Fungal Infection

Classification
1. Fungi are multicellular eukaryotes
2. Aerobic organisms
3. Require external energy sources (unable to perform photosynthesis)
Layers
1. Cell membrane (inner layer)
  1. Bilayered membrane encases the cytoplasm (innermost layer)
  2. Composed of ergosterol (see below)
2. Cell Wall
  1. Middle layer between the cell membrane and the capsule
  2. Composed of Carbohydrates, and to a lesser extent, Proteins
  3. Fungal cell wall is Antigenic, stimulating the human immune response
  4. Fungal cell walls stain with Grocott's methenamine silver (GMS), Periodic Acid-Schiff (PAS)
3. Capsule (outer layer)
  1. Polysaccharide outer fungal coating, surrounding the cell wall
  2. Capsule protects the fungal organism from Phagocytosis
  3. Fungal capsules stain with India ink, mucicarmine, and Alcian blue
  4. Only certain fungal organisms have a capsule
    1. Cryptococcus neoformans (most common)
    2. Malassezia furfur
    3. Rhinosporidium seeberi
    4. Trichosporon beigelii
    5. Blastocystis hominis
    6. Sporothrix schencki
Ergosterol
1. Ergosterol is the key sterol in fungal cell membranes
  1. Ergosterol is a target of many Antifungal agents
  2. Antifungals bind, disrupt or interfering with ergosterol synthesis
2. As with other organisms, sterols are a key component of the cell wall
  1. Contrast with Cholesterol cell walls in humans and hopanoids in Bacteria

Yeast
1. Unicellular, spherical forms of yeast assumed during growth
2. Yeast form may slowly reproduce through budding
  1. May form pseudohyphae when buds do not separate
3. Examples
  1. Candida albicans (e.g. Candidiasis)
  2. Cryptococcus neoformans (e.g. Cryptococcosis)
  3. Saccharomyces
Hyphae
1. Tree-like, thin branching tubules of fungal cells, with growth from the tubule ends
2. Examples: Tinea
Molds (Mycelia)
1. Clumps of intertwined, branching hyphae
2. Reproduce via spore formation
3. Examples: Aspergillosis
Spores
1. Molds form spores for reproduction
Dimorphic fungi
1. Fungi that grow in one of 2 forms: Yeast or Molds
2. Environmental factors (including Temperature) dictate the form assumed
  1. Mycelial form (with spores, infectious by inhalation) at 25 C
  2. Yeast cell form at 37 C
3. Yeast is the typical form found at Body Temperature
4. Examples
Saprophytes
1. Rely on organic matter (rotting vegetation, soil) for energy (food source)
2. Examples
  1. Aspergillus
  2. Candida
  3. Cryptococcus
  4. Pneumocystis

Background
1. Similar to Tuberculosis (although these organisms are fungal, and not contagious)
  1. Primary lung infections that may cause cavitary lesions and may cause disseminated disease
Transmission
1. Transmitted by inhaled spores (e.g. aerosolized from soil, bird droppings)
2. No human to human transmission
3. Dimorphic fungi (spores at 25 C, yeast cells at 37 C)
Spectrum of disease
1. Asymptomatic in most patients
2. Acute Pneumonia (typically mild)
3. Chronic cavitary Pneumonia (rare)
4. Disseminated Fungal Infection (rare, but esp. Immunocompromised patients)
Organisms
1. Blastomycosis
  1. Most pathogenic when compared with Coccidioidomycosis, Histoplasmosis
2. Coccidioidomycosis
3. Histoplasmosis
Other organisms
1. Aspergillosis is also a Fungal Lung Infection, opportunistic and described below