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Image of bacteria for article on colony morphology.
Published On: October 5th, 2021

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While microbial colonies have been observed on moldy food since the beginning of time, the invention of the petri dish in 1887 by microbiologist Julius Richard Petri pushed forward the study of bacterial colony morphology. However, what is the definition of colony morphology? Here we will answer that question as well as the question on how to identify different morphologies.

Concise Definition of Colony Morphology

Bacterial (or fungal) colony morphology refers to the visual appearance of colonies on an agar plate. Observing and describing morphologies of a bacterial colony are a core part of a microbiologist’s research process. Since there can be a diverse range of characteristics across different types of bacteria, practicing the observation of colony morphologies is often crucial in a microbiologist’s skill development or any researcher who usually handles bacteria in the lab. The variables in morphology include (but are not limited to) their size (diameter), separation from other colonies, ovality, color, halo-forming or not and the fuzziness or sharpness of the outline around the colony.

How To Identify And Describe Different Colony Morphologies

Now that it’s clear what the definition of colony morphology is, how does one then identify them? Colonies are distinguished by different sets of characteristics as detailed below. To identify a colony, a scientist simply has to match its appearance to pre-existing data or knowledge.

Colony Form

This refers to the overall shape of the colony, such as circular, irregular, filamentous, and so on. For example, Staphylococcus aureus often shows up as perfectly circular colonies on an agar plate.

Colony Elevation

Another way to describe colony morphology is, elevation is defined as the shape displayed by a colony when observed from the side. Generally, elevations are described as flat, raised, convex, pulvinate, umbonate, and crateriform.

Colony Edge/Margin

This refers to the structure of the colony on its edges that is exposed to the air. Some common forms that are often noted are entire, undulate, filiform, lobate, curled, scalloped, and serrated/erose. One type of colony that has a distinct filiform edge is the one formed by Bacillus anthracis. 

Colony Size

This is an important characteristic in fungal and bacterial colony morphology. This metric is typically described in millimeters. However, the size of a colony is also sometimes described in relative terms such as punctiform, small, medium, or large.

Colony Chromogenesis/Color

Color is a simple but precise way to describe colony morphology. A colony can display a single color or have different tones and hues from the middle to the outer part. It is important to note that the color seen on an agar plate may not be the actual color of the bacteria but the pigment that is produced due to the immersion of the bacteria in media. Nevertheless, when combined with other descriptive characteristics of a colony, the pigment produced can help with identification.  colony, can help with identification.

When noting down the color, it is worthwhile to also note down the opacity of an observed colony. Opacity is typically described as transparent, translucent, opaque, or iridescent.

Colony Surface and Consistency

Observing the surface and texture/consistency of the bacterial colony also helps with its identification; surfaces are described with terms like shiny, dull, smooth, rough, veined, glistening, wrinkled, and so on. On the other hand, consistency or texture is often described after touching or scraping at the colony. Some examples of texture descriptions include dry, brittle, mucoid, butyrous, and viscid.

Once a colony has been observed and described, a microbiologist can easily identify it using their prior knowledge or by using references. Identified colonies can then further undergo the colony picking process if they are to be duplicated.

Bacterial Colony Morphology: An Important Precursor

Colony morphology observations are an important pre-step in the identification and further use of cultivated colonies. Hopefully, this short piece on the definition of colony morphology and how to identify colonies has been enlightening. For an efficient identification and picking of colonies in the lab, contact Hudson Robotics to learn more about our colony picking robot solutions.