|I don't understand: 1.) people claim that exposure
to oxygen kills bacteria, fungi & viruses. Why is
that if bacteria, fungi & viruses are living & all
living organisms require oxygen?|
|Question Date: 2018-11-06|
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
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
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
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
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
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
Thanks for asking,
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
There are quite a few species of bacteria (and
some fungi) that have evolved to be
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
Viruses are another matter - the virus
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
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
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
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
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), O3,/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.
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