Types of microbes


The history of life dates back to countless eons. Today, there are an impressive number of organisms on Earth, and more and more species are discovered each year.

The complexity of life, from the smallest microorganisms to the largest trees and animals, is astonishing. The most common family of organisms is that of microbes.

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Main types of microbes

There are several types of microbes, including bacteria, archaea, protozoa, fungi, algae, lichens, slimy molds, viruses, and prions. Most of these organisms can survive outside a host in air or soil, with the exception of viruses, which can only survive for a short time outside their host cells.


Bacteria are single-celled organisms with a much simpler cell structure than other organisms. A key difference between bacteria and other biological organisms is that they lack membrane-bound organelles and lack a nucleus.

The genetic material of bacteria is contained in a single loop of deoxyribonucleic acid (DNA). Notably, some bacteria have an additional circle of genetic material known as a plasmid. The plasmid is important for the bacteria that contain it because it contains genes that confer an advantage, such as antibiotic resistance, over other species.

Bacteria are classified into five types according to their shape. These include bacilli (rods), cocci (spherical), vibrios (comma), spirillae (spirals), and spirochetes (corkscrew.) Bacteria can exist as individual cells, paired, in chains or in clusters.

Bacteria can be found in all habitats on Earth, from soil and ocean to arctic snow. Bacteria also live inside the body, where they perform an essential function, as evidenced by the gastrointestinal microflora.

Bacteria also play an important role in several critical environmental processes such as the nitrogen cycle. While some bacteria are involved in food production processes, others are pathogenic and have caused epidemics and pandemics throughout human history.


Archaea are unicellular organisms which form the third domain of life. Although these organisms are evolutionary distinct from bacteria, they share several similarities with bacteria.

There are key biological differences between archaea and bacteria. These include a lack of peptidoglycan in the cell wall, as well as the presence of phytanyl instead of fatty acids on the cell membrane.

The cell membrane of bacteria is always a lipid bilayer, whereas in archaea it can sometimes be a monolayer. Archaea also contain distinctive translational ribonucleic acids (tRNAs) and ribosomal RNAs (rRNAs).

Archaea are obligate anaerobes that live in oxygen-poor environments such as water or soil. Some examples of archaea include Aeropyrum pernix, Ignisphaera aggregans, and Metallosphaera sedula.

Archaea can survive in some of the most inhospitable environments on Earth, including salt deposits, deep-water thermal vents, and hot springs. These are called extremophiles.


Protozoa are unicellular eukaryotic organisms belonging to the protist kingdom. These organisms are often considered to be more complex than bacteria and archaea.

The reproduction of protozoa is asexual and carried out by budding, fission or schizogony; however, some protozoa are capable of sexual reproduction. A key difference between protozoa and bacteria / archaea is the presence of a nucleus.

Protozoa are mobile and able to move by the movement of cilia, flagella or amoeboids. Amoeboid movement is achieved through the use of pseudopodia, which are temporary protrusions of the cell.

Protozoa can reside in a wide range of moist habitats such as soil, marine environments, and freshwater. Some examples of protozoan species include Amoeba and Paramecium.

Although very few protozoa cause disease, some are known to be parasitic in nature. Parasitic pathogenic protozoa include Plasmodium, which is the organism that causes malaria.

You Are Your Germs – Jessica Green and Karen Guillemin


The group of eukaryotic organisms known as fungi includes fungi, yeasts, and molds. These organisms can be unicellular or multicellular and their size can range from micro to macroscopic.

Mushrooms do not contain chlorophyll and must instead absorb nutrients from their surroundings. Among the fungi classified as microorganisms, yeasts are unicellular, while molds are multicellular and produce microscopic filamentous structures.

Some fungi are pathogenic, while others are beneficial and can be used for medicinal purposes or for fermentation. Notably, some lower mushrooms also belong to the Protista Kingdom.


Algae are eukaryotes which, like plants, use chlorophyll for photosynthesis and have rigid cell walls. Found in moist environments such as soil and aquatic environments, algae can be microscopic and unicellular or can be multicellular and large. In fact, some species of algae can reach a length of 400 feet.

Multicellular algae can come in various shapes and degrees of complexity. Some form colonies, which can be simple aggregates of cells or contain specialized cell types, much like higher life forms.

Mud molds

Slime molds are enigmatic, both taxonomically and biologically. These organisms are neither protozoa nor fungi, although they share characteristics with these two organisms at different stages of their life cycle.

Slime molds can also resemble protozoa during their growth stage, as they lack cell walls and exhibit amoeboid movement. Comparatively, during their stages of propagation, slimy molds form fruiting bodies and sporangia like typical fungi.

The two groups of slime molds are cell-free and cellular slime molds.


Lichens are symbiotic organisms made up of a photosynthetic microbe such as a cyanobacterium or an algae which is closely associated with a fungus. Lichen bacteria provide nutrients to the fungus, while the fungus provides protective cover to the microorganism, which is mutually beneficial for both.

The typical structure of a lichen is an upper layer of fungal mycelium, an intermediate layer in which the microbe lives, and a lower layer of mycelium. One of the ecologically important roles of lichen is its ability to turn rock into soil.


Viruses are one of the main pathogens that have been responsible for a huge number of different diseases in humans, plants and animals. These organisms have a much simpler structure than other organisms because they have no cells or organelles.

Viruses can only replicate inside a host cell. When not resident in the host, viruses exist as virions. Virions have a simple structure made up of genetic material, a protein envelope (capsid) and, in some cases, a lipid envelope. Virions are the most numerous biological entities on Earth and mutate much faster than bacteria.

The main difference between viruses and the other types of microbes discussed here, other than prions, is that scientists disagree on whether viruses are technically “alive”. This classification depends on the definition of life.

Although there is as yet no distinct phylogeny identified, viruses exhibit functions essential for life. Some can even form protective structures for survival outside the host.

let’s pray

Prions are even simpler than viruses and therefore much smaller. These organisms are obligate parasites that survive for up to 2 years in the environment.

Prions have no genetic material and are rather self-sustaining proteins. These organisms have been implicated as the cause of various diseases such as bovine spongiform encephalopathy (BSE) and Creutzfeldt-Jakob disease (CJD).


The study of microbes is an incredibly complex but important field of study. Bacteria contribute 13% of the total biomass of all organisms, just behind plants.

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