Bacteria grow as colonies on solid media. A colony is a visible mas of microorganism that originated from a single mother cell. Hence, a colony of bacteria is a clone of genetically alike bacteria.
There are various types of bacteria and each type produces differently looking colonies. They vary in color, share, pigmentation, and other characteristics.
Colony morphology is a way of identifying bacteria. By observing the colony of bacteria, the identity of bacteria will be determined. Basic elements you can identify in a colony morphology include the following:
Image 1: The image shows the colony morphology of bacteria.
Picture Source: microbeonline.com
Form – it pertains to the shape of the colony such as filamentous, circular, and the likes. The form of the colony is subdivided into following:
- Size of the colony – The size of the colony is determined by its diameter and measured in millimeters. The terms used to describe the size of the colony are pinpoint, small, medium, and large. If the size of the colony of the organism is larger than 5 mm, it is an indicator that the organism is motile.
- Surface – it pertains to the appearance of the colony’s surface such as rough, smooth, glistening, dull, or wrinkled.
- Color – Some bacteria produce pigment when grown in a medium. A perfect example is Pseudomonas aeruginosa which produces a green pigment and Mycobacterium tuberculosis which produces a buff colored colony. On the other hand, Serratia marcescens produces a red colony. (1, 2, 3, and 4)
Image 2: Elevation of the bacterial colony.
Picture Source: microbeonline.com
Elevation – it pertains to the cross-sectional shape of the colony or side view of the colony such as:
- Flat
- Raised margin
- Raised
- Umbonate
- Crateriform
- Convex (4, 5)
Image 3: Margin of a bacterial colony.
Picture Source: slidesharecdn.com
Margin – It pertains to the magnified shape of the colony’s edge which includes:
- Entire – smooth
- Filamentous – filament-like
- Undulate – wavy
- Lobate – lobe-like structure
- Scalloped – scallop-like
- Curled – concentric (5, 6)
Image 4: Opacity of bacterial colony.
Picture Source: slidesharecdn.com
Opacity – it pertains to the characteristics of the colony such as:
- Translucent – clear
- Opaque – not clear
- Iridescent – shiny (7)
Image 5: Images of the bacterial colony with a varying degree of pigmentation.
Picture Source: bacteriainphotos.com
- Chromogenesis/pigmentation – Examples are white, red, buff, and purple, to name a few.
Image 6: The image shows the consistency and texture of bacteria.
Picture Source: microbiologyinpictures.com
Consistency/texture – The texture and consistency of the organism can be described as:
- Moist
- Dry
- Viscid – stick to loop
- Mucoid- mucus-like
Image 7: The different shapes of bacteria and the corresponding bacteria species.
Picture Source: ytimg.com
Shape – A bacterial colony can manifest in various shapes which include but not limited to the following:
- Coccus/cocci – Bacterial cells that are spherical in shape and resemble a tiny ball. A perfect example is the Streptococcus aureus.
- Bacillus/bacilli – They are rod-shaped bacteria which resemble a pill. A perfect example is Bacillus anthracis.
Spiral – Bacteria that are twisted or helical in shape which looks like little corkscrews. (4, 6, 7, and 8)
Image 8: The image shows the arrangement of cells, specifically coccus/cocci.
Picture Source: wikimedia.org
Arrangement of cells – Bacterial cells can be arranged in the following orders:
- Singly – single cell
- Diplococcic – arranged in pairs and are attached to each other.
- long chains and attached to each other.
- Staphylococcus – irregularly arranged in clusters/grape-like (5, 7)
Image 9: The image shows the typical characteristics of the bacterial colony on agar slants.
Picture Source: ruf.rice.edu
Characteristics of the bacterial colony on agar slants
- Filiform – straight
- Diffuse – spreading
- Echinulate
- Arborescent – tree-like
- Beaded
- Rhizoid (10)
Image 10: Growth of bacteria colony in nutrient broth.
Picture Source: slidesharecdn.com
Characteristic of bacteria in nutrient broth
- Clear – no growth
- Turbid – cloudy
- Flocculent
- Pellicle
- Sediment (11)
Microorganisms that can grow on a nutrient agar plate
Microorganisms such as bacteria grow on solid media in the form of a colony. What is a colony? It is a visible mass of microorganism that originates from a single mother cell.
Colony morphology of an organism is necessary for its identification. Various organisms can grow on solid media. They include the following:
- Bacteria – A colony of bacteria usually appear in colors white, cream, and yellow. As with the shape, bacteria colony is usually fairly circular.
Image 11: A colony of yeast on the agar plate.
Picture Source: eurekabrewing.files.wordpress.com
Yeasts – The colony of yeast, which is a type of fungi, is somewhat similar to that of the colony of bacteria. They can grow as a white patch with a glossy surface.
Image 12: A petri dish containing mold growth.
Picture Source: inspectapedia.com
- Mold – It is a type of fungi that usually appears whitish grey with fuzzy edges. Mold usually turns into a different color. (1, 4, 7, and 9)
Image 13: A colony morphology of bacteria on MacConkey agar.
Picture Source: microbiologyinfo.com
Factors affecting the colony morphology of bacteria
- Type of media – The cultural characteristics of bacteria can be affected by the type of media and the nutrient it contains. Keep in mind that some types of media are more nutritive than others. The more nutritive the media is the more it encourages hearty growth. On the other hand, some types of media can restrict growth.
- Temperature – Some bacteria grow more rapidly at body temperature while others are weak at room temperature. More so, some bacteria form pigment under favorable temperature.
- Length of time – The incubation time/period may affect the colonial characteristics and colonial size of bacteria.
- Presence of other organisms – The growth of bacteria can be affected by the presence of other organisms. (2, 4, 7, and 11)
References
- https://bio.libretexts.org
- https://microbiologyonline.org/teachers/observing-microbes/observing-bacteria-in-a-petri-dish
- https://www.sciencebuddies.org/science-fair-projects/references/interpreting-agar-plates
- https://microbeonline.com/colony-morphology-bacteria-describe-bacterial-colonies/
- https://www.scienceprofonline.com/microbiology/bacterial-colony-morphology-identification-unknown-bacteria.html
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4671912/
- https://www.ruf.rice.edu/~bioslabs/BIOC318/morphology.asp
- https://www.researchgate.net/figure/The-bacterial-identification-a-colony-morphology-in-agar-plate-b-cell-morphology_fig23_303029363
- https://guides.baker.edu/c.php?g=303096&p=2022446
- https://www.slideshare.net/HiwrHastear/culture-characteristic-of-bacteria
- https://www.sciencedirect.com/topics/medicine-and-dentistry/bacterium-colony
