The 3 types of bacteria (characteristics and morphology)

Beyond its scientific interest, some types of bacteria are of special concern to humans in a practical sense, either for health issues, to develop industrial processes or for their use as markers of the quality of the environment.

It is a type of microbe that is everywhere and takes a great variety of forms, in addition to make viable organisms, which often depend on them (for example, in the case of the digestive system human).

For this reason, there has always been a need to seek criteria to classify bacteria and to offer tools for their identification. In this article we will see what these main types of bacteria are, with summary explanations of each of them.

• You may be interested: "Major cell types of the human body”

What is a bacterium?

Microorganisms that are called bacteria can be found everywhere. The immense variety of species has allowed these life forms to colonize almost anywhere in the world. Nor is it necessary to emphasize that they have been and continue to be essential elements for today's life; for example, they are in charge of organic decomposition.

The domain of bacteria is made up of an immense group of living beings that are, in general, unicellular (made up of a single cell) and prokaryotes. A prokaryote is a type of cell that does not contain membranous organelles inside and its genetic content is found freely in it. These qualities are differentiated from the cells that form, for example, animals, since these are eukaryotic.

The general structure of bacteria consists of a cell membrane that separates its interior from the exterior. In addition to this, they also have a cell wall that surrounds the membrane, providing greater protection and stability to the bacteria. The sum of the two is known as the bacterial cell envelope, and there are differences in both composition and shape between different types of bacteria.

On the other hand, you also have to keep in mind that looking at size is not a good way to distinguish between bacteria and animals. There are microscopic animals, such as rotifers or tardigrades, which are made up of cells and even have a nervous system.

• You may be interested: "15 recommended medical books for the curious”

The main types of bacteria

Having criteria to be able to identify bacteria is a very useful tool for their study, even in some essential cases, such as in the identification of the cause of the infection in a disease human. Due to this importance, throughout the history of microbiology (a science that studies microorganisms, among them bacteria) a multitude of criteria have been generated to achieve a good classification of the cells prokaryotes.

There are many ways to classify the types of bacteria, such as according to their food source, according to their respiration, by the presence or absence of certain enzymatic activity (activity of a specific protein), or by its mobility. Moreover, for a correct identification it is convenient to combine different criteria.

One of the most classic and traditional criteria that exist when it comes to differentiating types of bacteria is to do so based on morphological characteristics. Although these are only based on the structure visible through the microscope, they have been very important in the taxonomy of bacteria; even many species of bacteria are named after their shape.

Mainly, these classification considers three fundamental forms:

1. Coconuts

This type of bacteria is characterized by having a spherical cell envelope. That is to say, when they are observed by the microscope they are circular cells. In this way they are easily identifiable, and it is easy to distinguish between them as individuals and the environment.

The subtypes that exist within this category are based on how cells are grouped, how they arrange themselves next to each other.

The solitary spherical bacteria are known as the coco shape. However, if instead of one they are two round cells united, then they are known as diplococci. There are more complex junctions that create a chain (streptococci) or irregular shapes that look like a bunch of grapes (staphylococci).

2. Bacilli

The main characteristic of this type of bacteria is that they have the shape of elongated rods.. As in cocci, the subtypes are based on how cells are grouped.

The solitary form is what is called as a bacillus. If two cells are joined together, then it is a diplobacillus. In the most massive unions they can be differentiated according to whether they join at the ends forming a chain (streptobacilli) or at the sides, forming a wall (palisade).

There is a form that is between the first two that has been seen; it is not as spherical as a coconut, but it is not as elongated as a bacillus. This is called coccobacillus.

3. Helical

In this last type of bacteria, different forms are grouped that present curvatures in their structure.. They can be understood as if they were bacilli that have twisted on themselves, reaching a helix shape. Thus, it is another of those types of bacteria that are easily recognizable using the microscope, due to their appearance.

They are mainly divided into two, rigid spirals (spirillae) or flexible spirals (spirochete). The difference is whether the spirals that draw its cell envelope remain the same or can change over time (the spiral moves).

Curiously there is another form that belongs to this type: the vibrio. This class of bacteria has a shape similar to a bean seed. Despite not drawing spirals, this type of bacteria is considered to be within this group, since the curvature of its cell envelope is representative of a genus of bacteria ("Vibrio") and are not temporary, as can occur in bacilli or coconuts.

Bibliographic references:

• Alberts et. to the. (2004). Molecular biology of the cell. Barcelona: Omega. ISBN 54-282-1351-8.
• Denamur, E.; Matic, I. (2006). Evolution of mutation rates in bacteria. Molecular Microbiology. 60 (4): pp. 820 - 27. Kojima, S .;
• Blair, D.F. (2004). The bacterial flagellar motor: structure and function of a complex molecular machine. International Review of Cytology. 233. pp. 93 - 134.
• Koch, A.L. (2002). Control of the bacterial cell cycle by cytoplasmic growth. Critical Reviews in Microbiology. 28 (1): pp. 61 77.
• Slonczewski, J.L.; Foster, J.W. (2013). Microbiology: an Evolving Science. New York: W W Norton