A microbial colony is a macroscopic, visible cluster of millions of identical microorganisms, typically bacteria or fungi, that originated from a single progenitor cell on a solid growth medium, such as an agar plate. The physical appearance of these colonies, known as colony morphology, is a genetically determined characteristic used as the foundational first step in identifying and characterizing an unknown organism. Macroscopic observation acts as a preliminary screening tool, allowing microbiologists to quickly narrow down potential identities before moving on to more time-consuming microscopic, biochemical, or genetic analyses.
Interpreting Colony Shape and Structure
The geometric features of a single, well-isolated colony are described using standardized terms that reveal the organism’s inherent growth pattern. The overall two-dimensional outline, or Form, can be classified into distinct shapes, such as circular or irregular. Other forms include filamentous, characterized by a thread-like, tangled appearance, and rhizoid, which displays root-like, branched extensions spreading across the agar surface.
The Margin or edge describes the appearance of the colony’s perimeter. A smooth or entire margin indicates a regular, unbroken edge, while a lobate margin features distinct, rounded projections. Undulate margins are slightly wavy, and a filiform margin appears very delicate and thread-like.
The third structural dimension is the Elevation, which describes the colony’s profile when viewed from the side. Common elevations include flat, where the colony barely rises, and raised, which forms a low dome. A convex elevation is smoothly rounded and dome-shaped, while an umbonate elevation features a prominent, central bump, resembling a button.
Pigmentation, Texture, and Optical Properties
Beyond the physical structure, a colony image provides information on the microbe’s metabolic processes through its color and surface qualities. Pigmentation, or color, is often a result of specific metabolic pathways that produce colored compounds. For instance, the bacterium Serratia marcescens produces a deep red pigment called prodigiosin, while Pseudomonas aeruginosa can produce a characteristic green pigment that often diffuses into the surrounding agar medium.
The Texture or consistency of the colony surface provides insight into the composition of the microbial cell envelope and its secreted products. A mucoid or slimy texture often suggests the overproduction of an extracellular polysaccharide (EPS) layer, which can protect the organism. Other colonies may appear smooth and glistening, indicating a moist, buttery consistency (butyrous), or they may be dry and wrinkled (rugose) or rough.
The colony’s interaction with light determines its Optical Properties. Colonies are described as opaque if they completely block light, translucent if they allow some light to pass through but appear distorted, or transparent if light passes through clearly. A shiny or glistening appearance often correlates with a smooth, moist texture, while a dull or matte surface is typically seen in rough or dry colonies.
Analyzing Population Density and Purity
A colony image, especially of an entire agar plate, also reveals crucial information about the quantity of the microbe in the original sample and the Purity Assessment of the culture. A pure culture, which is necessary for accurate identification and study, will show only one type of colony morphology across the entire plate. The presence of multiple, distinct colony types—differing in color, shape, or texture—immediately signals a mixed culture or contamination, requiring a re-isolation step.
For Quantification, colonies are counted to estimate the concentration of viable organisms in the original sample, a value expressed as Colony Forming Units (CFU). This method relies on the assumption that each colony arose from a single viable cell or clump of cells. Samples are typically diluted serially before plating to ensure a countable plate, generally defined as having between 25 and 250 colonies. Scientists determine the CFU per milliliter or gram of the initial sample by multiplying the number of counted colonies by the dilution factor.
The overall Growth Distribution on the plate can also provide functional information, such as whether the organism is motile. Non-motile bacteria will remain confined to the area where they were initially deposited, leading to distinct, isolated colonies. Conversely, highly motile species may exhibit a swarming growth pattern, where a thin, uniform layer of cells spreads rapidly across the entire agar surface, demonstrating their capacity for coordinated movement.