mycology

• Mycology


  The study of fungi and their multiple functions in nature

  
• What mycology studies

  • Beer
  • Wine
  • Bread
  • Cheese
  • Mushrooms
  • Biodegradation
  • Disease
• Fungi


  • Eukaryotic organisms that do not contain chlorophyll
  • Have cell walls
  • Have filamentous structures
  • Produce spores
  • Grow as saprophytes and decompose dead organic matter

  
• There are between 100,000 to 200,000 species of fungi depending on how they are classified
• About 300 species of fungi are presently known to be pathogenic for man
• Five kingdoms of living things


  • Monera
  • Protista
  • Fungi
  • Plants
  • Animals
• Man is an example of an animal kingdom organism
• Types of mycotic diseases


  • Hypersensitivity
  • Indoor air pollution
  • Mycotoxicoses
  • Mycetismus
  • Infection
• Hypersensitivity


  An allergic reaction to molds and spores
• Mycotoxicoses


  Poisoning of man and animals by feeds and food products contaminated by fungi which produce toxins from the grain substrate
• Mycetismus


  The ingestion of pre-formed toxin (mushroom poisoning)
• Most common pathogenic fungi do not produce toxins
• Factors contributing to pathogenicity of fungi


  • Thermotolerance
  • Ability to survive in tissue environment
  • Ability to withstand host defenses
• Increased frequency of mycotic diseases, increased awareness by physicians, better trained laboratory personnel, more invasive procedures used on patients, increased use of immunosuppressive drugs, increase in immunosuppressive disease, and better laboratory diagnostic tools have led to a revived interest in mycology
• Yeasts


  Unicellular organisms that reproduce by budding (a form of asexual reproduction)
• Hyphae


  Multicellular filamentous structures, constituted by tubular cells with cell walls
• Mycelium


  A mass of hyphae
• Coenocytic


  Mycelium without septa (cross walls)
• The terms "hypha", "mycelium" and "mold" are frequently used interchangeably
• Dimorphic fungi


  Fungi that have two forms: a yeast (parasitic or pathogenic) form and a mycelial (saprophytic or mold) form
• Conversion to the yeast form appears to be essential for pathogenicity in the dimorphic fungi
• Types of fungal spores


  • Sexual spores
  • Asexual spores (macroconidia, chlamydospores, microconidia, blastospores, arthroconidia)

  
• Fungi are ubiquitous in nature and human beings are constantly exposed to them
• Most mycotic agents are soil saprophytes and mycotic diseases are generally not communicable from person-to-person (occasional exceptions: Candida and some dermatophytes)
• Outbreaks of fungal disease may occur, but these are due to a common environmental exposure, not communicability
• Colonization of an intravenous catheter


  Demonstration of fungi in blood drawn from the catheter
• Transient fungemia


  Demonstration of fungi in blood drawn from an intravenous catheter
• True infection


  Demonstration of fungi in blood drawn from an intravenous catheter
• The physician must decide the clinical status of the patient based on clinical parameters, general status of the patient, laboratory results, etc. as the decision is not trivial since treatment of systemic fungal infections requires the aggressive use of drugs with considerable toxicity
• Diagnostic methods for fungi

  • Wet mount
  • Direct fluorescent microscopy
  • Biopsy and histopathology
  • Culture
  • Serology
  • DNA probes
• Wet mount


  Skin scrapings suspected to contain dermatophytes or pus from a lesion can be mounted in KOH on a slide and examined directly under the microscope
• Direct fluorescent microscopy


  May be used for identification of fungi, even on non-viable cultures or on fixed tissue sections
• Biopsy and histopathology


  A biopsy may be very useful as a source of the tissue-invading fungi and for the identification of the organism. The Gomori methenamine silver (GMS) stain is used to reveal the organisms which stain black against a green background. The H&E stain demonstrates the inflammatory cells but does not always tint the organism.
• Culture


  A definitive diagnosis requires a culture and identification. Pathogenic fungi are usually grown on Sabouraud dextrose agar (SDA). Each specimen is inoculated on duplicate sets of media and incubated separately at 25ºC and 37ºC to reveal dimorphism. The cultures are examined macroscopically and microscopically. The cultures are not considered negative for growth until after 4 weeks of incubation.

  
• Serology


  This tool may be helpful when it is applied to a specific fungal disease; there are no screening antigens for "fungi" in general. Because fungi are poor antigens, the efficacy of serology varies with different fungal agents. The most common serological tests for fungi are based on latex agglutination, immunodiffusion, complement fixation and enzyme immunoassays.
• DNA probes


  Ribosomal DNA is hybridized to a labeled DNA probe. This test is rapid (1 Hour) and species specific.
• Mammalian cells do not contain the enzymes which will degrade the cell wall polysaccharides of fungi, making these pathogens difficult to eradicate by the animal host defense mechanisms
• Because mammals and fungi are both eukaryotic, the cellular milieu is biochemically similar in both, so most substances which may impair the invading fungus will usually have serious side effects on the host
• Although one of the first chemotherapeutic agents was an anti-mycotic used in 1903, the further development of such agents has been slower than the development of anti-bacterial agents due to the difficulty in establishing selective toxicity to inhibit the invading organism with minimal damage to the host within eukaryotic cells