Basic Elements in Identifying Colonies

• Form - What is the basic shape of the colony? For example, circular, filamentous, etc.

• Elevation - What is the cross sectional shape of the colony? Turn the Petri dish on end.

• Margin - What is the magnified shape of the edge of the colony?

• Surface - How does the surface of the colony appear? For example, smooth, glistening, rough, dull, rugose, etc.

• Opacity - For example, transparent (clear), opaque, translucent (almost clear, but distorted vision, like looking through frosted glass), iridescent (changing colors in reflected light), etc.

• Chromogenesis - For example, white, buff, red, purple, etc.

• Consistency

• Butyrous (butter-like)

• Viscous or stringy (a portion of it may come off the agar surface with the transfer needle)

• Rubbery (whole colony comes off the agar surface with the transfer needle)

• Dry, brittle or powdery (colonies that break when touched by a needle)

• Odor

• Sweet

• Putrefactive

• Fruity

Figure 3. Comparison of the Gram positive and Gram negative bacterial cell walls. A, a Gram positive bacterium has a thick peptidoglycan layer that contains teichoic and lipoteichoic acids. B, a Gram negative bacterium has a thin peptidoglycan layer and an outer membrane that contains lipopolysaccharide, phospholipids, and proteins. The periplasmic space between the cytoplasmic and outer membranes contains transport, degradative, and cell wasll synthetic proteins. The outer membrane is joined to the cytoplasmic membrane at adhesion points and is attached to the peptidoglycan by lipoprotein links.

Figure 4. Gram stain morphology of bacteria. A, the crystal violet Gram stain is precipitated by Gram iodine and is trapped in the thick peptidoglycan layer in Gram positive bacteria. The decolorizer disperses the Gram negative outer membrane and washes the crystal violet from the thin layer of peptidoglycan. Gram negative bacteria are visualized by the red counterstain. B, bacterial morphologies.

Figure 5. Bacteria and Bacilli. Highly magnified.

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