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Microbiology

Morphology & Growth

Colony morphology and growth characteristics are crucial aspects of mycology, as they often provide the first clues to identifying a particular fungus. We’ll cover the key features to observe, how temperature and incubation time affect growth, and then highlight the characteristics of some major pathogens

Key Features of Colony Morphology

When examining a fungal colony, consider the following characteristics:

  • Growth Rate: How quickly does the colony grow? (e.g., rapid, moderate, slow)
  • Texture: What is the surface texture of the colony? (e.g., cottony, woolly, granular, velvety, glabrous/smooth, waxy)
  • Elevation: How does the colony rise above the agar surface? (e.g., flat, raised, heaped, rugose/wrinkled)
  • Margin: What is the edge of the colony like? (e.g., smooth, irregular, filamentous, lobate)
  • Color (Top and Reverse): What color is the colony on the surface? What color is the underside of the colony (visible through the agar)? Pigment production can be key for identification
  • Other Features: Are there any other notable features, such as:
    • Exudate: Production of droplets on the colony surface
    • Diffusible Pigment: Pigment that spreads into the agar
    • Zonation: Concentric rings of different colors or textures

Impact of Temperature and Incubation Time

  • Temperature
    • Most fungi grow optimally at temperatures between 25-30°C, which is why this is the standard incubation temperature for fungal cultures
    • Some fungi grow better at 35-37°C, which mimics body temperature and can help in identifying the tissue form of dimorphic fungi
    • Temperature can also affect colony morphology and pigment production
  • Incubation Time
    • Fungi generally grow slower than bacteria, so fungal cultures require longer incubation times
    • Most fungal cultures are incubated for up to 4 weeks (28 days), with regular observation for growth
    • Some slow-growing fungi may require even longer incubation times

Colony Morphology and Growth Characteristics of Major Pathogens

Here’s a breakdown of the colony morphology and growth characteristics of some common fungal pathogens:

Yeasts

  • Candida spp.
    • Growth Rate: Rapid
    • Texture: Creamy, smooth
    • Color: Typically white or cream-colored. Some species produce distinctive colors on chromogenic agar
    • Temperature: Grows well at 25-37°C
  • Cryptococcus neoformans
    • Growth Rate: Moderate
    • Texture: Mucoid (slimy)
    • Color: White to tan
    • Temperature: Grows well at 25-37°C
  • Malassezia furfur
    • Growth Rate: Slow
    • Texture: Creamy, smooth
    • Color: Cream-colored to slightly yellow
    • Lipid Dependence: Requires lipid supplementation in the culture medium (e.g., olive oil)
    • Temperature: Grows well at 30-35°C

Dimorphic Fungi

  • Histoplasma capsulatum
    • Mold Form (25-30°C)
      • Growth Rate: Slow
      • Texture: Cottony or woolly
      • Color: White to brown
    • Yeast Form (35-37°C)
      • Growth Rate: Moderate
      • Texture: Smooth, creamy
      • Color: Tan to brown
  • Blastomyces dermatitidis
    • Mold Form (25-30°C)
      • Growth Rate: Slow
      • Texture: Fluffy or cottony
      • Color: White to tan
    • Yeast Form (35-37°C)
      • Growth Rate: Moderate
      • Texture: Wrinkled, creamy
      • Color: Cream-colored to tan
  • Coccidioides immitis
    • Mold Form (25-30°C)
      • Growth Rate: Rapid
      • Texture: Fluffy or cottony
      • Color: White to gray
      • Hazardous: Produces highly infectious arthroconidia
    • Spherule Formation (in vivo or specialized media): Not typically observed on routine culture
  • Sporothrix schenckii
    • Mold Form (25-30°C)
      • Growth Rate: Moderate
      • Texture: Initially moist and white, becoming dark brown or black with age
      • Color: White to dark brown/black
    • Yeast Form (35-37°C)
      • Growth Rate: Moderate
      • Texture: Creamy
      • Color: Cream-colored to tan

