The short answer is that oxygen is toxic to some organisms, but many organisms cannot live without it. Others don’t need it, but can live with it. Here’s the longer story:

Almost all of us animals are aerobes, meaning that we need oxygen (“aero” = oxygen). When we don’t have enough, we can still get some energy out of our food. We make lactic acid (think sore muscles) as a byproduct when we do this. But it’s not very efficient. We only get a fraction of the energy we could harvest from food if we have oxygen. (Have you studied ATP? Aerobic respiration gives us about 34 ATP per glucose molecule. Aerobic respiration gives us 2 ATP per unit of glucose). In a few minutes without oxygen, we pass out and die.

Not all living things need oxygen. Anaerobic (“no oxygen”) bacteria live in places like deep in mud, in deep water, and even inside our own intestines, where they help us break down our food at the chemical level.

Organisms that are harmed by oxygen are called obligate anaerobes. “Obligate” comes from the same word as obligation, meaning there’s no choice. They vary in how much oxygen it takes to kill them. The air around us is about 20% oxygen. Some can survive if the oxygen level is less than that. A group of well-known obligate anaerobes is Clostridium, the bacteria that cause paralyzing food poising in canned goods. If the canning temperature is not hot enough to kill them, they thrive in the cans, multiplying and making toxins.

Organisms that can tolerate oxygen, but don’t use it are called “aerotolerant.” The bacteria in our mouths that cause tooth decay, Streptococcus mutans, fall into this group. This makes sense. Obviously, your mouth is full of oxygen if it’s open, but leaving your mouth open won’t stop cavities. When you’re breathing through your nose with your mouth shut, oxygen levels might get pretty low, but that won’t kill them either.

“Facultative aerobes” can use oxygen if it’s around, but they don’t need it. Yeast are a good example of this. If they have oxygen, they take it in and give off carbon dioxide. If you trap them in dough, or containers that air can’t enter, they do fermentation, making alcohol and carbon dioxide. This is the key to making bread rise or making wine and beer. The bacteria that make yogurt (Lactobacillus) are also facultative anaerobes. They make the lactic acid that gives yogurt its sour taste.

Kombucha is a drink that has been around for centuries, but is becoming very popular in the US. Both fungi and bacteria are used to make it. I have never even tried it, but I looked up how it’s made. People cover the jar with the ingredients, fungi, and bacteria in open containers with just a cloth on top, so air can enter. After a while, a sort of “skin” of the bacteria and fungi cover the liquid, keeping the oxygen out. So, do you think the organisms involved in making kombucha are obligate anaerobes, aerotolerant, or facultative anaerobes? Think about it before you look it up.

I saw your question about the effect of oxygen on bacteria, viruses and fungi. I am really glad you asked this question because it's a great example to illustrate that biology is more complex than most people appreciate. You've picked up on that and are asking the right question!

In essence, not all bacteria (or fungi) are the same.

There are quite a few species of bacteria (and some fungi) that have evolved to be "anaerobic" - that is, they grow best in low or even no oxygen environments. Some of them are considered "obligate" anaerobes - oxygen actually poisons them and they die. Others are "facultative" and can grow with or without oxygen. Given a choice, they will use oxygen and do aerobic respiration - this type of metabolism generates more ATP than anaerobic respiration. It's a bit cumbersome to go into detail here on these aspects of respiration and metabolism, but you can investigate more on your own in any Introductory level college Biology textbook and also on line (the Wiki site for "obligate anaerobes" is well done).

Some interesting anaerobic bacteria from a human health standpoint include Prevotella, Actinomyces, Bacteroides, Clostridium, Fusobacterium, Peptostreptococcus. (There are more).

If you had a wound that was infected with bacteria that were anaerobic, oxygen might actually help. You may have heard about wound debriding and oxygen treatment for helping wounds heal, and this is part of that (in addition to just providing the growing human cells with "easy" oxygen so they can heal).

