Does Bar Soap Carry Germs?
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A bar of soap sits beside the sink. We pick it up with dirty hands, rub it between our palms, put it back on its dish—and later someone else uses it.
So it's reasonable to wonder:
Can germs live on a bar of soap?
Yes. A used bar of soap is not sterile.
But that isn't quite the same question as whether using a bar of soap spreads those germs.
And researchers have actually tested that.
Can Bacteria Live on a Bar of Soap?
Microorganisms can be found on used bars of soap, just as they can be found on many of the things around them—the faucet, sink, soap dish, towel, shampoo bottle, and our own skin.
Our everyday environment is full of microorganisms.
So simply finding bacteria on an object doesn't necessarily tell us whether using that object presents a meaningful risk.
With bar soap, the more interesting question is:
Do bacteria on the bar transfer to your hands while you're washing?
What Happens When People Wash With a Contaminated Bar?
In a 1988 study, researchers deliberately contaminated soap bars with E. coli and Pseudomonas aeruginosa.
The commercial soap used in the experiment contained an antibacterial ingredient, but importantly, that ingredient was not active against the two types of bacteria being tested.
And the researchers didn't use just a tiny amount of bacteria.
The bars contained bacterial levels about 70 times higher than those previously reported on normally used soap bars.
Sixteen people then washed their hands with the contaminated bars using their normal handwashing technique.
Afterward, researchers could not detect either of the test bacteria on any of the participants' hands. The results agreed with an earlier study that had also found no detectable transfer from deliberately contaminated bars during washing.
That's an important distinction.
A microorganism being present on a bar of soap does not mean it will necessarily be transferred to you during washing.
How Can Soap Remove Germs If It Doesn't Kill Them?
Because killing isn't the only way to get rid of something.
As we learned in How Does Soap Make Things Clean?, soap helps loosen oils, dirt, and other material clinging to a surface so that water can carry them away.
And then comes an enormously important part of washing:
You rinse it away.
Microorganisms caught up in that material can be removed along with it.
Soap doesn't need to turn everything it touches into a sterile surface in order to clean effectively.
Is Liquid Soap More Hygienic Than Bar Soap?
Liquid soap has an obvious practical advantage in some situations: many people can use a dispenser without handling the same bar.
But that doesn't make an ordinary bar of soap inherently unhygienic.
For use at home, there's no reason to assume that switching from bar soap to liquid soap automatically makes washing safer simply because no one touches the soap itself.
What does make sense is taking care of the bar.
Let it drain. Let it dry between uses. Don't leave it sitting indefinitely in a puddle of water.
Conveniently, that's also how you make a good bar of soap last longer.
Clean, Sanitized, Disinfected, and Sterile Are Not the Same Thing
These words often get tossed around as though they all mean "really, really clean."
They don't.
Cleaning removes dirt, oils, and microorganisms from a surface.
Sanitizing reduces the number of microorganisms on a surface to a specified level.
Disinfecting uses an appropriate chemical or process to kill or inactivate many microorganisms on a surface.
Sterilizing goes much further, eliminating all viable microorganisms under defined conditions.
For ordinary life, we generally aren't trying to sterilize our hands, our bathtubs, or everything in our kitchens.
We're trying to clean them.
What About Washing Dishes With Traditional Soap?
Traditional soap has been used for household cleaning for generations. Marseille soap in particular has a long history of use for laundry and household tasks.
When you wash a dish with soap, the soap helps loosen grease, food residue, dirt, and other material so that it can be carried away with the wash water.
That is cleaning.
It would be inaccurate to say that washing a plate with Marseille soap sterilizes it. Ordinary dishwashing isn't intended to turn our kitchens into sterile environments.
There are situations in which sanitizing or disinfecting is appropriate. That's a different job, requiring an appropriate method or product.
And that distinction matters.
Ordinary soap is very good at the job it was made to do:
clean.
Soap School Fact
A bar of soap doesn't have to be sterile in order to clean.
In a 1988 experiment, researchers deliberately contaminated soap bars with bacterial levels about 70 times higher than those previously reported on normally used bars. After 16 people washed with them, neither of the test bacteria could be detected on their hands.
The presence of microorganisms and the transfer of microorganisms are two different things.
Clean is not the same thing as sterile.
Continue Learning
Previous Lesson: How Does Soap Make Things Clean?
Source
Heinze, J. E. & Yackovich, F. (1988). Washing with contaminated bar soap is unlikely to transfer bacteria. Epidemiology and Infection, 101(1), 135–142.