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Describing Colony Morphology

As we document our discoveries it is absolutely essential that we use common adjectives to describe the physical characteristics of the colonies that our isolates form when cultivated on an agar surface. Below is an illustrated guide to the terms we will use.

1. Culture conditions. Because colony morphology may be influenced by the conditions under which the species is cultured, it is important to describe those conditions accurately. Include the medium (e.g., R2A agar, Tryptic soy agar), temperature of incubation, age of the culture.

2. Form. Irregular colonies that spread rapidly over a plate should be identified as spreading in addition to irregular. A branching structure distinguishes rhizoid from filamentous. Punctiform colonies are distinguished from circular colonies by their very small size. Spindle colonies are lens-shaped.

3. Elevation. There are no clear distinctions among raised, convex, and pulvinate. Convex would be close to semi-circular in cross section, while raised and pulvinate colonies have shallower and deeper profiles, respectively. The "bump" on an umbonate colony is called an umbo.

4. Margin. Erose is synonymous with serrated.

5. Surface. Surface can be smooth, glistening, rough, wrinkled, dry, powdery, moist, mucoid (forming large moist sticky colonies, brittle, viscous (difficult to remove from loop), butyrous (buttery).

6. Opacity. Check opacity in a dissecting microscope. Colonies may be transparent (clear), translucent (can see through them), or opaque (colony blocks transmitted light).

7. Color. White, buff, brown, red, yellow, pink, purple, etc.

8. Other. Describe any distinctive odor, any diffusible pigments (staining the agar), or any other likely distinguishing characteristic.

Created by David R. Caprette, Teaching Professor, Rice University ( June 7, 2016 for the course BIOC 318, Laboratory Studies in Microbiology. Please feel free to copy and/or modify these materials for use in your own academic or other nonprofit program. If you don't mind letting me know of such intentions I'll be happy to hear from you.
Last updated Jan 3, 2017