Colony Morphology of Bacteria; How to describe Bacterial Colonies?

Bacteria grow on solid media as colonies. A colony is defined as a visible mass of microorganisms all originating from a single mother cell. Key features of these bacterial colonies serve as an important criteria for their identification.

Characteristics of Bacterial colonies
Characteristics of Bacterial colonies

Colony morphology can sometimes be useful in bacterial identification. Colonies are described as to such properties as size, shape, texture, elevation, pigmentation, effect on growth medium. In this blog post you will find common criteria that are used to characterize the bacterial growth.

  1. Colony Shape: It includes form, elevation and margin of the bacterial colony.
    1. Form of the bacterial colony: – The form refers to the shape of the colony. These forms represent the most common colony shapes you are likely to encounter. e.g. Circular, Irregular, Filamentous, Rhizoid etc.Elevation of bacterial colony
    2. Elevation of bacterial colony: This describes the “side view” of a colony. These are the most common. e.g. Flat, raised, umbonate (having a knobby protuberance), Crateriform, Convex, Pulvinate (Cushion-shaped)
    3. Margin of bacterial colony: The margin or edge of a colony may be  an important characteristic in identifying an organisms.   Common examples are Entire (smooth), irregular, Undulate (wavy), Lobate, Curled, Filiform etc.
      Colonies that are irregular in shape and/or have irregular margins are likely to be motile organisms. Highly motile organism swarmed over the culture media. Such as Proteus spp.
    4. Size of the bacterial colony: The size of the colony can be a useful characteristic for identification. The diameter of a representative colony may be measured in millimeters or described in relative terms such as pin point, small, medium, large.  Colonies larger than about 5 mm are likely to be motile organisms.

Appearance of the colony surface:  Bacterial colonies are frequently shiny and smooth in appearance. Other surface descriptions might be: dull (opposite of glistening), veined, rough, wrinkled (or shriveled), glistening.

Mixed growth of mucoid Lactose fermenting colonies and NLF colonies in MacConkey Agar
Mixed growth of mucoid Lactose fermenting colonies and NLF colonies in MacConkey Agar

Consistency/Texture : Several terms that may be appropriate for describing the texture or consistency of bacterial growth  are: dry, moist, viscid (sticks to loop, hard to get off), brittle/friable (dry, breaks apart), mucoid (sticky, mucus-like)

Color of the colonies (pigmentation):  Some bacteria produce pigment when they grow in the medium e.g., green pigment produces by Pseudomonas aeruginosa,  buff colored colonies of Mycobacterium tuberculosis in L.J medium, red colored colonies of Serratia marcescens.

Opacity of the bacterial colony: Is the colony transparent (clear), opaque (not transparent or clear), translucent (almost clear, but distorted vision–like looking through frosted glass), iridescent (changing colors in reflected light).

Some important terminologies

Draughtsman colonies

Draughtmans colonies of S. pneumoniae
Draughtmans colonies of S. pneumoniae

Young colonies of Streptococcus pneumoniae (pneumococci) have raised center but as the culture ages, they become flattened, with a depressed central part and raised edges giving them a ringed appearance also known as ‘draughtsman colonies’.

3 thoughts on “Colony Morphology of Bacteria; How to describe Bacterial Colonies?

  1. Can you explain how or why certain microorganisms grow into a crateriform colony instead of let say a raised colony? What is the explanation behind each form, elevation and margin?

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