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Why are viruses considered microbes?

Why are viruses considered microbes?


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My question is simple. Why is a virus considered a microbe? Considering a microbe is considered to be a "living" unit of life, which viruses are not.


What is a microbe?

A microbe (or microorganism) is a microscopic organism. Anything that is considered alive and that is small enough is called a microbe.

Note that this definition has two issues.

  1. There is no universally accepted definition of life.
  2. There is no universally accepted size threshold for being called a microbe (to my knowledge). I would go with a threshold of about $10^{-5}$ meters.

Is a virus a microbe?

A virus IS and IS NOT alive depending on the definition. Note btw, that the definition of what is alive is not a matter of Biology but a matter of Philosophy. Most of the time, viruses are considered as not being alive. It is important to understand that the definition of life has absolutely no impact on biology and is nothing but a question of nomenclature.

If you want to call a virus a living thing, then yes, viruses are microbes. As stated on the wiki article:

Some microbiologists also classify viruses (and viroids) as microorganisms, but others consider these as nonliving.

You can find a discussion of why viruses are generally considered as not being alive here.

Unit of life

The concept of "unit of life" has not much meaning in biology and, to my experience, is most often used as a nice image for teaching young students what a cell is. Most of the time that I heard of "unit of life" (mostly when I was in secondary school and eventually high school) was used to describe a single cell making up a multicellular individual such as a cell of your blood for example and not a unicellular individual.


Viruses are not alive until they are inside the body of the host organism (plant or animal). Viruses are active once they are inside the host and reproduce inside the host. They are a link between living and non-living.


Viruses are small bits of genetic code in a protective covering. Viruses are not "alive," that is, they cannot replicate, unless they are inside another organism. A virus is definitely too small to be seen without a microscope. Since viruses are so small (tinier than bacteria) they may be considered microbes. However, since they are not "alive" outside of a host organism, it is debatable whether they are really organisms at all. For convenience since they are neglected in other areas of biology, viruses are discussed in microbiology. Most viruses are known because they cause disease.


What are microbes?

Created: October 6, 2010 Last Update: August 29, 2019 Next update: 2022.

Microbes are tiny living things that are found all around us and are too small to be seen by the naked eye. They live in water, soil, and in the air. The human body is home to millions of these microbes too, also called microorganisms.

Some microbes make us sick, others are important for our health. The most common types are bacteria, viruses and fungi. There are also microbes called protozoa. These are tiny living things that are responsible for diseases such as toxoplasmosis and malaria.


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Concept in Action

Figure 6. A molecule, like this large DNA molecule, is composed of atoms. (credit: “Brian0918″/Wikimedia Commons)

Some cells contain aggregates of macromolecules surrounded by membranes these are called organelles. Organelles are small structures that exist within cells and perform specialized functions. All living things are made of cells the cell itself is the smallest fundamental unit of structure and function in living organisms. (This requirement is why viruses are not considered living: they are not made of cells. To make new viruses, they have to invade and hijack a living cell only then can they obtain the materials they need to reproduce.) Some organisms consist of a single cell and others are multicellular. Cells are classified as prokaryotic or eukaryotic.Prokaryotes are single-celled organisms that lack organelles surrounded by a membrane and do not have nuclei surrounded by nuclear membranes in contrast, the cells of eukaryotes do have membrane-bound organelles and nuclei.

In most multicellular organisms, cells combine to make tissues, which are groups of similar cells carrying out the same function. Organs are collections of tissues grouped together based on a common function. Organs are present not only in animals but also in plants. An organ system is a higher level of organization that consists of functionally related organs. For example vertebrate animals have many organ systems, such as the circulatory system that transports blood throughout the body and to and from the lungs it includes organs such as the heart and blood vessels. Organisms are individual living entities. For example, each tree in a forest is an organism. Single-celled prokaryotes and single-celled eukaryotes are also considered organisms and are typically referred to as microorganisms.