Dermatophytes

  • Trichophyton rubrum
    • Growth Rate: Slow to moderate
    • Texture: Variable (cottony, granular, fluffy)
    • Color (Top): White to reddish
    • Color (Reverse): Red
  • Trichophyton mentagrophytes
    • Growth Rate: Moderate to rapid
    • Texture: Variable (cottony, granular, fluffy)
    • Color (Top): White to cream-colored
    • Color (Reverse): Tan to brown
  • Microsporum canis
    • Growth Rate: Rapid
    • Texture: Fluffy or cottony
    • Color (Top): White to yellowish
    • Color (Reverse): Bright yellow

Mucorales (Zygomycetes)

  • Rhizopus spp.
    • Growth Rate: Very rapid (fills the plate in a few days)
    • Texture: Cottony, fluffy
    • Color: White to gray
  • Mucor spp.
    • Growth Rate: Very rapid
    • Texture: Cottony, fluffy
    • Color: White to gray

Hyaline Molds

  • Aspergillus fumigatus
    • Growth Rate: Rapid
    • Texture: Velvety or granular
    • Color: Blue-green to gray-green
    • Temperature: Grows well at 37°C
  • Fusarium spp.
    • Growth Rate: Rapid
    • Texture: Cottony or fluffy
    • Color: Variable (white, pink, purple)
  • Penicillium spp.
    • Growth Rate: Rapid
    • Texture: Velvety or powdery
    • Color: Green to blue-green with a white border
  • Scedosporium apiospermum
    • Growth Rate: Rapid
    • Texture: Cottony or velvety
    • Color: Gray to brown

Dematiaceous Molds

  • Alternaria spp.
    • Growth Rate: Rapid
    • Texture: Woolly or cottony
    • Color: Dark brown to black
  • Cladosporium spp.
    • Growth Rate: Slow to moderate
    • Texture: Velvety or powdery
    • Color: Dark green to black
  • Fonsecaea spp.
    • Growth Rate: Slow
    • Texture: Velvety or waxy
    • Color: Dark green to black

Important Considerations

  • Variability: Colony morphology can vary depending on the culture medium, temperature, and other factors
  • Microscopic Examination: Colony morphology is just the first step in identification. Microscopic examination of the fungal structures (hyphae, conidia) is essential for accurate identification
  • Reference Materials: Use reference materials (e.g., textbooks, online resources) to compare your observations with known characteristics of different fungal species
  • Experience: With experience, you’ll become more familiar with the colony morphology of common fungal pathogens

Key Takeaways

  • Colony morphology and growth characteristics are important tools for identifying fungi
  • Consider growth rate, texture, elevation, margin, color, and other features
  • Temperature and incubation time can affect colony morphology
  • Use reference materials and experience to accurately identify fungal species

Key Terms

  • Colony Morphology: The macroscopic appearance of a fungal colony on a culture medium
  • Growth Rate: The speed at which a fungal colony grows
  • Texture: The surface appearance of a fungal colony (e.g., cottony, granular)
  • Elevation: The height of a fungal colony above the agar surface
  • Margin: The edge of a fungal colony
  • Top Color: The color of the top surface of a fungal colony
  • Reverse Color: The color of the underside of a fungal colony (visible through the agar)
  • Exudate: Liquid droplets produced on the surface of a fungal colony
  • Diffusible Pigment: Pigment that spreads into the agar surrounding a fungal colony
  • Zonation: Concentric rings of different colors or textures within a fungal colony
  • SDA: Sabouraud Dextrose Agar, a common fungal culture medium
  • BHI: Brain-Heart Infusion agar, an enriched fungal culture medium
  • Lipid Supplementation: Addition of lipids (e.g., olive oil) to a culture medium to support the growth of lipid-dependent fungi (e.g., Malassezia)
  • Arthroconidia: A type of asexual spore formed by the fragmentation of hyphae, characteristic of Coccidioides
  • Hazardous: Potentially dangerous or infectious, requiring special handling precautions
  • Reference Materials: Textbooks, online resources, or other materials used to compare observations and identify fungal species