Submitted by Chairman_Mittens t3_y96dfg in askscience
I was thinking about this while rising my mouth out with mouthwash this morning. Why does alcohol kill 99% of bacteria in my mouth, but not the cells that make up my cheeks, gums and tongue?
Isn't a single cell essentially the same basic structure as a bacteria, just joined together with other cells? Or am I actually killing millions of cells every time I rinse, but they regrow each time?
To add to this, you can use alcohol to kill cells in tissue culture quite easily. A good part of our resistance to many chemicals is basically dilution, as you said.
To add to that, there are bacteria Enterococcus faecium, that are not susceptible to alcohol.
Yeah this is part of why I added the “chemicals they’re not specifically adapted to” bit, because to my knowledge you can name practically any harmful chemical and there will be at least one bacterial species that is extremely tolerant of it, if not completely unaffected. There are bacteria that can survive in heavy metal-laden mine runoff so contaminated that it looks like blood and has the pH of white vinegar, there are bacteria which live in steaming sulfuric acid-filled volcanic caldera lakes, there are halobacteria that thrive in bodies of water over ten times the salinity of the ocean, and there are even bacteria that can survive being irradiated (apparently in part by having lots of redundancies in their DNA and being really good at repairing DNA with minimal errors).
And that’s just actual bacteria, because there are also archaea (not actually bacteria, but historically conflated with them) that can survive in such extremely high temperatures that their specially heat-adapted enzymes are actually an indispensable tool in biotech (in particular, PCR makes use of a thermophilic archaeal DNA transcriptase so that it can speed things up with higher temperatures, iirc). Obviously this is different from chemical resistance, but it just goes to show how there are microbes that can survive almost anything.
Thanks for your very complete and excellent response
great explanation! its crazy the environments that bacteria can survive. Waters bears too.
this is admittedly really pedantic (although this might actually be more of a popular misconception than I’m thinking), but technically water bears can actually survive (at least in terms of like, actually actively living in) only in a fairly narrow range of conditions, requiring freshwater aquatic habitats in a fairly moderate temperature range (they can live in extremely thin films or tiny droplets of water in otherwise terrestrial environments, but that’s about the most extreme they get when actually living, breathing, eating, and mating), it’s more that they can enter a state of cryptobiosis (called a “tun”) which is incredibly resilient to extremes, up to and including the conditions of outer space or near-absolute zero temperatures (and then having their frozen tun state used in a quantum double-slit experiment, and still being successfully revived after lol).
So with water bears it’s more that they’re really good at weathering extremes, but not necessarily actually living in them per se, whereas bacteria and archaea can often actually exhibit active cellular metabolism and other processes even in some really extreme habitats.
I mostly just mention this because I’ve seen a lot of people say that water bears are “extremophiles”, when really they’re actually somewhat on the fragile side while actively alive, they just have a very resilient hibernation-like state they can enter, basically.
Yes they are not actively growing in those environments, but as long as they can return to that active state, they are alive. Many organisms (especially “simple” organisms like bacteria and lower eukaryotic life) are able to enter these non-metabolically active states, sometimes with extra protection (like the water bears, spore forming bacteria, and lots of fungi and yeast) allowing them to survive those extreme conditions. Arguably this is ability is more interesting and without a doubt it’s more important for the survival and dissemination of life across earth, and potentially across the universe. We may have already spread them ourselves :)
They are already on the Moon, thanks to an Israeli lunar probe that crashed there.
I watched that stream live. That thing must have hit the moon at like 500 km/h. Maybe some of the ejecta will land on another planet?
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Isn’t that why permafrost melting in places like Siberia could be kind of alarming? A bunch of hibernating prehistoric bacteria that we’ve never had to deal with before “waking up” again or something like that?
When were tardigrades put through a double slit experiment?
Bacteria are amazingly adaptive, but nothing shows the absolute persistence of life like the radiotrophic fungus found in the reactor at Chernobyl. This fungus actually uses melanin to produce energy for the organism. It isn’t just surviving radiation, it is feeding off of it.
The original poster does not seem to understand how ephemeral the boundary between organisms and their biome is in practice. The colony of bacteria is equally unaffected as the macro organism is to the death of individual cells destroyed by alcohol. If you don’t brush and floss, but do use mouthwash you will have major problems with gum disease. Less then someone who does nothing but still severe enough to reduce your life expectancy.
More to the point if you did kill all the bacteria in your body you would die from malnutrition if nothing else.
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They can resist 10x the alcohol as other tolerant bacteria (which is hella impressive), but they will still die if you dropped one in 95% alcohol.
