Classification of Bacteria The 16 Main Types

Philip Kelley

There are various classifications of bacteria and these vary depending on the criterion that is of interest to the researcher. Bacteria can be classified according to many criteria: according to their morphology, according to the characteristics of their cell walls, according to their tolerance to certain temperatures, according to their method of cellular respiration and according to their way of nourishing themselves, among many other classifications..

Bacteria are characterized by being formed by a single cell without a nucleus; This is why they are called prokaryotic unicellular organisms. These organisms are also characterized by having a firm cell membrane, which surrounds and protects them. Its reproduction is asexual, it occurs when the cells generate other identical cells and it is usually a very fast reproduction, if the conditions are favorable..


Bacteria are abundant on Earth. They exist in practically all environments and are very varied. This variability allows the existence of bacteria that can survive at high and low temperatures, in great depths of the sea, in the absence and abundance of oxygen, and in other characteristic environments of the planet..

There are bacteria that transmit diseases, but there are also those that help carry out certain processes that would be impossible to carry out were it not for these organisms. For example, bacteria participate in the digestion processes of some animals.

The five most relevant classifications of bacteria

1- Classification by cell wall

The specific characteristics of the cell walls of bacteria indicate differences between bacteria..

To determine these characteristics of cell walls, an experiment is carried out using a dye called Gram, in honor of its discoverer, Christian Gram. Through this technique two classes of bacteria emerge: gram positive and gram negative.

Gram positive

Corynebacterium diphtheriae laboratory culture. Taken and edited from: Copacopac [CC BY-SA 3.0 (].

They are those that maintain the coloration of the dye, even when said dye has been dissolved with alcohol. Cell walls, largely made up of a component called peptidoglycan, are thicker.

Gram negative

Pink Gram negative bacteria. Taken and edited from: Microrao [CC BY-SA 4.0 (].

They are those that do not retain the Gram dye after washing with alcohol. In this case, the levels of peptidoglycan are lower, so the cell walls are thinner.

2- Classification by shape

This classification has to do with the morphology of the bacteria. There are basically four types: bacilli, cocci, helical and coccobacilli.


They are bacteria that have an elongated, rod-like shape.

There is another classification that has to do with the number of bacilli combined. The structure that has two bacilli in a chain is known as diplobacillus.

If the structure has several bacilli connected at the ends in the form of a chain, it is called streptobacillus.

The conditions that bacilli can cause are linked to a decrease in blood pressure, meningitis, pneumonia, bronchitis, conjunctivitis, sinusitis, among others..


They are those bacteria whose shape is rounded. The classification made of the bacilli by the number of individuals in each structure also applies to cocci..

If the structure is made up of two cocci, it is called a diplococcus. The chain-shaped structures are called streptococci; and those that are irregular in shape are known as staph.

Cocci can cause throat infections, postoperative infections, endocarditis, toxic shock syndrome, and peritonitis, among other diseases..


These bacteria are spiral shaped, similar to a helix. When they are rigid they are called spirils; and when they are flexible they are called spirochetes. There is a third group called vibrio, which is characterized by not having a spiral shape, but a curved one..

Helical bacteria can cause syphilis, cholera, gastroenteritis, and leptospirosis, among other conditions.


Chlamydia trachomatis. Source: User Marcus007 on de.wikipedia [Public domain]

Coccobacilli are bacteria that are both elongated and rounded; are considered a midpoint between cocci and bacilli.

Coccobacilli can cause vaginal or uterine infections, endocarditis and respiratory infections, among other diseases.

3- Classification by power supply

Bacteria have different ways of absorbing the nutrients that feed them. According to this classification, there are two types of bacteria: autotrophic and heterotrophic.


They are those bacteria that can generate their own food. This production of food on their own can be done, for example, thanks to sunlight or to obtain carbon from the environment.


They are the bacteria that obtain the carbon dioxide necessary for their subsistence from organic compounds, among which proteins and carbohydrates stand out..

They are abundant in water and play a leading role in the decomposition of the elements.

4- Classification by cellular respiration

Depending on the way they breathe, four main types of bacteria can be found: aerobic, anaerobic, facultative and microaerophilic.


They are those bacteria that require oxygen for their development. Among aerobic bacteria, those responsible for generating tuberculosis, and those that generate lung or skin conditions stand out..


They are bacteria that do not need oxygen to survive: they can survive with little or no oxygen. They are abundant in the human intestine.


They are those bacteria that can develop and subsist both in the presence of oxygen and in its total absence; that is, they can be aerobic or anaerobic at the same time. They have very different shapes.


It refers to bacteria that can grow in spaces with very small amounts of oxygen, or very high tensions of carbon dioxide. They can cause stomach and intestinal diseases.

5- Classification by the temperature in which they grow

Some bacteria tolerate high temperatures, while others thrive in very cold environments. According to the temperature at which bacteria are capable of developing, four types are defined: psychrophilic, mesophilic, thermophilic and hyperthermophilic..


These bacteria thrive at low temperatures, from -10 ° C to about 20 ° C. They can cause stomach, intestinal or urinary diseases.


Mesophilic bacteria are characterized by growing in environments with a temperature similar to that of the body; that is, between 15 ° C and 40 ° C. Its most common habitats are human organisms and some animals.


They are those bacteria that develop at high temperatures, above 45 ° C, in marine environments.


They are bacteria that grow in extremely high temperatures, above 100 ° C. They tend to multiply quickly.


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