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 important criteria for their identification.
Colony morphology can sometimes be useful in bacterial identification. Colonies are described on the basis of size, shape, texture, elevation, pigmentation, and effect on growth medium.
In this blog post, you will find common criteria that are used to characterize the bacterial growth.
- Colony Shape: It includes form, elevation, and margin of the bacterial colony.
- 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 the 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)
- Margin of bacterial colony: The margin or edge of a colony may be an important characteristic in identifying 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 organisms swarmed over the culture media, such as Proteus spp.
- 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 pinpoint, 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.
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
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’.