How can bacteria possibly grow in/on my soap?

Well, it seems like a conundrum to me: our use and promotion of antibacterial products which actually have bacteria growing in them.  It’s like finding out your bar soap has bacteria on it (ahh, unfortunately it does).  So what’s the purpose of washing if the soap we are using has bacteria?

There are some heavy duty questions.  We’ll tackle them by splitting them up into parts.

Part 1: The science of soap and its intended purpose

Let’s go back to the purpose and method of washing our hands (or actually any part of the body): to get us clean.  Clean means the removal of dirt and germs, but not the killing of germs.  Our bodies naturally produce oil, and a lot of dirty surfaces have oil in them, making dirt stick to our hands and difficult to remove with water alone.  Why? Because oil and water don’t mix.  Here’s where the chemistry of soap helps.  Soap molecules are elongated and have two ends: one end that loves water (hydrophilic) and one end that repels water (hydrophobic).  When you lather your hands with soap, the end that repels water sticks to the oily dirt, and the end that loves water, well of course sticks to the water, and the soap will help lift away the oily dirt from your skin.  Any germs in the dirt are loosened with it and flushed down the drain when you rinse well with water.  Thus, the purpose of washing with soap is not to kill germs, but to wash them away!  

Part 2: Antibacterial Soaps

Antibacterial hand soaps first came on the scene after 1984, when David Poshi and Peter Divone filed for a patent for “antimicrobial soap”, which used triclosan to kill microbes. (The FDA, Soap, and Superbugs)  After several decades, the safety of triclosan came under scrutiny (it is an endocrine-disrupting chemical that may cause cancer), and in 2013, the FDA called for data proving that triclosan was safe for everyday use and more effective than preventing infections than products that didn’t contain it.  After manufacturers had not provided the necessary proof, in 2016 the FDA ruled companies couldn’t sell triclosan-containing consumer products such as antibacterial soaps anymore and stated that washing with antibacterial hand soap is not any more effective than washing with soap and water. (Triclosan)

These rules from the FDA don’t apply to handsoaps used in healthcare settings, or hand sanitizers or antibacterial wipes.  So in these cases, triclosan is allowed because of the ability to kill germs even when water is not used to flush them away.  

Part 3: The ability of soaps and even antibacterial soaps to harbor bacteria (!)

Hopefully the understanding of how soap works to loosen dirt and germs from our hands will help to override the gross fact that soap, like most other moist substances in the house, can harbor bacteria.  For normal (non-antibacterial) soaps, It all goes back to biofilms.  Biofilms are thin, slimy layers of bacteria that adhere to surfaces and to each other.  (For more on biofilms you can read our article here.)   The slime on your teeth in the morning and the slime in the dog’s water bowl are both examples of biofilms. 

Biofilms are colonies that protect the bacteria from eradication by scrubbing, or drying out, or in the case of antibacterial soap, from agents that can directly kill it.  The slime is a coat of armor to the bacteria living beneath it!  

If bacteria can form a biofilm, it’s set to thrive, because biofilms are particularly hard to eradicate.  We brush our teeth in the morning, and wash out the dogbowl, but guess what–the biofilm is back the next day, if not a few hours later.  So, it’s not surprising that the longer a container is used and has water in or around it, the more likely there’s a biofilm on it!  This goes for our water bottles, contact lens storage cases, and yes, bar soap dishes, handsoap, shampoo and body wash containers.  In fact, a study in 2011 showed that about 25% of soaps sampled from public restrooms in the US were contaminated with more than 106 colony-forming-units (CFUs) per ml (for reference, the maximum amount of contamination for eye cosmetics or baby products is 100 CFUs per gram or ml).  

Refillable containers for many different products are still thought to be the best practice to reduce plastic waste, and they do accomplish that one goal.  However, the way they are refilled is very important and can substantially increase the risk for biofilms to grow and contaminate the soap inside.  In another 2011 study, 14 refillable soap dispensers were sampled in one elementary school, and all 14 had significant bacterial contamination, which resulted in a 26-fold increase of gram-negative bacteria on hands after washing with the contaminated soap!  In a school setting, of course there adults and children, and although hand-washing technique was not dictated, washing with bacteria-contaminated soap increased bacteria on the hands for both.  Bacteria levels dropped drastically after the soap containers were changed out to accept sealed refill packets (bladders of uncontaminated soap).  

What are the lessons here?  

  • Although the FDA’s position on antibacterial soaps is probably true (antibacterial soaps do not provide any measurable benefits over plain soap outside of a healthcare setting), it is made on the premise that the soap or the container is not harboring bacteria!  

  • Refilling your soap container without using sealed soap refills may expose you to increased bacteria.  This is because without extensive cleaning of the dispenser, humid air that fills a nearly-empty dispenser is probably enough to enable a biofilm to take hold, and without thoroughly cleaning your soap dispenser, the new liquid soap poured into it becomes contaminated, too.  This article by GoJo, a soap manufacturer, outlines the risks of refilling your soap dispenser from larger containers of soap and lists these facts about soap dispensers (from article Evaluation and remediation of bulk soap dispensers for biofilm):

    • Once dispensers are contaminated with biofilm, even cleaning and soaking in bleach has been proven ineffective, as biofilms are highly resistant. 

    • It takes only a tiny number of remaining bacteria from the biofilm community to recontaminate the soap and dispenser. Recontamination occurs rapidly – within two weeks.

    • Contaminants can be present even when not obvious or visible (that’s why we call them “microorganisms” – you need a microscope to see them).

    • Biofilms can be found in bulk dispensers made of any materials (plastic, stainless steel, etc.).

Therefore, if you use liquid soap, making the switch to sealed soap refills or single-use dispensers is safer for everyone who uses the soap!   The problem is that there are virtually no “natural” soaps for the home that use sealed soap refills.  There are many “solutions” online for cleaning soap dispensers, but they are more than likely ineffective– this article showing that even industrial chemicals such as sodium hypochlorite, sodium hydroxide and benzalkonium chloride were not able to completely eradicate a biofilm of salmonella after it was allowed to establish for only 7 days.

  • Many people prefer solid bar soap to liquid soap for washing.  However, solid bar soap that sits in water allows a slimy biofilm to form that nurtures bacteria with dead skin cells and water.  If you use solid bar soap, make sure to use a dish that allows water to drain off the soap (so it’s not sitting in a puddle of water) and one that is not porous, like a good ceramic or metal dish.  Wooden grate soap dishes are popular and very “zen”-looking, but think about wet wood and all the microbes that it can sustain!   Don’t be tempted to use that “soap water”, either.  To get rid of bacteria on the surface,  washing the bar soap thoroughly with water drastically diminishes the risk of bacteria surviving on the soap. It is also recommended to wash the container where the bar soap is kept often to keep the goo from developing.  (The Shocking Truth About Bar Soap And Germs)  Check out these soap dishes:

Although it seems like an insult to a frugal, ecologically conscious mind, the most healthy soaps come in liquid single use dispensers or a bar soap situated on a well-drained soap dish.  You might keep these in mind the next time you see a super deal on a liquid soap refill, and even reconsider the merits of old-fashioned bar soap.

Photo by Matthew Tkocz on Unsplash

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