You're right! Not very practical for wiping down surfaces. Tough lil' bastards.
I forget exactly which bacteria this includes, but I know there are some referred to as “hypolipotrophs” that are known for being able to survive on even microscopically-thin films of lipids on otherwise sterile metallic surfaces, and to survive many forms or sterilization and even some degree of autoclaving in this way. It’s estimated that many can actually survive even on the surfaces of otherwise sterile surgical tools (thankfully, none are known to be pathogenic).
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“But I guarantee you, if you somehow exposed every cell in your body at once to a high concentration of 99% ethanol, you would not survive it either. “
Actually you need less than a percent to kill most people if you expose all cells.
I mostly went with a really huge number to emphasize how even the probably 20-30% ethanol found in mouthwash (that’s a guesstimate, it may actually more or less than that, but I know 40% is around where common hard liquor tends to hover, and I kind of doubt most mouthwash has that much ethanol in it) effects bacterial cells really drastically due to their size, even the amount of alcohol in common wine or even beer is enough to kill a good share of microbes that aren’t specially adapted to be tolerant of alcohol, hence why beer and wine (and equivalents) were the drinks of choice for most people throughout a good portion of history (basically until people started to really figure out the whole water purification thing in the last couple centuries); it was better to be vaguely drunk/hungover all the time than get cholera or dysentery.
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Stealing the top post to add:
If you are cut, you are better off NOT disinfecting with alcohol since the alcohol will kill the cells at the edge of the cut and slow healing. You should wash with soapy water.
Also for bacteria, the most lethal concentration of alcohol is about 70%. This kills more bacteria than the 99%.
Just wanted to add a note that most of our cells are usually only part-way exposed to the alcohol. The rest of the surrounding tissue provides much of the pathways for nutrients, liquids, and other necessities for our cells to continue existing, cleverly regulated by the body as usual. Towards an exposed surface, most of our tissues have some sort of protection from the local environment like mucous in the lungs or the epidermal layers of the skin. If these defenses break down, the infrastructure layers also act as a means to repair or replace any damaged individual cells. Any tissue that doesn't routinely come into contact with the external environment will tend to lack the defensive features and be more susceptible to alcohol, but by the time alcohol filters to those tissues it is usually quite a bit more dilute.
Aren’t certain types of bacteria more resistant based on their cell membrane composition? Like gram-positive vs gram-negative or acid-fast bacteria?
Not saying they’re immune but stain retention after ethyl alcohol washes would indicate its membrane is less permeable with regards to ethyl, right? iirc the thickness of the layer peptidoglycan in gram-positive cells allows for simple stain retention; mycolic acid in acid aids in stain retention after acid-alcohol washes
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Labtech here, it does also kill cells when you use it to sterilize. Like you mentioned, membranes (since they are unprotected) and wounds and such. Thats where the intens stinging/burning comes from.
Your skin can handle it pretty well though. But if it is punctured, and you use high conevtration alcohol, you will kill the other most layers of cells in the tissue.
yeah this is why it’s better to use hydrogen peroxide to sterilize wounds rather than IPA or even ethanol (IPA especially tends to make injuries worse, I’ve noticed).
My medic friend tells me hydrogen peroxide is no longer recommended to sterilize wounds because it does too much tissue damage and can slow healing/increase scarring.
I still use it though. Id rather have a scar then infection.
Yup as with pretty much everything in existance, the root square law is king.
How does alcohol kill bacteria? Does it damage the lipid membrane?
I’m assuming it’s mostly by a combination of dissolving their cell membranes and pulling moisture out of them.
As someone else mentioned bacteria (unlike animal cells) do have protective cell walls made of a substance called peptidoglycan as well, and it varies how thick these walls are and whether they’re outside or inside the cell membrane (which leads to differences in how bacteria take in alcohol-based dye solutions and stain on microscope plates), so that also plays a role in how vulnerable different bacteria are to organic solvents like alcohol.
Thank you!
This. Ethanol is very hygroscopic, meaning it’s really good at pulling moisture from its surroundings at higher concentrations. It will pull this water from whatever source it can find, including living tissue; that’s why it’s deadly to bacteria, it literally sucks them dry. I had a TA in university who told me a story of a guy who was banned from every lab on campus since he decided to take a slug of pure ethanol from the jug and had to be sent to the hospital for chemical burns in his mouth and throat. The main reason we can drink it is because it’s so diluted in most cases, and like you said we are made up of far more material than bacteria.