Viruses are another matter - the virus itself does not have the ability to carry out metabolism on its own - it hijacks the host cell for its energy and biosynthesis needs. Now, if providing oxygen to the host cells helps them fight off the virus, then oxygenation would be considered "bad" for the virus. The ways in which mammalian cells use and scavenge different forms of oxygen are many and relates to all of this as well.

You may have heard the expression “the dose makes the poison” before, and that is exactly what is happening here! The exact mechanism would be unique to each type of bacteria or fungi, but essentially too much oxygen poisons the organism since its metabolic processes can only handle a certain amount. Sort of how if you eat too much candy you might start to feel sick. You might be interested to know that oxygen toxicity occurs in humans, too. Usually divers who breathe enriched oxygen during dives have to be aware of this type of thing.

That said, there are certain kinds of bacteria which do not require oxygen to live at all. They are called anaerobic bacteria, and they are not as rare as you might think. In fact, many of them probably live within you! Most of the bacteria found in the human gut is anaerobic bacteria. Additionally, viruses technically do not require oxygen as they are not living. However, they do require a host to reproduce, and if that host requires oxygen I suppose you could argue that that virus requires oxygen.

First, not all bacteria are killed by oxygen - some bacteria do require oxygen, just as you do.

Second, not all living things require oxygen; in particular, fungi and most bacteria do not require oxygen. For these organisms, oxygen is poisonous. In fact, oxygen is poisonous to all living things, including you, but the cells in your body are adapted by over a billion years of evolution and can confine the oxygen to specific regions of your cells where it can't do damage. Still, if you were to breathe an atmosphere that were pure oxygen, unlike the Earth's atmosphere that is 80% nitrogen, you would die from oxygen poisoning.

Bacteria that do not use oxygen to live have never evolved the ability to confine it because they live in environments that oxygen does not normally reach, which means that the oxygen can get into the vulnerable parts of the cell (such as its DNA) and destroys said vulnerable parts, killing the bacteria. The same applies to many viruses, but some viruses are protected by types of protein coatings that can keep the DNA and RNA inside safe from oxygen in the air.

Fungi are not killed by normal amounts of oxygen for the same reason as you: fungi are descended from ancestors that used oxygen to live, as you do. However, unlike animals (such as yourself), fungi have evolved an alternative means of getting energy through fermentation, which does not require oxygen in order to work. This means that while fungi can live in the presence of oxygen, they do not need it. During the daytime, the same is also true of plants, since they can get energy from photosynthesis, which also does not require oxygen (instead, it produces oxygen). Plants still need oxygen at night, however, unlike fungi.

Some products are marketed with claims that they can cure disease by delivering oxygen to cells. There is no scientific evidence to support these claims, and these products likely don’t have any beneficial effect. Typically, when scientists say that exposure to oxygen kills bacteria/virus, they are referring to a process called oxidation. Molecular oxygen can react with water inside the cell to form hydrogen peroxide, which is highly reactive. Hydrogen peroxide is a powerful oxidizing agent and can form a highly reactive entity called hydroxyl free radical. Some bacteria are anaerobic – meaning that they thrive in the absence of oxygen. These bacteria are particularly susceptible to oxygen since they do not have any natural defenses against oxidation, and so oxygen will be detrimental to them. Other bacteria require oxygen to grow, and have natural defenses against oxidation. Hydrogen peroxide that you can buy at the store can be used as a disinfectant for surfaces or to clean cuts to prevent infection. Hydrogen peroxide will kill bacteria and viruses, and even spores if treated long enough. Hydrogen peroxide is effective against external infection and for sterilizing instruments, but for bacterial infections in your body, you should see a doctor to get antibiotics. For example, a very common bacteria S. Pyogenes can cause a variety of serious conditions including toxic shock syndrome and should be treated with antibiotics.

Oxygen? It would be O₃, ozone, not O₂, the oxygen gas we breathe.

