The history of life stretches back untold eons. Today, there is a staggering array of organisms on Earth, with more species being discovered every year.
The complexity of life, from the smallest microorganisms to the largest trees and animals, is astonishing. The most widespread family of organisms is the microbes.
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Major types of microbes
There are several types of microbes, which include bacteria, archaea, protozoa, fungi, algae, lichens, slime molds, viruses, and prions. Most of these organisms can survive outside of a host in the air or soil, with the exception of viruses, which can only survive for a brief time outside their host cells.
Bacteria are unicellular organisms with a much simpler cell structure than other organisms. A key difference between bacteria and other biological organisms is that they have no membrane-bound organelles and a lack of nucleus.
The genetic material of bacteria is contained in a single loop of deoxyribonucleic acid (DNA). Notably, some bacteria have an extra circle of genetic material known as the plasmid. The plasmid is important for the bacteria that contain it, as it contains genes that confer an advantage, such as antibiotic resistance, over other species.
Bacteria are classified into five types depending on their shape. These include bacilli (rod), cocci (spherical), vibrios (comma), spirilla (spiral), and spirochaetes (corkscrew.) Bacteria can exist as single cells, paired, in chains, or in clusters.
Bacteria can be found in every habitat on Earth, from soil and the ocean to arctic snow. Bacteria also live inside the body, where they provide an essential function, as is evident by the gastrointestinal microflora.
Bacteria also play an important role in several critical environmental processes such as the nitrogen cycle. Whereas some bacteria are involved in food production processes, others are pathogenic and have caused epidemics and pandemics over the course of human history.
Archaea are single-celled organisms that form the third domain of life. While these organisms are evolutionarily distinct from bacteria, they share several similarities to bacteria.
There are some 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 translation ribonucleic acids (tRNAs) and ribosomal RNAs (rRNAs).
Archaea are obligate anaerobes that live in low-oxygen 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-sea thermal vents, and hot springs. These are known as extremophiles.
Protozoa are single-cell eukaryotic organisms that belong to the Kingdom Protista. These organisms are often considered to be more complex than bacteria and archaea.
The reproduction of protozoa is asexual and achieved 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 motile and capable of moving by cilia, flagella, or amoeboid movement. 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 disease-causing protozoa include Plasmodium, which is the organism that causes malaria.
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The group of eukaryotic organisms known as fungi includes mushrooms, yeast, and molds. These organisms can be either unicellular or multicellular and can range from micro- to macroscopic in size.
Fungi do not contain chlorophyll and must instead absorb nutrients from their surrounding environment. Of the fungi that are classified as microorganisms, yeasts are unicellular, whereas molds are multicellular and produce microscopic filamentous structures.
Some fungi are pathogenic, whereas others are beneficial and can be used for medical or fermentation purposes. Notably, some lower fungi also belong to the Kingdom Protista.
Algae are eukaryotes that, like plants, use chlorophyll to photosynthesize and have rigid cell walls. Occurring in moist environments like soil and aquatic environments, algae may be microscopic and unicellular or can be multicellular and large. In fact, some algae species may grow up to 400 feet in length.
Multicellular algae can occur in a variety of forms and degrees of complexity. Some form colonies, which can be simple aggregates of cells or contain specialized cell types, much like higher forms of life.
Slime molds are enigmatic, both taxonomically and biologically. These organisms are neither protozoa nor fungi, although they share features with both of these organisms at various stages in their life cycle.
Slime molds can also be protozoan-like during their growth stage, as they lack cell walls and exhibit amoeboid movement. Comparatively, during their propagation stages, slime molds form fruiting bodies and sporangia like typical fungi.
The two groups of slime molds are the acellular and cellular slime molds.
Lichens are symbiotic organisms that consist of a photosynthetic microbe like a cyanobacterium or algae that is intimately associated with a fungus. The bacteria of lichen provide nutrients for the fungus, whereas the fungus provides protective cover for the microorganism, which proves mutually beneficial for both.
The typical structure of a lichen is a top layer of fungal mycelium, a middle wherein the microbe lives, and a bottom 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 pathogenic agents that have been responsible for an enormous number of different diseases in humans, plants, and animals. These organisms are much simpler in structure as compared to other organisms, as they have no cells or organelles.
Viruses can only replicate within a host cell. When not residing within the host, viruses exist as virions. Virions have a simple structure that consists of genetic material, a protein coat (capsid), and, in some cases, a lipid envelope. Virions are the most numerous biological entities on Earth and mutate much more rapidly than bacteria.
The key difference between viruses and the other types of microbes discussed here, aside from prions, is that scientists disagree as to whether viruses are technically “alive.” This classification depends on one's definition of life.
Whilst there is no distinct phylogeny yet identified, viruses exhibit functions that are essential for life. Some can even form protective structures for survival outside the host.
Prions are even simpler than viruses and thus much smaller. These organisms are obligate parasites that have been found to survive for up to 2 years in the environment.
Prions possess no genetic material and are instead self-perpetuating 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 to 13% of the total biomass of all organisms, only second to plants.