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Petri dishes with bacteria for article on colony morphology
Published On: August 17th, 2021

Microplate Washer ELISA

Bacterial colonies are an important component of microbiology that remains relevant to this day and probably will always stay that way. These colonies are used for medical research, development of pharmaceuticals, use in the food industry (the safe ones, naturally) and more. Once a bacterial colony is cultivated, the first thing to do is identify it through the bacterial colony morphology it displays.

Bacterial Colony Morphology Definition And Importance

But first, how does one define colony morphology? Its definition is simple: colony morphology is simply the appearance of the colony once it grows on an agar plate. The visual cues provided by a cultivated bacterial colony serve as an important way for microbiologists to identify and isolate them via colony picking for other applications.

Categories of Bacterial Colony Morphology

Since the appearance of bacterial colonies can be varied, scientists describe colony morphology by using several characteristics. These include:

  • Size (often noted down in milimetres)
  • Colour – buff, white, red, black, etc
  • Opacity – transparent, translucent, iridescent, etc
  • Form (shape of the colony) – circular, rhizoid, irregular, etc
  • Elevation (the shape of the colony from the side angle) – flat, raised, convex, umbonate, etc…
  • Edge/Margin (shape of the outer part of the colony) – entire, undulate, lobate, curled, etc…
  • Surface appearance – shiny, wrinkled, glistening, moist, etc…
  • Texture (when manipulated with a sterile instrument) – dry, mucoid, brittle, viscid, etc…
  • Odor – grape-like, putrid, sweet-smelling, etc…

How to Observe And Describe Bacterial Colony Morphology

Observing bacterial colony morphology needs to be a systematic process. The steps below act as a good guideline.

  1. Before beginning to observe the grown colonies, note down the medium that was used in the agar plate to grow the bacterial colonies. It will act as a good reference when comparing the bacteria morphology in a different type of medium.
  2. Look at the agar plate and observe the different colonies on it. Pick a colony that is well-isolated and well sized so it’s easy to look at.
  3. Take a look at the colony’s form and color, followed by opacity and surface appearance. You can use a magnifying glass to do this if you wish. Alternatively, to get the best view of the colony, use a dissecting/stereoscopic microscope and put the agar plate under the lens with the cover closed. Wipe down the inside of the cover if it is condensed before observation.
  4. Tilt the plate to view the colony from the side to observe elevation and margin. Often, odor is an optional morphology to note down, but you can take a quick whiff from a bit of a distance if you’d like to note this down (and if it’s safe).
  5. Lastly, to determine texture, use an inoculating loop or a needle to pick at the colony to observe its consistency as it leaves the medium. Once you’ve noted down all of the above, you can successfully describe the morphology of the colony you’ve chosen.

Examples of Bacterial Colony Morphology

An example of a popular bacterial colony that is often studied is Staphylococcus aureus. On a Mannitol salt agar, this bacteria forms two to three milimetres wide and circular colonies with a smooth, shiny surface appearance. They are often golden yellow and opaque with an entire margin. The colonies also sometimes give off an “old socks” smell, and have a texture often described as “creamy”.

Another example of a colony that is used for observation is Escherichia coli (E. coli). On a nutrient agar, E. coli forms large, greyish-white, smooth, opaque/translucent discs. Some strains produce mucoid colonies.

If you are jogging your memory about bacterial colony morphology to assist the process of colony picking for research or commercial purposes, contact Hudson Robotics to get more info on our colony picking solutions that will make your job a lot faster and easier.