It is not true that all living organisms require oxygen, and not all fungi, bacteria, and viruses are the same. In fact, within each of these types of organisms, there are vast differences between individual species in terms of where they live, how they live, what they eat, and what kills them.

Some bacteria can only survive if no oxygen is present; others need oxygen to live. There is a class of microbes called obligate anaerobes, which are actually poisoned by oxygen. That is not to say that a single oxygen molecule in a bottle of these microbes would result in the death of every single one of the microbial cells; there are concentrations of oxygen at which certain microbes cannot survive, and these concentrations vary depending on the microbe. An example of such an anaerobic microbe is the Veillonella genus of bacteria that live in mammalian intestines.

Viruses are an interesting case. There is quite a bit of debate as to whether they can actually be classified as living things because they cannot reproduce themselves independently. The presence of oxygen does not kill viruses - that's why the flu virus can survive in atmosphere and be transmitted.

All of what I have written above assumes that we are discussing molecular oxygen (or O₂). There are oxygen species called "reactive oxygen species" or ROS for short. (Note that "species" here does not have the same meaning as it does in biology; "species" here basically means "molecules".) ROS can be dangerous to living organisms because they are reactive, by having extra negative charges (extra electrons), fewer electrons than the number that would a molecule stable, or certain types of electrons. What makes these species dangerous is their tendency to react with things in their surroundings and therefore break bonds that, for the health and well-being of the organism in which it resides, should not be broken. An example of damage is that a certain type of ROS can cause small but damaging changes in DNA. Some DNA damage can be repaired; others not so easily, and these mistakes will persist as cells divide.

The mistakes that propagate through generations of cells can eventually result in defective proteins and the death of cells with these defective proteins. These ROS may be what you mean. Either way, it is not correct to say that "exposure to oxygen kills ___".

What a challenging question! This is not my area of specialty, but I will tell you about what little I do know.

My understanding is that oxygen availability affects the ability of aerobic bacteria and fungi to respire, breathe, and reproduce. Among bacteria, some require oxygen in their respiration process. These are called aerobes. There are also bacteria that cannot tolerate oxygen, called anaerobes. A third category of bacteria can grow with or without oxygen, called facultative anaerobes. Most bacteria will use oxygen if it is available.

Like bacteria, most fungi require oxygen for respiration and successful reproduction. However, there are also anaerobic fungi, like yeast, which is useful in baking bread.

Viruses are a bit more complex. Viruses are RNA/DNA molecules surrounded by a protein coat which may or may not be enveloped by a lipid layer. So technically, they can live in the presence of oxygen. More important, though, they require a host for replication. So whether they live or die in the presence of oxygen, depends more on whether their host does or does not require oxygen. From this perspective, my understanding is that exposure to oxygen helps bacteria, fungi, and viruses, not killing them. The opposite, however, is considered a method for preventing microbes from living. This is why most food products are vacuum sealed, to prevent microbe growth.

One method I know of to kill microbes using oxygen is through ozone treating of water. Ozone is produced when oxygen is exposed to a high-voltage current. It is a molecule composed of three oxygen atoms, temporarily existing in a very unstable and reactive state. Ozone, also known as O₃, is a highly powerful oxidant that can cause pesticides, fungi, organic materials, contaminants, and viruses to become inactive. Unlike regular oxygen (the O₂ molecule), O_{3,/sub> is an extremely active oxidant.}

An oxidant is a reactant that oxidizes or removes electrons from other reactants during a redox reaction. That means that ozone, the oxidizing agent, removes one or more electrons from another reactant in a chemical reaction. Being an oxidant, ozone can also help remove metals (like manganese, iron, and sulfur) from water by oxidizing those molecules into insoluble particles that can be filtered out of water. Despite being oxygen, the reason ozone can kill these things is because it is an oxidant. Any pathogens or contaminant that can be disinfected, altered, or removed via an oxidation process will be affected by ozone.