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Alcohol-based hand sanitizers are an effective way to kill a myriad of potentially harmful microbes. But is there a risk of germs becoming resistant to this ubiquitous liquid?

Hosted by: Hank Green

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Sources:
https://www.ncbi.nlm.nih.gov/pubmed/9880479 https://www.ncbi.nlm.nih.gov/pubmed/27029301 https://www.ncbi.nlm.nih.gov/pubmed/23499305 https://www.ncbi.nlm.nih.gov/pubmed/30020626 https://www.ncbi.nlm.nih.gov/pubmed/29036196 https://www.ncbi.nlm.nih.gov/pubmed/28933406 https://www.ncbi.nlm.nih.gov/pubmed/2559658 https://doi.org/10.1126/scitranslmed.aar6115 https://doi.org/10.1016/S1473-3099(18)30542-5 https://www.cdc.gov/infectioncontrol/guidelines/disinfection/disinfection-methods/chemical.html

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[ ♪INTRO ].

You've probably heard of superbugs: infectious microbes that become resistant to our most effective treatments. So with the COVID-19 pandemic, when the whole world became obsessed with alcohol-based hand sanitizers, you might have started to wonder whether using too much of them could make the superbug problem worse.

But, as far as researchers can tell, that's not really a concern. And that's because alcohols chemically attack molecules that are fundamental to all cells. Bacteria generally can find ways to resist antibiotics because these drugs use focused strategies to kill or hinder those microbes.

Like, they block the action of one protein or something. But if the bacteria tweak that protein bit, or find a way to get rid of the antibiotic itself quickly, then the drug won't work. Alcohol-based hand sanitizer is different because it messes with a lot of things.

They usually contain isopropanol the same stuff as rubbing alcohol and/or ethanol, the alcohol found in booze. Both of these rip cells open and destroy the proteins inside. See, cells are surrounded by a membrane: a double layer of fatty molecules which keeps their insides in and their outsides out.

But ethanol and isopropanol are teeny. They are almost nothing compared to them. Just two or three carbon atoms, a hydroxyl group, and some hydrogens.

And, in part because of that, they can slip their way in between the membrane's layers. There, they mess with the fatty molecules a bit, ultimately making the barrier more porous. This may be damaging enough on its own, as cells with weakened membranes tend to leak important things, or get over-filled with water.

But also, it lets lots of alcohol get fully inside the cell, where it wreaks havoc on the proteins that keep the microbe functioning. And not just, like, one protein, like an antibiotic would. These alcohols can react with different parts of many proteins, disrupting the bonds that hold them together, and ultimately breaking them down in a process known as denaturation.

This is actually a major reason why the concentration of alcohol in hand sanitizer is so important. Below sixty percent or so, and it can't destroy the microbe effectively. But, you don't want to go overboard, either.

Ninety percent is usually the recommended maximum, because water helps speed up the reactions between alcohols and proteins. In the right concentration, these alcohols can kill most bacteria, although there are some holdouts - namely, the few that can hunker down in tough structures called endospores. Hand sanitizers can even destroy viruses that have a fatty outer layer, like coronaviruses, for similar reasons.

And becoming fully resistant to this level of destruction would involve much more than just a couple random mutations. The microbe would have to change fundamental parts of its structure - like, suddenly develop the ability to create endospores. And scientists don't think that's very likely to happen.

Now, one study published in Science Translational Medicine in 2018 did find that a species of bacteria was becoming more tolerant of the alcohol in hand sanitizers. That led to a whole bunch of headlines about how microbes were becoming resistant to alcohol and maybe we shouldn't use it anymore. But that's not what the study's authors actually said.

The microbes increasing in prevalence were tolerant of about twenty-three percent alcohol. They did not claim - and still don't - that microbes able to withstand seventy percent alcohol had evolved. Rather, their concern was that improper use of sanitizers or unreliable formulations could mean dangerous bacteria aren't exposed to high enough concentrations of alcohol, or aren't exposed for long enough to fully kill them.

And that may mean hospital staff may believe surfaces are sterile when they actually aren't. In fact, they stressed that people should continue using alcohol-based sanitizers, as they save millions of lives every year. They just urged more careful review of the products and protocols.

The point is, as far as we can tell, using alcohol-based hand sanitizer is not creating superbugs. So go forth and sanitize! Thanks to Paul, Fred, and Lorraine for asking us about hand sanitizers, and to everyone else who supports us on Patreon!

If you're a patron and you've got questions about how the world works, be sure to make good use of that QQ inbox. We love hearing what you're curious about, and hey you might just get your questions answered in an episode! And if you're not a patron, you can learn more about how to become one at Patreon.com/SciShow. [ ♪OUTRO ].