A colony is a visible mass of microorganisms that arises from the growth of a single cell or a small group of cells on a solid medium. Colonies are clonal, so their members share the same genetic makeup apart from occasional mutations.
Explanation
On agar plates microorganisms replicate by cell division to form clusters of millions of cells, creating colonies. Colony morphology – size, shape, margin, elevation, surface texture, opacity and colour – helps differentiate species. For example, pigmented bacteria such as Staphylococcus aureus form golden convex colonies; Pseudomonas aeruginosa produces blue‑green colonies from pigments. Fungal colonies often appear fuzzy or powdery. Colony appearance depends on growth rate, motility and production of extracellular substances.
In the laboratory, colonies are counted to estimate the number of viable cells in a sample by reporting colony‑forming units. Pure cultures are obtained by streaking to separate single cells so that discrete colonies develop. Observing colony characteristics helps identify and isolate microorganisms.
Colony morphology and examples
Colony features provide clues to identity. Streptococcus species produce small translucent colonies, whereas Bacillus species produce large spreading colonies with irregular edges. Yeasts such as Saccharomyces cerevisiae produce moist cream‑coloured colonies. Moulds such as Penicillium produce green fuzzy colonies with a white margin. On selective media, lactose fermenters like Escherichia coli produce dark colonies with a metallic sheen on eosin methylene blue agar, while non‑fermenters remain colourless. Recognising these patterns helps differentiate organisms in clinical and environmental samples.
Most microbiological counts are reported as colony‑forming units per millilitre or gram, acknowledging that each colony originates from one viable cell or a clump of cells. This concept is important for food safety testing, water quality assessment and evaluation of antimicrobial agents.
Observation of colonies builds experience for identifying organisms based on growth patterns. Colony morphology on its own cannot definitively identify species but it provides a useful starting point for further biochemical and molecular tests.
Related Terms: colony morphology, CFU, pure culture, streak plate, agar