Art Connection

Figure 7. From an atom to the entire Earth, biology examines all aspects of life. (credit “molecule”: modification of work by Jane Whitney credit “organelles”: modification of work by Louisa Howard credit “cells”: modification of work by Bruce Wetzel, Harry Schaefer, National Cancer Institute credit “tissue”: modification of work by “Kilbad”/Wikimedia Commons credit “organs”: modification of work by Mariana Ruiz Villareal, Joaquim Alves Gaspar credit “organisms”: modification of work by Peter Dutton credit “ecosystem”: modification of work by “gigi4791″/Flickr credit “biosphere”: modification of work by NASA)

Which of the following statements is false?

  1. Tissues exist within organs which exist within organ systems.
  2. Communities exist within populations which exist within ecosystems.
  3. Organelles exist within cells which exist within tissues.
  4. Communities exist within ecosystems which exist in the biosphere.

All the individuals of a species living within a specific area are collectively called a population. For example, a forest may include many white pine trees. All of these pine trees represent the population of white pine trees in this forest. Different populations may live in the same specific area. For example, the forest with the pine trees includes populations of flowering plants and also insects and microbial populations. A community is the set of populations inhabiting a particular area. For instance, all of the trees, flowers, insects, and other populations in a forest form the forest’s community. The forest itself is an ecosystem. An ecosystem consists of all the living things in a particular area together with the abiotic, or non-living, parts of that environment such as nitrogen in the soil or rainwater. At the highest level of organization (Figure 7), the biosphere is the collection of all ecosystems, and it represents the zones of life on Earth. It includes land, water, and portions of the atmosphere.


The population biology of bacterial viruses: why be temperate

A model of the interactions between populations of temperate and virulent bacteriophage with sensitive, lysogenic, and resistant bacteria is presented. In the analysis of the properties of this model, particular consideration is given to the conditions under which temperate bacteriophage can become established and will be maintained in bacterial populations. The effects of the presence of resistant bacteria and virulent phage on these "existence" conditions for temperate viruses are considered. It is demonstrated that under broad conditions temperate phage will be maintained in bacterial populations and will coexist with virulent phage. Extrapolating from this formal consideration of the population biology of temperate bacteriophage, a number of hypotheses for the conditions under which temperate, rather than virulent, modes of phage reproduction are to be anticipated and the nature of the selective pressures leading to the evolution and persistence of this "benign" type of bacterial virus are reviewed and critically evaluated. Two hypotheses for the "advantages of temperance" are championed: (1) As a consequence of the allelopathic effects of diffusing phage, in physically structured habitats, lysogenic colonies are able to sequester resources and, in that way, have an advantage when competing with sensitive nonlysogens. (2) Lysogeny is an adaptation for phage to maintain their populations in "hard times," when the host bacterial density oscillates below that necessary for phage to be maintained by lytic infection alone.


What is a Virus?

Because a virus is unable to reproduce without the help of the host cell, by a strict definition a virus cannot therefore be considered as a living organism. One of the required chapacteristics for a living organism being the capacity to reproduce. A virus is, however, capable of reproducing within the host cell by making use of the cellular processes of the host cell. Virtually all cells are susceptible to infection by viruses. Different types of viruses infect different host cells. Thus there are viruses that infect plants, others that infect bacterial cells (these viruses are called bacteriophage or phage), and viruses that infect humans and other mammals. Both eukaryotes and prokaryotes can be infected by viruses.

All viruses contain genetic material, either DNA or RNA, enclosed in a case made of protein. Viruses vary greatly in shape and size. Not all viruses carry disease. Antibiotics have no effect on viruses. Some anti-viral drugs are available that can be used to treat viral infections.

Viruses, bacteria, and amoebae are microorganisms. The study of microorganisms is called microbiology.

Recommended reading at Virology and current topics on microbiology research at the Microbiology Blog


Simmer Down: Viruses Not 'Fourth Domain' of Life

Biologists have categorized life into three large domains: Bacteria, Archaea (weird, bacteria-like microbes), and Eukarya (unicellular and multicellular organisms such as fungi, plants, and animals that possess nucleated cells). Under this classification system, viruses are left out in the cold. They certainly are not "alive" in the classical sense because they are not capable of metabolizing or replicating on their own. But it does not feel quite right to classify them as "inanimate," either, because they are built of biological molecules and contain genetic information. Thus, for the most part, viruses languish in the no man's land between the living and the dead.

