Microbiology Lab Study Guide: Colony Morphology and Gram Staining

Colony Morphology

Definition and Importance

Colony morphology refers to the visible characteristics of a microbial colony grown on solid media. Each colony arises from a single cell or group of identical cells (clones), making it a useful tool for identifying and differentiating bacterial species in the laboratory.

• Colony: A visible mass of microbial cells originating from one cell or group of identical cells.

• Significance: Observing colony morphology helps in the preliminary identification of bacteria and assessment of culture purity.

Environmental Factors Affecting Colony Morphology

• Temperature: Influences growth rate and appearance.

• Incubation Time: Affects colony size and development of features.

• Nutrient Availability: Determines colony size, color, and texture.

Describing Bacterial Colonies: The Seven Categories

Colony appearance is described using standardized terms from seven main categories:

• Size: Pinpoint, small, medium, large

• Shape: Round, irregular

• Color: Specific color (e.g., white, yellow, red); also note if opaque or translucent

• Margin (Edge): Entire (smooth), undulate (wavy), lobate (lobed), filamentous (unbranched strands), rhizoid (branched strands)

• Surface: Smooth, rough, wrinkled, shiny, dull

• Texture: Mucoid (sticky), moist, dry, butyrous (buttery)

• Elevation: Flat, raised, convex, pulvinate (very convex), umbonate (raised in the center)

Swarming Phenomenon

• Definition: Swarming is a rapid and coordinated movement of bacterial populations across the surface of solid media.

• Appearance: Concentric rings of growth, resembling a bullseye pattern.

• Example: Proteus species are well-known for swarming behavior.

Gram Staining

Principle and Purpose

The Gram stain is a differential staining technique that classifies bacteria based on differences in their cell wall structure. It is fundamental for bacterial identification and guides initial treatment decisions in clinical microbiology.

• Gram Positive Bacteria: Stain purple due to a thick peptidoglycan layer that retains the primary stain.

• Gram Negative Bacteria: Stain pink because of a thin peptidoglycan layer and an outer lipid membrane that does not retain the primary stain after decolorization.

Gram Stain Procedure and Steps

1. Primary Stain: Apply crystal violet (purple) for 1 minute. Stains all cells.

2. Mordant: Add iodine for 1 minute. Iodine binds with crystal violet to form a larger, less-soluble complex.

3. Decolorization: Rinse with alcohol (10-30 seconds) until runoff is clear. This step differentiates Gram positive from Gram negative bacteria.

4. Counterstain: Apply safranin (pink) for 1 minute. Stains cells that have lost the primary stain.

5. Rinse with water after each step.

Role of the Mordant (Iodine)

• Function: Iodine forms a complex with crystal violet, making it less soluble and harder to wash out of the cell.

• If Skipped: Crystal violet would wash out more easily, causing Gram positive cells to appear Gram negative.

Mechanism of Decolorization

• Gram Positive Cells: Alcohol dehydrates the thick peptidoglycan layer, trapping the crystal violet-iodine complex inside the cell.

• Gram Negative Cells: Alcohol dissolves the outer lipid layer and, due to the thin peptidoglycan, creates openings that allow the crystal violet-iodine complex to escape. These cells are then stained pink by safranin.

Effects of Decolorization Errors

• Over-decolorization: Excess alcohol can break down the cell wall and membrane of Gram positive cells, causing them to lose the primary stain and appear reddish.

• Under-decolorization: Insufficient alcohol fails to remove the primary stain from Gram negative cells, so they may appear purple instead of pink.

• Skipping Decolorization: All cells retain the crystal violet, and the lighter safranin cannot stain over it, so all cells appear purple.

Summary Table: Gram Stain Results and Cell Wall Structure

Bacterial Type	Cell Wall Structure	Stain Color	Response to Decolorization
Gram Positive	Thick peptidoglycan, no outer membrane	Purple	Retains crystal violet-iodine complex
Gram Negative	Thin peptidoglycan, outer lipid membrane (LPS)	Pink	Primary stain lost, counterstained with safranin

Key Terms

• Peptidoglycan: A polymer that forms a mesh-like layer outside the plasma membrane of most bacteria, providing structural strength.

• Mordant: A substance (iodine) that helps fix a dye in or on a substance by forming an insoluble compound.

• Decolorization: The process of removing the primary stain from some cells, allowing differentiation based on cell wall properties.

Example Application

• Clinical Relevance: Rapid Gram staining of a patient sample can guide initial antibiotic therapy, as Gram negative and Gram positive bacteria often differ in drug susceptibility.

Additional info: The Gram stain is named after Hans Christian Gram, who developed the technique in 1884. Some bacteria, such as Mycobacterium species, do not reliably stain with the Gram method due to unique cell wall structures and require alternative staining techniques.