When microorganisms like bacteria and fungi are grown in a laboratory, a single microscopic cell on a solid surface like an agar plate will multiply into a visible cluster of genetically identical cells. This visible mass is called a colony, and the study of its physical appearance is known as colony morphology. This practice allows researchers to quickly gather information about a sample.
Describing a Microbial Colony
The process of describing a microbial colony involves observing several distinct visual features. One of the first characteristics noted is the size, which is typically measured in millimeters. Colonies are often categorized with terms like pinpoint (less than 0.5 mm), small, medium, or large. This measurement provides a basic but informative data point about the organism’s growth rate under specific lab conditions.
Another feature is the colony’s overall shape, referred to as its form. A colony might be perfectly circular, or it could be irregular with no defined shape. Some colonies display a filamentous (thread-like) or rhizoid (root-like) form, which indicates a different type of growth pattern across the surface.
The outer edge of the colony, or its margin, is also examined closely. A margin can be smooth and even, described as “entire,” or it can be wavy (undulate), lobed, or filamentous. The elevation describes how the colony is raised from the agar surface when viewed from the side. A colony can be flat, raised, convex (dome-shaped), or umbonate, which features a raised center.
Finally, scientists assess the colony’s surface texture and color. The texture might be smooth, rough, wrinkled, or mucoid, which has a slimy or sticky appearance. Color, or pigmentation, is also noted; while many bacteria are cream or white, some produce distinct pigments, appearing yellow, red, or other colors. Opacity is also noted, describing the colony as transparent, translucent, or opaque.
Factors That Shape a Colony
The appearance of a microbial colony is not random; it is determined by a combination of internal and external factors. The primary influence is the organism’s genetic makeup. A microbe’s DNA contains the instructions for cellular processes that dictate cell wall composition, pigment production, and the secretion of substances that determine texture and shape.
The environment in which the microbe grows also shapes its final appearance. The growth medium, the nutrient-rich substance like agar used for cultivation, has a direct impact. The type and quantity of available nutrients can affect a colony’s growth rate, size, and color, as some media are designed to enhance pigment production or differentiate between metabolic activities.
Temperature is a major factor, as different microbes have optimal temperature ranges for growth; temperatures outside this range can slow growth and alter colony size or color. The amount of available oxygen is another condition, as some microbes require oxygen to grow (aerobic) while others do not (anaerobic), which affects their development and colony characteristics. The length of incubation time also impacts the final look, as colonies change in size and appearance as they mature.
Practical Applications in Microbiology
Observing colony morphology is a skill in microbiology with several practical uses. Its primary application is in the presumptive identification of microorganisms. The distinct appearance of a colony provides a rapid, cost-effective first clue to a microbe’s identity, helping scientists narrow down the possibilities and decide which subsequent, more specific tests are needed for confirmation. For example, large, mucoid colonies on a specific type of agar might suggest the presence of Klebsiella pneumoniae.
This technique is also instrumental for assessing the purity of a culture. In many experimental and clinical settings, it is necessary to work with a pure culture, meaning one that contains only a single type of microbe. By streaking a sample onto an agar plate, a scientist can see if all the resulting colonies look the same. If multiple different types of colonies appear, it indicates that the culture is contaminated with unwanted organisms.
In various industries, particularly food production and pharmaceuticals, monitoring for microbial contaminants is a routine part of quality control. Technicians use colony morphology to screen for the presence of specific spoilage organisms or pathogens. However, colony morphology alone is rarely enough for a definitive identification and is almost always followed by more advanced methods like biochemical tests or genetic sequencing.
Distinguishing Between Bacteria and Fungi
While there can be exceptions, bacteria and fungi generally display distinct colony morphologies. Bacterial colonies are typically smaller, with a shinier or smoother surface texture. Their margins are often more defined and regular, commonly appearing circular with a buttery or mucoid consistency.
Fungi, on the other hand, present differently depending on whether they are molds or yeasts. Molds, which are filamentous fungi, tend to form large colonies that are fuzzy or cottony in appearance. These colonies are composed of thread-like filaments called hyphae that spread across the agar, often resulting in colonies with indistinct, filamentous margins.
Yeasts are unicellular fungi, and their colonies can sometimes be mistaken for bacteria because they are often moist, creamy, and circular. However, yeast colonies are generally larger than bacterial colonies and may have a characteristic “yeasty” odor.