The debate about how to classify viruses received a jolt with the discovery of extremely large viruses (such as Pandoravirus) that are so gigantic they can be seen with a light microscope and contain more genetic information than some bacteria. It has been proposed, due to some intriguing similarities in DNA sequences -- specifically, in the gene that encodes for an enzyme called RNA polymerase -- that such large viruses actually constitute a "fourth domain" of life. If that is the case, then perhaps all viruses should be awarded this new status.

"Sacrebleu!" say French scientists in a recent issue of Trends in Microbiology. Considering viruses to be a fourth domain would unnecessarily complicate evolutionary biology. For instance, the authors indicate that using RNA polymerase to redraw the tree of life presents a gigantic challenge. Large viruses do not all cluster into a single new domain. Instead, classifying life based on RNA polymerase would likely demand the creation of several new domains. (See figure.)

Such a phylogenetic tree is unwieldy, or as evolutionary biologists call it, "non-parsimonious." A foundation of building evolutionary trees is that they ought to be as simple as possible. This is referred to by scientists as the principle of parsimony but is more colloquially known as Occam's Razor. Essentially, a model that requires fewer assumptions (in this case, evolutionary changes) is superior to a model that requires more.

This is not the only problem with a viral fourth domain. The biggest difference between cells and viruses is their method of replication. All three domains of life replicate by cell division, which implies that this trait was derived from the Last Universal Common Ancestor (LUCA). (In other words, LUCA is the theoretical ancestor of Bacteria, Arcahea, and Eukarya.)

Viruses, which do not replicate by cell division, probably evolved independently multiple times, "here, there, and everywhere," as the authors conclude. Some probably evolved before LUCA, and others well after LUCA. Many have likely exchanged genetic material via horizontal gene transfer. Lumping them all into a fourth domain, therefore, makes little sense.

Though the debate over the classification of viruses may at first seem to be purely academic, it touches upon underlying questions that are of much greater significance: What exactly is life, and how did it evolve? The answer to those questions may be partially found within the enigmatic world of the viruses.

Source: Patrick Forterre, Mart Krupovic, and David Prangishvili. "Cellular domains and viral lineages." Trends in Microbiology, 22 (10): 554-558. October 2014.


Why aren't viruses considered alive?

I get that we have a list of criteria for something being alive (i.e. it has to grow, reproduce, evolve, respond to stimuli, etc.) and that viruses fail several of these criteria. Here is my issue though, isn't this list arbitrary? Like, why draw the line at a place that excludes viruses. They sure seem alive by many standards. They even have genetic material, reproduce, and evolve. It has been proposed before that our definitions of life could exclude life on other planets that evolved differently from Earth. Doesn't that mean we are just arbitrarily choosing to exclude some from Earth itself too? Is there a reason or a benefit to exclude viruses from the tree of life?

It's semantics, someone drew a line in the sand and here we are.

Definitions get fuzzy in biology sometimes. There isn't a great way to determine how different two organisms need to be to be two different species, and biologist will often fall into two different camps on how to do it (called lumpers and splitters).

Your genome is filled with sequences that are basically retroviruses that have lost the ability to escape the cell (called LTR Retrotransposons) but still replicate and move around your genome. It's even possible that some viruses evolved from transposons. If you look hard enough you can even find LTR Retrotransposons that still have parts of the genes they needed to burst out of a cell (called Endogenous Retroviruses). Where are you going to draw the line here? Is your genome filled with living organisms? Or has it been part of the genome so long (all eukaryotes have transposons to some extent) that it is just part of who you are?

Dichotomies are useful for discussing biology, but the reality is that things are rarely black and white.

This is really helpful. Thank you!

Where are you going to draw the line here? Is your genome filled with living organisms? Or has it been part of the genome so long (all eukaryotes have transposons to some extent) that it is just part of who you are?

Well, don't we agree that our mitochondria are a part of our genome? Is there a difference?

As a virologist, this question comes up quite a bit. To be ɺlive' a few things have to be true: (1) it must maintain homeostasis, (2) it must make its energy, ie ATP, (3) it must be able to grow/divide by its own machinery. Now by their very definition viruses do not meet any of these points and as such are not considered alive.

how to viruses make their energy?

