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What are microorganisms: classification, characteristics and types

In the living world there are beings of all kinds, of which those that are visible to the naked eye are well known, such as large and small animals, invertebrate animals, plant organisms or insects, but there are other organisms that are not visible. at first glance but that is good among us: microorganisms.

In this AgroCorrn article we will focus on the latter and, with simple explanations, we will find out what microorganisms are, their classification, characteristics and types .

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  1. What are microorganisms and their characteristics
  2. Classification of microorganisms
  3. Types of microorganisms

What are microorganisms and their characteristics

Microorganisms are small living beings whose study is responsible for microbiology . In this other AgroCorrn article we explain everything about Environmental Microbiology: definition and importance .

In fact, they are so small that they cannot be seen by the human eye and a microscope is necessary to view and study them. The structure of microorganisms is very simple and houses beneficial, pathogenic and other species that vary according to circumstances, but we will deal with these details later.

Within microorganisms there are unicellular and multicellular species, both prokaryotes (such as bacteria) and eukaryotes (such as protozoa or fungi). They also include ultramicroscopic beings , only visible with microscopes such as electronic ones, such as viruses.

Among the general characteristics of microorganisms we highlight:

  • Their diet and survival requires nutrients such as protein or carbohydrates.
  • For their development they need water, although the amount varies between microorganisms. Thus, for example, bacteria require less water than molds or fungi.
  • Microorganisms tend to have narrow ranges of temperatures in which they develop optimally and vary from one group to another, but the vast majority develop between 30 and 50 degrees.
  • Bacteria may require oxygen to survive (strict aerobes), its complete absence (anaerobes) or to tolerate oxygen (facultative anaerobes). Fungi and other eukaryotic microorganisms are usually aerobic.
  • As with temperature, tolerant ranges of pH are often narrow. In this sense, we differentiate acidophilic microorganisms (acidic environment), neutral or alkalophilic (alkaline environments).
  • The time they require for their development and growth depends on the type of microorganism and whether the conditions are more or less optimal. Thus, we differentiate slow-growing and fast-growing microorganisms.

Classification of microorganisms

We can classify microorganisms into four large groups:


They are unicellular microorganisms , that is, they have a single cell and unorganized genetic material within a nucleus. They reproduce by bipartition and are capable of donating and receiving genetic material through transformation, transduction or conjugation. There are several bacterial forms, which can be rounded (cocci), helix-shaped (spirilla) or small and curved (vibrios).

Find out more about them in this other AgroCorrn post about Bacteria, good allies of ecology .


These are acellular forms that generally consist of a protein capsid with or without an envelope and a DNA or RNA molecule (never both). They are obligate parasitic microorganisms, that is, they need to infect a cell (animal or plant) in order to reproduce and survive. Depending on the shape of their capsid, they can be icosahedral (more or less rounded) or cylindrical. There are also viruses with a more complex structure and that have a head and a tail. Viruses can kill the cell they infect or use it as a reservoir, waiting for the right conditions to be released.


They are unicellular or multicellular eukaryotic microorganisms . They reproduce by budding, that is, the parent emits a bump that grows until it is ready to separate. They can also reproduce by sporulation or breaking into fragments. They can be yeasts (unicellular) or fungi with hyphae (multicellular).


Parasites are protozoa such as amoebae and helminths and worms. These are eukaryotic organisms . They differ in that protozoa are unicellular and helminths and worms are multicellular. Parasites are not usually classified as microorganisms, but microbiology is often studied in conjunction with parasitology.

Types of microorganisms

There are other more complex classifications of microorganisms, which take into account the immunity and danger they pose to the body. Thus, these are the types of microorganisms according to this criterion:

Beneficial microorganisms

These are microorganisms that produce benefits for humans, either as part of the human microbiota or through the production of products of interest to humans such as cheese, beer, antibiotics, yogurt or bread. Although it may seem that microorganisms are bad, most of them are beneficial, and some bacteria and fungi are included in this group (except viruses).

The microbiota of the human being is essential for the survival and well-being of the organism. Thus, they perform functions such as protection against infections or contributing to digestion processes.

Pathogenic microorganisms

We include in this group those microorganisms that are capable of producing diseases in their hosts and can be viruses, bacteria or fungi. Its mechanism consists of infecting cells, reproducing in them and killing them or damaging them and causing disease. Bacteria are also capable of producing toxins that cause damage. They are fought with antimicrobials, which can be antibiotics for bacteria, antivirals for viruses, and antifungals or antifungals for fungi. The degree of pathogenicity of a microorganism depends on the immune status of the infected person.

If you are interested in the world of microorganisms and also that of ecology and the environment, you may like to read this other AgroCorrn article about Microorganisms for the production of biodiesel .

If you want to read more articles similar to What are microorganisms: classification, characteristics and types , we recommend that you enter our Biology category .

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