Colony Morphology of Bacteria

By Acharya Tankeshwar •  Updated: 05/19/22 •  3 min read

Bacteria grow on solid media as colonies. A colony is defined as a visible mass of microorganisms originating from a single mother cell. Key features of these bacterial colonies serve as 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 on the basis of size, shape, texture, elevation, pigmentation, and effect on growth medium.

Find common criteria that are used to characterize bacterial growth;

Colony shape

It includes the 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 bacterial colony

Elevation of the bacterial colony: It gives information about, how much the colony rises above the agar. This describes the “side view” of a colony. These are the most common elevations; e.g. flat, raised, umbonate (having a knobby protuberance), crateriform, convex, pulvinate (cushion-shaped).

Margin of the 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, and large. Tiny colonies are also referred to as punctiform (pin-point). Colonies larger than about 5 mm are likely to be motile organisms.

Appearance of the colony surface

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

Bacterial colonies are frequently shiny and smooth in appearance. Other surface descriptions might be: dull (opposite of glistening), veined, rough, wrinkled (or shriveled), or glistening. Bacillus species give dry, wrinkled colonies. Pseudomonas stutzeri also gives similar-appearing wrinkled colonies.

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

The opacity of a bacterial colony can be described as transparent (clear), opaque (not transparent or clear), translucent (almost clear, but distorted vision–like looking through frosted glass), or iridescent (changing colors in reflected light). A pinpoint translucent β-hemolytic colonies on blood agar is most probably a Streptococcus species. Staphylococci give opaque, smooth, and circular colonies on the agar plate surface.

Some important terminologies

Draughtsman colonies

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

Young colonies of Streptococcus pneumoniae (pneumococci) have raised centers 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’.

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.