I don’t know if this is exactly true, but I’ve always heard that 100% ethanol basically doesn’t exist unless it’s stored under some extremely specific conditions, as literally any trace amount of humidity in the environment dilutes it (this is why I said 99% in the above comment, although again, I’m actually not totally certain this is the case, just what I’ve always heard).
Even everclear (which I think is like 75-80%?) will definitely mildly burn your mouth if you try to drink it neat. Personally, I find often even regular hard liquor kind of irritates my mouth and throat (part of why I don’t drink it), and I think this is why mouthwash tends to not actually have all that much alcohol overall (also of course because, even with all the additives to make it unpalatable, desperate people will still try to get drunk off of it).
That’s because ethanol likes to form an azeotrope with water! When you’re distilling ethanol, there’s a point at about the 95% mark where the combination of hydrogen bonding between the OH groups in the water and ethanol as well as the vapour pressure of the solution make it so that the boiling ethanol will pull the water with it, making it basically impossible to distill 100% ethanol using regular non-chemical means. Because of the OH group wanting to interact with water, ethanol is super hygroscopic, meaning you are correct that if concentrated enough it will pull moisture from the air.
Pure ethanol absolutely exists though, and it’s a really useful solvent in a lot of labs. However, because of the reasons stated above, it’s generally kept in a sealed glass bottle over what’s known as molecular sieves. They’re these little beads of metal-silicates that basically form a microscopic mesh that can trap and “sieve” the water from the ethanol since the ethanol molecule is too big to get through.
Everclear comes in multiple proofs/percentage abv:
In college I did a shot of 190 and within a few minutes the outer layer of tissue began peeling off the inside of my cheeks is disturbingly large sheets. It's not really meant to be taken straight and should be used as a mixer.
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This made me reconsider my heavy drinking pattern... then I forgot it again quickly after :)!
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Why does the size of a cell affect its vulnerability to a chemical when there is so much of the chemical?
Because the same amount of ethanol is going to be less concentrated in a larger volume, since cells are basically just globs of watery liquid (cytoplasm) inside a bubble of mostly lipids (a cell membrane, although bacteria do also have cell walls, unlike human cells) the same solution of alcohol ends up somewhat more diluted in a larger volume of liquid than it does in a smaller one (you also have to account for the mechanics of osmosis on a larger surface area and other factors like that, but the end result is pretty much the same either way). Even then it’s not a huge difference, and any solution of ethanol concentrated enough to kill a given bacterium would have a decent chance of killing an isolated human cell also, but this hardly matters with the comparatively small concentration of ethanol in mouthwash, especially since the epithelial cells that line your mouth are constantly dying off and sloughing away in large numbers anyway.
And remember that much of the body's defence and much medicine theory is based on the banking that the cause of the problem (bacteria, cancer cell, parasite, virus, whatever) dies before too much of you do to be able to recover.
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Also: the cells in the epithelium most exposed to the alcohol are already dead.
Would the multiple layers of cells also play also have a say in this?
>But I guarantee you, if you somehow exposed every cell in your body at once to a high concentration of 99% ethanol, you would not survive it either.
It takes much less than that to die of alcohol poisoning. Blood Alcohol Content of 0.40% is more than enough
I don’t think I could swish vodka around in my mouth for longer than 2 seconds without yakking.
Drinking a lot can age you, as your body just churns through the cells that are being killed off and needing replaced.
So can I kill someone by putting him/her in a vat of highly concentrated alcohol and provide them with oxygen to breathe
Possibly, at least in theory. The epidermal layer of their skin would provide enough protection it would take a very long time, like unless you’re also keeping them alive with an IV as well dehydration or even starvation would definitely get them first. However, if we’re talking a solution of above 90% ethanol here, the process might be quicker, as that would be enough to debride their outer skin cells pretty significantly given some time (and once ethanol penetrated into their internal tissues, then there would start to really be some problems, not least of which that ethanol pulls a lot of water out of cells besides also eating away at their membranes).
There are probably literally tens of millions of more efficient ways to kill someone, but with high enough concentration of ethanol it probably could be done (keep in mind though that ethanol above 90% isn’t like alcohol people drink, like even everclear isn’t more than 80% iirc, and because of how hygroscopic it as and the way it pulls moisture from tissue, the alcohol would rapidly dilute itself during the process).
Damn thanks a lot What do you think is the most efficient and unique way to kill someone?
"We not only have way more cells in our body"; this is incorrect, there are more bacteria in/on our bodies than human cells... it's just that our cells are larger.
Edit, source: https://pubmed.ncbi.nlm.nih.gov/27541692/
> Our analysis also updates the widely-cited 10:1 ratio, showing that the number of bacteria in the body is actually of the same order as the number of human cells, and their total mass is about 0.2 kg.
