The bacterial colonial morphology are those descriptive characteristics that help microbiologists to determine and complete the "profile" of a cultivable bacterial species. It must be taken into account that many types of bacteria in an agarized medium can be easily distinguished by the characteristics of their cellular aggregates in the form of colonies..
This attribute of bacterial colonies is easily visible in solid culture media, whether they have been "seeded" or inoculated with pure cultures (a single isolated species) or with mixed cultures (a mixture of unknown species), in which case many times they are used as a character for taxonomic identification.
The morphology of a bacterial colony is highly variable, both from the macroscopic and microscopic point of view, a fact that is demonstrated from the observation of colonies by means of scanning electron microscopy, where amazing details of their ultrastructure can be appreciated..
Since both bacteria and other microorganisms are capable of growing on solid surfaces in the form of colonies, knowledge of the characteristics of this type of growth is very important for those who study microbes in their natural environment and their relationships with the environment (“ microbial ecologists ”).
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Most of the bacterial species that are grown in a laboratory and found in natural environments have the ability to grow in both liquid and solid media..
Growth in liquid media is usually "tracked" experimentally through measurements of the optical density of the culture over time..
This process consists of inoculating a sterile nutrient medium with the bacterial species of interest and monitoring the increase in “turbidity” over time, which is determined as an increase in optical density, which is measured with an electronic device called a spectrophotometer..
Once it is evident that the optical density values at a certain wavelength do not increase any more, the researcher usually graphs the values obtained as a function of time and obtains what is known as a bacterial growth curve..
In the curves thus obtained, a regular behavior is easily discernible (because it occurs in practically all the species of bacteria analyzed), since four well-defined phases are observed:
- A phase "lag”Or delay.
- A logarithmic or exponential phase (spike).
- a stationary phase (the set of the curve).
- a death phase (decrease in optical density).
Bacterial growth in a solid medium is somewhat different than in a liquid medium, since the cells are not dispersed in a moving fluid, but rather aggregate, forming well-defined colonies..
Normally, the growth in solid medium is faster towards the extremes of the colony or, in other words, the cells that divide more actively are in the periphery, while those in the central region are more “old”, they are inactive and undergo processes of autolysis (death).
Some authors attribute these growth differences in the colonies to the existence of gradients of oxygen, nutrients and even toxic products produced by bacteria inside the colonies, stating that towards the extremes there are higher concentrations of nutrients and oxygen than toward the center.
In view of the fact that the borders of the colonies are less thick than the central portion, oxygen and nutritive material diffuse more easily in these areas than in the center, where, on the contrary, the diffusion processes are so slow that they prevent efficient cell division.
It is also important to comment that the definition of a given morphological pattern in a bacterial colony is a highly controlled process, not only metabolically, but also in relation to gene expression, intercellular communication processes, etc..
In addition, the morphology of a colony depends on numerous environmental factors such as the composition of the environment, temperature, percentage of humidity, among others..
The morphology of a bacterial colony can be analyzed from a macroscopic perspective (with the naked eye) or microscopic (using observation instruments such as microscopes).
From the macroscopic point of view, the morphology of the bacterial colonies can be analyzed according to the characteristics of the general shape, the elevation and the margins or edges..
The appreciation of the general shape and the characteristics of the margins or edges is achieved by looking at the colonies from the bottom up (when these are grown in a Petri dish, under controlled conditions); while the type of elevation is distinguished by looking at the colony in profile or from the side keeping the plate at eye level.
In this case, the bacterial colonies can be:
- Punctiform: those that grow as small aggregates of points close to each other.
- Circular: they are very uniform colonies, completely round.
- Filamentous: colonies that grow as filaments projecting from a central region or nucleus.
- Irregular: those colonies that do not have defined shapes and that are rather amorphous.
- Rhizoids: as the name implies, these colonies grow similar to the roots of a plant.
- Fusiform: those colonies that have an elongated shape, as if it were an ellipse whose edges have been stretched longitudinally.
Colonies can present different types of margins or borders, among which are:
- Whole
- Curly
- Lobulated
- Eroded
- Filamentous
- Curly (those that look like tree rings).
Finally, depending on the elevation of these bacterial cell aggregates on a solid medium, the colonies can be:
- Flat: those with little or no elevation.
- Elevated: they project slightly on the surface, but do so in a regular way, that is, the elevation is uniform throughout the diameter of the colony.
- Convex: those that rise more noticeably in the center, but whose margins remain rather attached to the surface.
- Pulvinate: those that resemble a “dome” that protrudes prominently from the surface.
- Umbonadas: those colonies that have raised edges but are characterized by “projecting” a greater mass of cells towards the center, acquiring a shape similar to a breast (“mamiliform”).
In addition to the characteristics mentioned, the bacterial colonies can also have different textures that can be seen with the naked eye, so that the colonies have been defined.
- Soft and shiny
- Rough
- Wrinkled
- Dry or dusty in appearance.
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