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» OCW Home » Microbiology » Student Notes 
Lecture 19 - Fungi
Author: Amelia Virostko
Color Key
Important key words or phrases.
Important concepts or main ideas.

1. Fungi Vocabulary

  1. Yeast: single fungal cell with a rigid cell wall that divides by budding
  2. Pseudohyphae: long chains of yeast resulting from incomplete separation during budding
  3. Hyphae: threadlike, branching filaments composed of fungal cells attached end to end
  4. Molds: multicellular colonies composed of clumps of branching hyphae that produce spores
  5. Spores: reproducing bodies of molds
  6. Dimorphic fungi: fungi that can grow as either a mold or a yeast

2. Introduction

  1. Aerobic eukaryotes free-living in the environment or part of the normal human flora
  2. Anti-fungal medicines target ergosterol, the key sterol in the fungal cell membrane
  3. Cell wall antigenic to humans, and some fungi have polysaccharide capsule that protects them from the immune response

3. Histoplasma capsulatum

3.1. Encounter

  1. Commonly found in the southeastern U.S. that borders the Mississippi River and many tropical countries
  2. Grows in bird and bat excrement, therefore found in old barns and caves

3.2. Entry

  1. Small, aerosolized spores are inhaled
  2. Transform into yeast when exposed to normal body temperature

3.3. Damage and Clinical Manifestations

  1. Often asymptomatic or resembles a minor respiratory illness
  2. Mild pneumonia can case lung infiltrates and calcified granulomas
  3. Disseminated disease can occur in immunocompromised hosts, that can cause meningitis, bone lytic granulomas, and skin granulomas

3.4. Key Virulence Factors

  1. Damage caused by direct invasion and displacement rather than toxins

3.5. Identification and Diagnosis

  1. Found within histiocytes and granulomas
  2. Lung biopsy can be performed and stain specimen with Silver stain

3.6. Treatment

  1. Acute infection does not require treatment
  2. Chronic or disseminated infections require itraconazole or Amphotericin B
  3. Resistance is not a problem

3.7. Outcome

  1. Usually self-limited infection or response to treatment
  2. Can be chronic or life-threatening in immunocompromised hosts

4. Blastomyces dermatiditis

Rarest form of systemic fungal infection and most likely to become chronic and disseminated

4.1. Encounter

  1. Anywhere in the U.S. independent of bird or bat excrement

4.2. Spread

  1. Similar to Histoplasma

4.3. Damage and Clinical Manifestations

  1. Tends to present in disseminated form than Histoplasma
  2. Present with weight loss, night sweats, lung involvement and skin ulcers

4.4. Identification and Diagnosis

  1. Large yeast that divide by broad based budding

4.5. Treatment

  1. Itraconazole, ketoconazole, or amphotericin-B

5. Coccidioides immitis

  1. Dimorphic yeast that enters by inhalation
  2. Causes San Joaquin Valley fever
  3. 60% of infections are asymptomatic or present as a mild upper respiratory tract infection
  4. 35% develop symptoms 1-3 weeks after exposure in bones, joints, and/or skin
    1. Symptoms include: cough, sputum production, chest pain, fever, chills, night sweats, anorexia, weakness, erythema nodosum, and arthralgia
    2. Meningitis can occur within 6 months and presents with headache
  5. Disseminated infection occurs with increased frequency in African-Americans and Filipinos, and pregnant women
  6. Similar to Histoplasma in its mode of entry, spread, damage, diagnosis, and treatment

6. Opportunistic Fungi: Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus

Infect immunocompromised hosts or neutropenic patients

6.1. Candida albicans

6.1.1. Encounter

  1. Part of normal flora
  2. Patients at high risk are patients with malignancies, organ or bone transplants, AIDS, burns, operations, trauma, long-term use of IV or intra-arterial catheters, or being treated with broad spectrum antibiotics

6.1.2. Spread

  1. Phagocytosis by neutrophils is the main mechanism to prevent spread of infection

6.1.3. Damage and Clinical Manifestations

  1. Can cause mucous membrane infectcions, cutaneous candidiasis, disseminated candidiasis, endocarditis, cystitis, or peritonitis

6.1.4. Identification and Diagnosis

  1. Microscopic exam reveals budding yeast cells, pseudohyphae and hyphal elements
  2. Can be cultured from blood in disseminated infection

6.1.5. Treatment

  1. Amphotericin-B or fluconazole

6.2. Cryptococcus neoformans

  1. Encapsulated yeast present in the environment
  2. Found only in the yeast form
  3. AIDS is the predisposing factor

6.2.1. Encounter

  1. Found in pigeon droppings and inhaled into lungs

6.2.2. Spread

  1. After causing asymptomatic infection in the lungs, the yeast can spread through blood to the brain
  2. Brain abscesses cause damage by displacement, not inflammation

6.2.3. Damage and Clinical Manifestations

  1. Can cause pneumonia, skin lesions, or bone lesions like other fungi
  2. Most common presentation is meningitis
  3. Symptoms include: headache, lethargy, coma, personality changes, memory loss, cranial nerve deficits

6.2.4. Identification and Diagnosis

  1. Cerebral spinal fluid stained with India ink
  2. Cryptococcal antigen test

6.2.5. Treatment

  1. Amphotericin-B and flucytosine

6.3. Aspergillus fumigatus

6.3.1. Encounter

  1. Found in air, soil, and moldy vegetation
  2. Not part of normal human flora
  3. Initially invade lung or paranasal sinuses

6.3.2. Spread

  1. Invasive infection can spread to the brain, bone, skin, liver, and breast

6.3.3. Damage and Clinical Manifestations

  1. Lung infection causes consolidation, lobar pneumonia, and lung cavities
  2. Can form fungus balls that occlude bronchi
  3. Angioinvasive that can cause ischemia and necrosis
  4. Allergic bronchopulmonary aspergillosis from Type I hypersensitivity

6.3.4. Virulence Factors

  1. Toxic metabolic products called aflatoxin that are hepatotoxic and carcinogenic

6.3.5. Identification and Diagnosis

  1. X-ray or CT scan can reveal aspergillomas
  2. Sputum exam or blood culture show characteristic branching hyphae

6.3.6. Treatment

  1. Allergic reactions treated with corticosteroids
  2. Fungus balls must be surgically removed
  3. Invasive disease treated with Amphotericin-B
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