Microscopic organisms, such as bacteria and fungi, are generally too small to be seen individually without magnification. When grown in a laboratory setting, these microbes multiply rapidly, creating large, visible structures called microbial colonies. Studying the formation and appearance of these colonies provides foundational information for identifying and classifying microorganisms in biology and medicine. Observing these growth patterns is one of the first steps scientists take to understand a microbe’s characteristics.
Defining the Microbial Colony
A microbial colony is a visible aggregation of microorganisms that develops on the surface of a solid growth medium, such as an agar plate, following incubation. This mass is a population of cells that has multiplied to a density sufficient to be seen by the unaided eye. The entire colony originates from a single viable cell or a small cluster of cells, referred to as a Colony Forming Unit (CFU).
The concept of the CFU reflects the uncertainty about whether a colony started from one isolated cell or a small group of cells stuck together. Regardless of the exact starting number, the resulting colony is considered a clonal population because all cells within it are the direct descendants of the initial progenitor organism. This ensures that all cells in an isolated colony share the same genetic makeup, making it a pure culture suitable for further study.
The rapid appearance of a colony is due to exponential growth. Microorganisms like bacteria divide through binary fission, where one cell splits into two, creating a geometric progression. Under optimal conditions with sufficient nutrients, some bacteria can double their population size in as little as 20 minutes.
This relentless doubling causes the population size to increase logarithmically, quickly transforming an invisible starting cell into a mass containing millions of cells within 24 to 48 hours. The plate culture method allows for the formation of these isolated colonies. It is a standard laboratory procedure used to estimate the concentration of viable microorganisms in a liquid sample, typically expressed as CFU per milliliter.
Physical Traits for Identification
Once colonies appear on the growth medium, researchers characterize them by observing their physical traits, collectively known as colony morphology. This systematic visual assessment provides the first clues for preliminary identification, as the appearance of a colony is often species-specific. The general shape, or form, is a primary characteristic recorded, with common examples including perfectly circular, irregular, or filamentous (thread-like and spreading).
The characteristics of the colony’s edge, called the margin, are also noted. A smooth, unbroken edge is described as entire, while a wavy edge is called undulate, and a lobed edge has pronounced, finger-like projections. Tilting the plate allows for observation of the colony’s profile, known as the elevation.
Elevation profiles are described using terms such as flat, raised, convex (domed), or umbonate (having a distinctive raised center resembling a knob). The surface appearance and texture offer further distinguishing details. A colony may be smooth and glistening, dull and rough, or wrinkled; a slimy, mucus-like consistency is often termed mucoid.
Beyond shape and texture, the production of color, or pigmentation, is an important identifier for certain species. While many colonies are white or cream-colored, others produce distinct pigments, such as the red pigment of Serratia marcescens or the yellow pigment of some Staphylococcus species. These observable characteristics can be influenced by the type of nutrient medium, temperature, and incubation time used in the laboratory.
The Colony’s Natural State: Biofilms
While the laboratory colony provides a simple, isolated model for identification, microbial growth in the natural world is typically far more complex, existing as a community known as a biofilm. A biofilm is defined as a collection of microorganisms that adhere to a surface and are encased in a self-produced matrix. This structure represents the preferred, functional state for most bacteria in their natural environment.
The defining feature of a biofilm is the Extracellular Polymeric Substance (EPS), a slimy, protective matrix. This EPS is a polymeric conglomeration primarily composed of polysaccharides, proteins, and extracellular DNA, which binds the cells together and anchors the community to the surface. Biofilms are often described as “cities for microbes” due to their three-dimensional architecture, which includes internal channels that allow water, nutrients, and waste products to move through the structure.
Biofilms have significance in both health and industry because the EPS matrix provides substantial protection. Cells within a biofilm exhibit increased tolerance to antimicrobial agents and the host immune system compared to their free-floating, or planktonic, counterparts. In human health, biofilms are responsible for chronic infections, the contamination of medical devices, and the formation of dental plaque on teeth.
In industrial settings, biofilms cause damage and reduced efficiency in water distribution systems, heat exchangers, and pipelines through biocorrosion. They also pose a persistent contamination risk in food processing and manufacturing equipment, making their study a major focus for scientists aiming to develop new control strategies.