Did you not actually read the abstract on the paper you posted? This literally says the exact same thing as the link I posted, that there’s basically a 1:1 ratio (“on the same order as the number of human cells”), not that there are more. Considering these are both from the same year, I’m assuming they’re maybe actually referencing the same study.
Once again, this isn’t actually relevant because OP clearly wants to know the effects of alcohol on individual bacteria versus human cells (which as I note, are not really going to be very different), as they state at the end of the post, not on the generalized effect of alcohol on the entire bacterial human microbiome.
But, if they did want to know the latter, everything I said still 100% holds anyway (if you were to take just the entire 0.2 kg mass of bacteria from an average human body and expose it to high concentrations of ethanol, it would still pretty definitely die???), and even as a pedantic nitpick your comment states something blatantly incorrect, and the study you posted to supposedly back it up actually contradicts your claim pretty clearly.
Of the same order is not the same as "equal number"... 3.8x10^13 is still 26% more than 3.0x10^13. If a football match is 4-3, would you call it a draw? They mean it's not a factor 10 more...
https://www.sciencealert.com/how-many-bacteria-cells-outnumber-human-cells-microbiome-science/amp
It’s about 1.3:1 at most, so very slightly more, but definitely not vastly more, and again (as I’ve said three times now) irrelevant to anything I said. This article includes links to two other sources, including one noting how it’s actually nearly impossible to give a completely accurate general human number on this due to the variance between individuals. In any event, the common citation that there’s 10:1 or some other huge number (which you strongly imply in your comment) is derived from a single discredited study, and the actual number is far less impressive.
But yet again, a bacterium is a single-celled organism, a human is multicellular one. It’s pretty obvious which is going to more vulnerable to being killed by alcohol. Exactly how many individual bacteria are inside a given human body has little to nothing to do with anything I said.
I never meant to answer OP's question, I just meant to correct your incorrect statement. Hence the quotes, then the correction. Why are you treating all this as a personal attack?
Where did I say anything about a personal attack? I never once said “stop personally attacking me”, or even mentioned myself at all.
I am kind of confused which “incorrect statement” you think you’re correcting, though, because I never said “there are more human cells than bacterial cells in a human body”, I never even said “bacteria make up but a small fraction of the mass or volume of the human body” (although the latter statement would be inarguably correct), I merely said that a human (as in our own cells) has a lot more cells than a single bacterium (one cell). Hundreds of trillions is clearly more than one. So I don’t entirely understand what exactly you thought you were “correcting”, because you were essentially addressing a claim that wasn’t even made (regardless of how accurate it is).
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Depends on what your metric for “more” is.
The very paper they posted a link to above (as well as the one mentioned in the Nature article I posted, because they both say the same thing) makes it pretty clear that bacteria wins neither. There are about the same number of bacterial cells as human cells in an average human body, and they make up only around 0.2 kg of a human body’s mass.
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If you say "more cells" you're implying number of cells. If you say "more cell mass", then you're implying mass.
The explain to me like I'm 5 explanation is that your epidermis, the outermost layer of your skin, is waterproof and composed of dead cells composed most of keratin. The alcohol would kill your living cells same as the bacteria but since you have a layer of dead cells protecting you it will mostly just irritate the area and dry out your skin.
So, a lot of great answers. And I didn’t think I saw it in any of the comments. Contrary to what you would see in tv and movies, tough characters cleaning wounds with alcohol… It’s better to clean wounds with soap and water. Alcohol, and hydrogen peroxide as that’s used a lot, can actually further damage the already damaged cells. If for some chance you don’t have access to clean water, it’s best to just let the blood flush out the wound. Alcohol and hydrogen peroxide do make great surface cleaners, though!
The outer layers of the cells that line your cheek are in constant turn over. Meaning it gets regenerated as you shed them and more grow underneath. In addition, bacteria are simply single cell organisms while your cheek has several layers of cells so you theoretically simply destroy the top layer but there is more underneath.
Since the bacteria has only 1 layer, its pretty much it. Its dead when the cell wall gets destroyed.
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I thought alcohol caused the burning feeling because it was a TRPV1 agonist, the same as capsaicin, not because of any cellular damage.
That... is a whole new realm of reading I'll have to do. It looks like there's both epithelial non-neuronal, and neuronal presentations of the protein, but I can't tell if the epithelial activation results in a brain-aware pain signal, or simply impacts the local biology such as increasing glandular activity, etc.
