Colony and colony formation

A colony is population of a single type of microorganism that is growing on a solid or semi-solid surface. Bacteria , yeast , fungi , and molds are capable of forming colonies. Indeed, when a surface is available, these microbes prefer the colonial mode of growth rather than remaining in solution.

On a colonized solid surface, such as the various growth media used to culture microorganisms , each colony arises from a single microorganism. The cell that initially adheres to the surface divides to form a daughter cell. Both cells subsequently undergo another round of growth and division. This cycle is continually repeated. After sufficient time, the result is millions of cells piled up in close association with each other. This pile, now large enough to be easily visible to the unaided eye, represents a colony.

The appearance of a colony is governed by the characteristic of the organism that is the building block of that colony. For example, if a bacterium produces a color (the organism is described as being pigmented), then the colony can appear colored. Colonies can be smooth and glistening, rough and dry looking, have a smooth border or a border that resembles an undulating coastline, and can have filamentous appearance extensions sticking up into the air above the colony.

The visual appearance of a colony belies the biochemical complexities of the population within. For example, in a bacterial colony, the organisms buried in the colony and those near the more aged center of the colony are not as robustly growing as those bacteria at the periphery of the colony. Indeed, researchers have shown that the various phases of growth found when bacteria grow in a liquid growth medium in a flask (fast-growing and slower-growing bacteria, dying bacteria, and newly forming bacteria) all occur simultaneously in various regions of a colony. Put another way, within colonies, cells will have different phenotypes (structure) and genotypes (expression of genes).

Variations of phenotype and genotype have been elegantly demonstrated using a variant of Escherichia coli that differentially expresses a gene for the metabolism of a sugar called lactose depending on the growth rate of the bacteria. Growth on a specialized medium produces a blue color in those cells were the gene is active. Colonies of the variant will have blue-colored sectors and colorless sectors, corresponding to populations of bacteria that are either expressing the lactose-metabolizing gene or where the gene is silent.

The nature of the solid surface also affects the formation of a colony. For example, nutrients can diffuse deeper into a semi-solid growth medium than in a very stiff medium. Colonies of Bacillus subtilis bacteria tend to form more wavy, fern-like edges to their colonies in the semi-solid medium. This is because of uneven distribution of nutrients. Those bacteria in a relatively nutrient-rich zone will be able to grow faster, and often grow in the direction of the nutrient source. Even the shape of the bacteria changes from an oval to a longer form in these fast growing regions. The molecular basis of this shape transformation remains unresolved.

In another example of the influence of the surface on colony dynamics, the periphery of colonies grown on wet surfaces contains very motile (moveable) bacteria. Their motion is constrained by the high number of bacteria. The results is the formation of so-called "whirls and jets" that form, disappear and re-form. These motions, which appear under the light microscope to be very random and chaotic, are in fact very highly organized and helps drive the further formation of the colony.

Another phenomenon of colony formation is the communication between constituent cells. This is also known as "cross-talk." Cells of the amoeba Dictyostelium discoideum, for example, can actually signal one another when growing in a colony, especially in nutrient-poor environments. Cells that encounter nutrients emit a compound called cyclic adenosine monophosphate (cAMP). The subsequent growth of cells is in the direction of the increasing cAMP concentration. Visually, a spiraling pattern of growth results. Mounds of amoebas also form. The microbes at the top of the mounds produce spores that can become dispersed by air movement, allowing the colonization and new colony formation of other surfaces.

Chemical signalling within a colony has also been demonstrated in yeast, such as Candida mogii and in bacteria, such as Escherichia coli.

See also Agar and agarose; Biofilm formation and dynamic behavior