermatitidis, and Paracoccidioides brasiliensis
• Coccidioidomycosis


  Known as the "Valley Fever", caused by Coccidioides immitis where the arthrococonidia are inhaled and convert in the lung to spherules. Most cases are clinically occult or mild infections, but some have progressive pulmonary infection and dissemination to the brain, bone, and other sites
• Histoplasmosis


  A primary pulmonary infection resulting from inhalation of conidia of Histoplasma capsulatum which convert in vivo into the blastoconidial (budding yeast) form. Dissemination to the hilar and mediastinal lymph nodes, spleen, liver, bone marrow, and brain may be life-threatening in infants and other immunocompromised patients
• Blastomycosis


  Similar to histoplasmosis, a primary pulmonary infection resulting from inhalation of conidia from the mycelial phase of Blastomyces dermatitidis which convert in vivo to the parasitic yeast phase. May cause chronic pneumonia and disseminated infection to the skin, bones and prostate in males
• Paracoccidioidomycosis


  Caused by Paracoccidioides brasiliensis, a dimorphic fungus that exists as a mold in soil and as a yeast in tissue. The yeast is thick-walled with multiple buds, in contrast to Blastomyces dermatitidis which has a single bud

  
• Opportunistic fungi


  Cause deep mycoses in immunocompromised patients
• Blastomyces dermatitidis


  Fungus that converts in vivo to the parasitic yeast phase and causes a primary pulmonary infection
• Blastomyces dermatitidis infection


  • Causes a granulomatous reaction often associated with a marked fibrotic reaction
  • Can spread to the skin, bones and prostate in males (disseminated blastomycosis)
• Paracoccidioides brasiliensis


  Dimorphic fungus that exists as a mold in soil and as a yeast in tissue, with thick-walled yeast cells having multiple buds (unlike B. dermatitidis)

  
• Paracoccidioides brasiliensis causes paracoccidioidomycosis, also known as South American blastomycosis
• Paracoccidioides brasiliensis grows in the soil and is endemic in rural Latin America. Disease occurs only in that region.