You are absolutely correct that ethanol is considered a strong agonist for TRPV1 though. I'll have to add an edit about the specifics of the burning sensation and read up some more on this.
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Thank you that was educational and fun!
>Isn't a single cell essentially the same basic structure as a bacteria, just joined together with other cells? Or am I actually killing millions of cells every time I rinse, but they regrow each time?
No there are a lot of different types of cells, bacteria has much simpler cell walls so alcohol dissolves them easily.
But our skin and mucous membranes have evolved specifically to resist the outside environment, through the use of condensed keratin at the surface, that forms a very touch layer, still if you put your hand in concentrated alcohol long enough it will damage your skin. mucous membranes are similar to skin except with less dry keratin on the outside.
Internally, the cells would be far more vulnerable, think of what a drop of alcohol do to your eye or to an open wound. it would be bad. alcohol injection is used sometimes when you need to damage certain tissues on purpose, such as in nerve ablation procedure.
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> No there are a lot of different types of cells, bacteria has much simpler cell walls so alcohol dissolves them easily.
Human cells do not even have cell walls, unlike bacteria.
If you're thinking of the cell membrane, then I wouldn't say the bacterial membrane is any simpler. The lipids used are slightly different, but that's not a question of simplicity or complexity. And for example Gram negative bacteria would have two membranes, which is more complex than the single membrane of human cells (though we have additional membranes around some organelles).
In any case, alcohol would damage our cells just as easily as bacterial cells. If they were exposed to the same concentration.
It does. It kills brain cells like they're going out of fashion, you know those cells that cannot really be replaced once they're lost, yeah those guys.
Drink up, less brains and more cancer, what's not to love???</s>.
It does... but your skin has a layer of DEAD skin cells that act as a protective layer. However, as others have said, >90%/vol alcohol would give you chemical burns in your mouth.
Some bacteria and some other unicellular organisms can survive high concentrations of alcohol.
Because bacteria are not protected by tens and thousands of other cells, and the protective system that follows.
It's a bit like how it's easy to break a single stick, but a bundle of stick is hard to break, but if you try, some of the sticks will still be damaged.
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So many weird takes. Isn't the simple answer just that you can't kill what's already dead? Our outer skin isn't alive. It's dead armor, basically. If it wasn't, hand sanitizer would hurt like a motherfucker, as it does if you have a cut or scrape. Same with our mucus membranes in our mouth etc. Our living cells are generally not exposed to the outer world because of the infection risk.
In hospitals they use that reddish stuff (bedadectine? I definitely butchered that) because it kills bacteria but doesn't work in the same way as rubbing alcohol, which if used on a healing wound for a week or more (when changing bandages) will slow down healing and could even cause the wound to become infected from staying open longer and constantly having the scab rubbed off and doused in alcohol over and over.
Alcohol "pickles" the human body. Not only the liver but it does a good job of killing brain cells. Also, there is a distinctive look to newborns with FAS (Fetal Alcohol Syndrome). Newborns whose mothers drank heavily during pregnancy do not develop as well as other babies. They are underweight, their growth is stunted, and depending on how much the mother drank during pregnancy they may be born addicted to alcohol. FAS is irreversible. Many other symptoms may be present at birth. So very sad when the mother abuses drugs or alcohol while pregnant. But I am getting off on a tangent here because that wasn't your question. Just thought you might be interested.
regular_modern_girl t1_it3x4qt wrote
It does kill cells in our body to some extent, like high concentration ethyl alcohol (like 90% or higher) can cause mild chemical burns in your mouth if you drink it neat. Not to mention the scarring of the liver (although tbf that’s more because of acetaldehyde, the fairly heavily toxic chemical ethanol gets mostly metabolized into).
This being said, it mostly is just a difference of scale; we not only have way more cells in our bodies, but eukaryotic cells are many times larger than (most) bacteria (this obviously varies a lot, but generally it is the case), so they’re far more vulnerable to damage or death by chemical means unless they’re specifically adapted against it.
But I guarantee you, if you somehow exposed every cell in your body at once to a high volume of 99% ethanol, you would not survive it either. The outer membranes of our cells are mostly just a thin film of lipids which organic solvents like ethanol can dissolve, and we don’t even have cell walls like bacteria do to add an extra layer of protection, it’s just that we normally only come into contact with alcohol in comparatively small amounts in a diluted form.
EDIT: also, should probably be mentioned, mouthwash usually does contain other antibacterial ingredients besides just ethanol, and if you just used, say, vodka as mouthwash, it would likely kill some bacteria in your mouth but overall not be nearly as effective (plus everyone would think you were a drunk).