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Short answer
Colony morphology is the visual description of a bacterial colony growing on an agar plate: its size, shape, surface, color and texture. The CDC defines it as “the visual culture characteristics of a bacterial colony on an agar plate.” It is one clue toward identifying an organism, not an identification on its own, and it is only reliable when the colony is well isolated and the growth conditions are recorded.
Definition and scope
A CDC REACH job aid on colonial characteristics, part of the CDC course on biochemical and gram-negative organism identification, states that colonies need to be well isolated from other colonies so that their characteristic shape, size, color, surface appearance and texture can be observed. Two university laboratory manuals (the Miller Microbiology Laboratory Manual and the Ohio State University microbiology lab course, both hosted on LibreTexts) teach the same descriptive vocabulary. Miller adds that observing colony characteristics “is one piece of information that microbiologists can use to identify unknown bacteria.”
This page covers the vocabulary and how to record it. It does not cover plating technique, staining or biochemical identification; see identifying bacteria for the wider workflow.
How colonies are described in practice
The sources describe a colony by a fixed set of attributes. The CDC aid names size, shape (form and margin), surface appearance, color and opacity, and hemolysis on blood agar. Miller and Ohio State name the terms for each attribute.
Form, elevation and margin
Form is the overall outline of the colony. Miller lists circular, irregular and rhizoid (branched); Ohio State describes forms running “from round to irregular to filamentous and rhizoid (root-like).”
Elevation is the profile seen from the side. Miller lists raised, flat, convex, concave and umbonate. Ohio State gives the practical method: judge elevation by turning the plate on end.
Margin is the edge. Miller lists entire (smooth), undulate (wavy), lobed (a more pronounced wave) and spreading (no distinct colony), and notes that filamentous and rhizoid can also describe a margin. Ohio State adds that the edge should be examined magnified under a dissecting microscope.
Surface, color and opacity
Surface and texture terms differ slightly by source. Pigment, per Miller, may come from the cells, from pigments produced under certain conditions, or from reactions between metabolites and the medium. Ohio State distinguishes pigment through the whole colony from a “bull’s eye” pattern, and notes that some pigments are water-soluble and others are not.
Hemolysis on blood agar
The CDC aid lists three hemolysis categories (alpha, beta, gamma) and explains hemolysis as the result of lysis of the blood cells that are a component of blood agar. For the reaction itself, see alpha-hemolysis on blood agar.
Vocabulary compared across three sources
No single source gives the whole vocabulary. This table shows which source names which terms; a dash means that source does not list terms for that attribute.
| Attribute | CDC REACH job aid | Miller lab manual | Ohio State lab course |
|---|---|---|---|
| Size | Large (>1 mm), medium (about 1 mm), small (<1 mm), pinpoint (<0.5 mm) | Measure in millimeters, or describe as “pin point” | Measure with a millimeter rule; colonies under 1 mm are punctiform (pin-point) |
| Form | Named as a shape feature, no terms listed | Circular, irregular, rhizoid | Round, irregular, filamentous, rhizoid |
| Elevation | – | Raised, flat, convex, concave, umbonate | Judge by turning the plate on end |
| Margin | Named as a shape feature, no terms listed | Entire, undulate, lobed, spreading; filamentous and rhizoid also possible | Examine the magnified edge |
| Surface | Smooth, rough, glistening, dull, wrinkled, spreading, mucoid | Dry, ridged or wavy, mucoid/shiny, dull, filamentous | Smooth, glistening, rough, dull, rugose (wrinkled) |
| Color and opacity | Pigmentation (white, pink, purple, etc.); opaque, transparent | Pigment origin discussed | Transparent, opaque, translucent, iridescent |
| Hemolysis | Alpha, beta, gamma | – | – |
The size bands do not line up: the CDC aid reserves “pinpoint” for colonies under 0.5 mm, while the Ohio State course calls colonies under 1 mm punctiform (pin-point). When recording size, state which convention is used.
A recording checklist
Following the order the CDC aid implies, a complete colony record covers:
- A colony that is well isolated from its neighbors.
- Size, stating which size convention is used.
- Shape: form, elevation and margin.
- Surface and texture.
- Color and opacity, noting whether pigment is in the whole colony or in a pattern.
- On blood agar, hemolysis (alpha, beta or gamma).
- The medium, incubation time and incubation temperature alongside the description.
The last item matters because, in Miller’s words, “the colony characteristics of a microbe may change depending on the medium, time and temperature of incubation.”
Where it matters for labs and supply decisions
Because appearance depends on the medium, the same organism can look different on different plates. Selective and differential media each have their own page here, for example HE agar and MacConkey agar. Colony counting has its own requirements; see how to count colonies on an agar plate. The dish the colonies grow in is covered in what is a Petri dish.
Common misunderstandings
- “Morphology identifies the bacterium.” Miller describes colony characteristics as one piece of information among several.
- “A colony always looks the same.” Medium, time and incubation temperature can change it.
- “Any colony can be described.” The CDC aid requires well-isolated colonies; crowded or merged growth hides shape and size.
FAQ
What is colony morphology?
It is the visual appearance of a bacterial colony on an agar plate, covering size, shape, surface, color and texture (CDC REACH job aid, checked 2026-11-01).
What are form, elevation and margin?
Form is the colony’s outline, elevation its side profile, and margin its edge. Miller’s manual lists example terms for each, such as circular, raised and entire.
Can colony morphology identify bacteria?
Not alone. Miller describes observing colony characteristics as one piece of information that microbiologists can use to identify unknown bacteria, and the CDC aid calls colony morphology an important skill for identifying microorganisms.
Why does a colony look different on another medium?
Miller notes that colony characteristics may change with the medium, incubation time and temperature, and that pigment can arise from reactions between metabolites and the medium.
Sources
- CDC REACH, “Colonial Characteristics” job aid: reach.cdc.gov (checked 2026-11-01).
- Miller, Microbiology Laboratory Manual, 1.07 Colony Morphology: LibreTexts (checked 2026-11-01).
- Ohio State University, Microbiology Lab SP25, 3.02 Bacterial Colony Morphology: LibreTexts (checked 2026-11-01).
