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Image of bacteria for article on colony morphology.
Published On: September 14th, 2021

Did you know that the first microscope was essentially just two types of lenses stacked on one another? Before the invention of even a barebones microscope in the 16th century, scientists often relied on the power of observation via the naked eye to come to their hypothesis. Bacterial colonies, for example, were identified based on their physical characteristics. Microbiologists even today have to learn how to describe colony morphology well without a microscope.Describing bacterial colony morphology is not that complex and just requires a systematic approach so you can get the broadest number of descriptions possible to pinpoint the type of colony accurately. We’ll go through that systematic process below for your reference.


Steps to Describe Bacterial Colony Morphology


Step 1: Pick a Colony and Measure Its Size

Look at the agar plate you have grown the colonies on and observe the colonies with the naked eye or through other magnifying lenses. Pick the colony you want to focus on, and measure the diameter of the colony in millimeters. Alternatively, you can describe the colony’s size as a pinpoint, small, medium, large, etc though this will not be very precise.

Step 2: Observe the Basic Morphology of the Bacterial Colony

Once you’ve noted down the size, it’s time to take a closer look. Note down the color (and iridescence) of the colony and its opacity (is it transparent, translucent, or opaque?). Next, describe colony morphology by noting down the following characteristics:

  • Shape (otherwise known as form) – look at the overall shape of the colony from the top. Is it circular, irregular, rhizoid, or some other shape?
  • Colony margin – this refers to the colony’s edges. This is called an’ entire’ margin if the colony has a clean edge that covers the whole colony. Other margin types include undulate, lobate, curled, and filiform. For this, it’s useful to use a magnifying glass or a microscope to get a better look.
  • Surface – note down the appearance of the surface of the colony. Is it smooth, dull, glistening, or wrinkled?
  • Elevation – observe the colony from a side angle, and take note of the elevation of the colony. Is the colony flat on the agar plate, or is it raised? Does the elevation have a concave, convex, or umbonate shape? What about a crateriform shape?

Step 3: Test the Texture of the Colony

Now that you’ve done all the main observations, it’s time to “disturb” the colony. Use an inoculating loop, toothpick, or even a needle to scrape slightly at the colony. Identify the texture you feel; scientists typically refer to the texture or consistency of a colony with terms like dry, ridged, butyrous, viscid, and mucoid.

(Optional) As a final step, if it’s safe and absolutely necessary, you can take a quick whiff a distance away from the agar plate to identify any scents that the colony is releasing. Sometimes, the odor of a colony can help you identify what it is (but this is rarely necessary).

Why Learn To Describe Colony Morphology?

Identifying the species represented by colonies is the main purpose of learning how to describe colony morphology. This is especially important in the process of colony picking, which is a precursor to many microbiological processes in the lab and in industry. However, these days many labs use colony picking robots to automate the process, such as the ones offered by Hudson Robotics.

Contact us today to learn more about any lab automation tools you may be procuring for your lab!