The 3 types of bacteria (characteristics and morphology)

Cocci, Bacilli, Helicoidal... we explain their main characteristics.

Beyond their scientific interest, some types of bacteria are of special interest to human beings in a practical sense, either for health in a practical sense, either for health issues, to develop industrial processes or for their use as markers of environmental quality.

It is a type of microbe that is everywhere and takes a wide variety of forms, as well as making organisms viable, which often depend on them (for example, in the case of the human digestive system).

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

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. It goes without saying that they have been and continue to be essential elements for life today; for example, they are in charge of organic decomposition.

The bacterial domain is composed of an immense group of living beings that are, in general, unicellular (consisting of a single cell) and prokaryotic. A prokaryote is a type of cell that does not contain membranous organelles in its interior and its genetic content is found in free form in it. These qualities differentiate them from the cells that form, for example, animals, since these are eukaryotes..

The general structure of bacteria consists of a cell membrane that separates their interior from the exterior. In addition to this, they also have a cell wall that surrounds the membrane, providing further protection and stability to the bacterium. 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, it should also be noted 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 composed of cells and even have a nervous system.

The main types of bacteria

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

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

One of the most classic and traditional criteria for differentiating types of bacteria is based on morphological characteristics. Although these are only based on the structure visible under the microscope, they have been very important in the taxonomy of bacteria; many bacterial species are even named after their shape.

Mainly, these classifications consider three fundamental forms:

1. Cocci

This type of bacteria is characterized by a spherical cell envelope.. That is, when observed under 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 the cells are grouped, the way in which they adopt an arrangement next to each other.

Solitary spherical bacteria are known as cocci. However, if instead of one, two round cells are joined together, then they are known as diplococci. There are more complex unions that originate 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 are elongated rod-shaped.. As with cocci, the subtypes are based on how the cells are grouped.

The solitary form is called a bacillus. If two cells are found joined together, then it is a diplobacillus. In the more multitudinous unions they can be differentiated according to whether they are joined at the tips 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 neither is it as elongated as a bacillus. It is called coccobacillus.

3. Helicoidal

In this last type of bacteria are grouped different forms that present curvatures in its 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 (spirochetes). The difference is whether the spirals that draw their cell envelope remain the same or can change over time (the spiral moves).

Interestingly, there is another form that belongs to this type: the vibrio. This class of bacteria has a silhouette resembling a pinto bean seed. Despite not drawing spirals, this type of bacteria is considered to be within this group, since the curvature of their cell envelope is representative of a genus of bacteria ("Vibrio") and they are not temporary, as may occur in bacilli or cocci.

Bibliographic references:

• Alberts et. al. (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. Nueva York: W W Norton