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Microbiology

Colony Morphology

This is a critical first step in identifying bacteria in the clinical lab. This section will cover the theory, interpretation, and application of this essential technique

Theory: The Science Behind the Shapes

  • What is Colony Morphology?
    • Colony morphology refers to the visual characteristics of bacterial colonies as they grow on solid media (e.g., agar plates). It’s the first and often most crucial step in the identification process
    • It’s the art and science of observing and describing the appearance of bacterial colonies
  • Why is it Important?
    • Provides initial clues to the identity of the organism
    • Helps differentiate between different bacterial species
    • Guides selection of further identification tests
    • Offers insights into the pathogenicity of the organism
  • Factors Influencing Colony Morphology
    • Agar Type: Different agar types (e.g., blood agar, chocolate agar, MacConkey agar) provide different nutrients and selective/differential properties, affecting colony appearance
    • Nutrient Availability: The composition of the media impacts growth rate and colony characteristics
    • Temperature: Incubation temperature can influence colony size and pigment production
    • Atmospheric Conditions: Aerobic, anaerobic, or microaerophilic conditions affect growth and colony appearance
    • Organism-Specific Factors
      • Capsule production: Can lead to mucoid or slimy colonies
      • Pigment production: Produces characteristic colors (e.g., Serratia marcescens red pigment)
      • Motility: Can cause swarming or spreading growth
      • Cell wall composition: Impacts colony texture and opacity
      • Metabolic byproducts: Can influence colony odor

Interpretation: Deciphering the Clues

  • Key Characteristics to Observe and Describe
    • Size
      • Measured in millimeters (mm)
      • Descriptive terms: pinpoint, small, medium, large, very large
    • Shape
      • Round, irregular, filamentous, rhizoid (root-like), spindle-shaped, etc
    • Elevation
      • Flat, raised, convex, pulvinate (cushion-shaped), umbonate (raised in the center)
    • Margin
      • Entire (smooth), undulate (wavy), lobate (lobed), filamentous, curled, erose (irregular, jagged)
    • Surface
      • Smooth, rough, glistening, dull, mucoid (slimy), wrinkled, granular
    • Opacity
      • Transparent, translucent, opaque, iridescent
    • Color
      • White, gray, yellow, red, green, etc. Note the intensity of the color
    • Odor
      • Describe the smell (e.g., musty, fruity, putrid). Important for certain organisms
    • Hemolysis (on Blood Agar)
      • Alpha (α): Partial hemolysis, greenish discoloration around the colony (e.g., Streptococcus pneumoniae)
      • Beta (β): Complete hemolysis, clear zone around the colony (e.g., Streptococcus pyogenes, Staphylococcus aureus)
      • Gamma (γ): No hemolysis, no change in the agar (e.g., Enterococcus faecalis)
  • Putting it All Together: Examples of Descriptive Language
    • “Large, round, convex, entire, smooth, opaque, white colonies.”
    • “Small, pinpoint, irregular, flat, rough, translucent, gray colonies.”
    • “Beta-hemolytic, medium, round, raised, entire, smooth, opaque, creamy-white colonies.”
  • Creating a Systematic Approach
    • Lighting: Ensure adequate lighting (use a good light source, ideally with adjustable intensity)
    • Magnification: Use a magnifying lens if needed
    • Standardized Vocabulary: Use consistent terminology for descriptions
    • Record Keeping: Document all observations accurately and completely

Application: Putting Knowledge into Practice

  • Procedure
    1. Incubation: Ensure plates are properly incubated at the correct temperature and atmospheric conditions for the suspected organism
    2. Observation
      • Hold the plate up to the light and observe the colonies
      • Use a systematic approach to describe each characteristic
      • Note the presence of any unusual features (e.g., swarming, satelliting)
      • Use a magnifying lens for detailed examination
    3. Record Keeping
      • Document all observations, including the media used, incubation conditions, and the descriptive characteristics of the colonies
      • Include a representative drawing or photograph of the colony morphology
    4. Correlation
      • Correlate colony morphology with Gram stain results and other preliminary tests (e.g., catalase, oxidase)
      • Use colony morphology to guide the selection of appropriate biochemical tests or automated identification systems
  • Examples of How Colony Morphology Guides Identification
    • Gram-positive cocci in clusters on blood agar
      • If colonies are large, beta-hemolytic, and golden-yellow, Staphylococcus aureus is highly suspected
      • If colonies are small, non-hemolytic, and white, a coagulase-negative staphylococcus (e.g., Staphylococcus epidermidis) is more likely
    • Gram-negative rods on MacConkey agar
      • If colonies are lactose-fermenting (pink/red), Escherichia coli or another lactose-fermenting Enterobacteriaceae is suspected
      • If colonies are non-lactose fermenting (colorless), Salmonella or Shigella might be considered
    • Mucoid colonies: Often associated with Klebsiella pneumoniae or Pseudomonas aeruginosa
  • Troubleshooting
    • Inconsistent Results: Ensure proper media storage and preparation. Check incubation conditions
    • Contamination: Isolate colonies from a pure culture. Review aseptic technique
    • Atypical Morphology: Consider unusual growth conditions or antibiotic exposure

Key Terms

  • Colony: A visible mass of microorganisms (bacteria, fungi, etc.) growing on a solid surface, originating from a single cell or a small group of cells
  • Morphology: The study of the form and structure of organisms, in this context, referring to the visual characteristics of bacterial colonies
  • Size: The overall dimensions of a colony, typically measured in millimeters (mm)
  • Shape: The general form of the colony, such as round, irregular, filamentous, or rhizoid
  • Elevation: The height of the colony relative to the agar surface, described as flat, raised, convex, pulvinate, or umbonate
  • Margin: The edge or border of the colony, which can be entire (smooth), undulate (wavy), lobate (lobed), filamentous, or erose (irregular, jagged)
  • Surface: The texture of the colony’s upper surface, which can be smooth, rough, glistening, dull, mucoid (slimy), or wrinkled
  • Opacity: The degree to which light is able to pass through the colony, described as transparent, translucent, opaque, or iridescent
  • Hemolysis: The breakdown of red blood cells in the agar surrounding a colony, typically observed on blood agar. Types include alpha (partial), beta (complete), and gamma (none)
  • Mucoid: A descriptive term for colonies that have a slimy or gummy consistency, often due to the production of a polysaccharide capsule