By why are those the criteria we demand for life? We haven’t always used those as cutoffs have we? Why not a use broader definition?

To me it is very simple, a virus doesn't do anything on it's own. It will sit there forever unless it comes into contact with a living cell. Like a bear trap it is mechanically triggered. When it comes in contact with the right shaped protein on the surface of a cell it will inject its load of DNA or RNA. An infected cell is alive untill it is totally destroyed and then it is a sack of dead but deadly virus

All living things are able to reproduce on their own. Viruses reproduce by hijacking a living cell, and inserting their viral DNA so that the cell creates more viruses. Since a virus needs a host to reproduce, it is not considered alive.

But don’t other things need hosts too? Parasites can’t reproduce without the proper host. Is the different cell division alone?

All living things are able to reproduce on their own.

There isn't a single living thing that can reproduce on its own.

All species depend on a plethora of other species.

The dependency of a virus may seem more straightforward or feel "more dependent" than say the dependency of a parasite on its host or the cow on a human, but that doesn't make a useful defining differentiation.

Mainly because there has to be a point somewhere. There are even smaller structures than viruses that could be called alive - viroids for example. Like a virus at least has a cover of sorts, a viroid on the other hand is just a rogue chain of nucleic acids just wandering around and infecting cells. It does in theory procreate, but it doesn't do anything else, no chemical processes happen on their own in this one. Then there are prions, they are just proteins, that have the ability to turn normal proteins into themselves - make them misfold. That could count as reproduction, if you oversimplify what that is. Are these two alive? Well they are organic, they multiply, they either procreate or dissapear, so maybe yes?

But then when you think about it, some crystals act similarly, as to if they are put in the roght conditions, they grow, if a part splits off, it will start to collect chemically identical mollecules to it, but it's not really alive, but it's doing basically the same thing as a virus.

It's really just to make it easier to understand, because in general, the line is artificial and made for our own ease of mind, because it likes to group things. Everything is gradual in reality. But damn, this is more philosophical than empirical :D srr if this makes no sense


Why Does It Matter?

Suffice to say that the word &lsquomicrobe&rsquo has been used to draw some broad circles around multiple, divergent categories. This is particularly noticeable in journal articles: many studies that involve 16S rRNA gene sequencing make inferences about microbes or microbial communities when they are really referring to bacteria. Such titles can be misleading referring to bacteria as &lsquomicrobial communities&rsquo would be similar to studying human beings and calling it a study of animal communities. This terminology can also lead to confusion about different microbial groups for trainees who are new to the field.

This isn&rsquot merely a pedagogical debate: words have power, especially the power to shape public view of microbiology. As the COVID-19 pandemic ravages communities worldwide, the word 'microbe' conjures up images of spike proteins, masks and hand washing. Messaging about public health could be more effective if science communicators emphasized the differences between bacterial and viral infections, and didn&rsquot paint microbes as a broad, interchangeable category: antibiotics target bacterial pathogens and they are ineffective against viral infections. Viruses need to enter host cells to reproduce, while many bacterial pathogens can also mount infections by secreting toxic compounds that affect host metabolism. These facts influence treatment strategies for different types of infections.

Adding specificity to microbial conversations also allows for the introduction of nuance into the concept of a microbe, which can be especially helpful for introductory science classrooms. While &lsquomicrobe&rsquo is a broad umbrella term that is helpful to introduce beginners to the field, understanding the differences between microbial groups builds competency in identifying the roles of specific sub-groups in human and environmental health. The role of soil microbes in sequestering carbon is a hot topic in climate research, but scientists typically measure bacterial and fungal activity to understand soil carbon storage.

Definitions and names need to evolve with our ever-expanding knowledge, which is no easy task. &lsquoMicrobe&rsquo is a convenient and practical term to introduce novices to the multitudes of the microbial world, but professional microbiologists might want to ask themselves what they mean when they say &lsquomicrobe&rsquo: did they study the fungal community? Or the bacterial community? Or the phages that infect bacteria? In the microbial world, the devil is in the details.


Watch the video: Virenfallen im Alltag. Galileo (November 2022).