A bacterial colony is a visible mass of millions of genetically identical microorganisms that have grown from a single initial cell on a solid nutrient medium, such as an agar plate. The study of the physical form and structure of these masses is known as colony morphology. Analyzing morphology is a foundational practice in microbiology, providing the first clues to an organism’s identity before more sophisticated testing is performed. These characteristics are specific to the bacterial species, allowing scientists to rapidly differentiate between various types of microorganisms.
The Core Descriptors of Colony Morphology
Microbiologists rely on a standardized vocabulary to describe colony characteristics, starting with shape and size. Shape, or form, is viewed from the top and categorized as circular, irregular, or filamentous. Tiny, pinpoint colonies are described as punctiform. Size is measured in millimeters, but can also be described qualitatively as small, medium, or large.
Observing the colony’s edge, or margin, provides detail reflecting the bacteria’s growth pattern. A smooth, unbroken edge is described as entire, which is the most common form. A wavy edge is called undulate, a deeply lobed edge is lobate, and a fuzzy, root-like edge is called rhizoid or filamentous.
Elevation describes the height and profile of the colony as viewed from the side. A colony barely rising above the agar surface is considered flat. A raised colony is slightly higher with vertical sides. A dome-shaped colony is convex, and an umbonate colony has a raised center resembling a small knob.
The surface texture and color offer further distinguishing characteristics. Texture can be smooth, rough, or wrinkled. Some bacteria secrete extracellular material, making the colony appear mucoid or slimy. Color, or pigmentation, is a clear differentiator; some species produce distinct pigments, such as the orange-red color produced by Serratia marcescens or the greenish sheen from Pseudomonas aeruginosa.
How Morphology Aids Bacterial Identification
Colony morphology analysis serves as a preliminary screening tool in the laboratory, providing a rapid and cost-effective method for identification. Each bacterial species produces a consistent and characteristic colony appearance under standard growing conditions, acting as a visual fingerprint. This allows researchers to narrow down the potential identity of an unknown organism.
By assessing the combination of features—such as a circular, convex, opaque, and yellow colony—a microbiologist can often place the organism within a specific genus or species group. This initial assessment guides the selection of subsequent biochemical or genetic tests. For instance, a colony with a spreading, motile edge might immediately suggest a species of Proteus, directing the next phase of the investigation.
Morphology is also used to verify that a culture is pure, meaning the agar plate contains only one type of bacterium. The goal is to isolate colonies originating from a single cell, resulting in a genetically uniform population. If multiple distinct colony types are observed, it indicates a mixed culture, requiring re-isolation before proceeding with definitive identification tests.
External Factors That Change Colony Appearance
While colony morphology is genetically determined, its expression can be altered by environmental conditions. The composition of the culture medium is a major factor, as the availability of specific nutrients, such as sugars or proteins, influences a colony’s size, texture, and pigmentation. For example, some bacteria produce a distinct color on one type of agar but remain white or cream-colored on a medium with a different nutrient base.
Incubation temperature also plays a role in the final appearance of a colony, particularly affecting pigment production. Some bacteria synthesize their characteristic pigment only within a narrow temperature range. A colony grown at a warmer temperature may be colorless, while the same species grown cooler displays a bright color. Temperature changes can also affect metabolic pathways responsible for motility, influencing the margin and overall shape.
The length of the incubation period is another factor that causes colonies to change appearance as they age. Colonies increase in size over time, and their margins can become more irregular as the population expands and nutrient depletion occurs. These environmental influences highlight the need for standardized laboratory conditions when using morphology for bacterial characterization.