Would bacteria grow on a bar of soap?

30 May 2010



Would bacteria grow on a bar of soap? So would it be better to wash my hands using the dirty soap bar, or not wash them at all?


Chris - The answer is actually, yes, they can - because soap isn't actually very toxic for bacteria. The reason that washing your hands with soap and water works so well to decontaminate them is actually the physical decontamination. When you rub your hands together, the soap helps to prise away various oils and other layers from the skin that the bacteria are clinging to and it therefore detaches the bacteria. It's not actually being necessarily antibacterial. Now some soaps will kill bacteria. The majority don't because the bacteria have got quite a tough cell wall around them, so they're resistant, but it's the physical washing that gets rid of them.

One interesting thing though, when we've had lots of outbreaks of winter vomiting disease, the norovirus, you see lots and lots of these alcohol dispensers springing up all over the place, saying, "Clean your hands. This will help to stem outbreaks of norovirus." Actually, norovirus doesn't have around its outside an oily bag which can be attacked by alcohol. It's a very tough little protein husk that the virus is made of and as a result, the virus is completely immune to alcohol hand wipes, and therefore, all you're doing when you're using alcohol is you're producing a pure culture of norovirus on your hands. Soap and water on the other hand does work. So the best thing to do, is always wash your hands. Helen.

Helen - Well that is rather lovely, but I just wanted to ask. I've always wondered. If you go to a public loo, there's a nasty bit of soap sitting on the side and you think, "Yuck!" Is it better to use that soap if it's covered in bacteria and wash your hands and rub off your own bacteria, or should you leave it alone? What do you think?

Chris - This is a bugbear of mine actually because when you go to public conveniences, the first thing you have to do to get into them is open the door of course, and the door always opens inwards. You can push the door open which means you don't have to touch any part of it. But when you're coming out again, you have to touch the handle on the door of the loo then you got to touch all over the taps, and then you've got to touch the door handle to open it and pull it inwards again.

Now okay, lots of people will be good in the toilet. They won't make a mess, but they will also wash their hands diligently afterwards. You then go out, leaving the toilet with clean hands, but you touch the door handle that the one in a million people who haven't washed their hands has just touched and decorated with a nice culture of bacteria, of faecal origin probably but also other things are possible. It's now on your hands - which were nice and clean, so there are no other bugs there to compete with them, so now you've got a nice pure culture of pathogens all over your hands.

Why are the doors are not organised so that you can push the door open on the way out or have some kind of automatic door? More lavatories these days are getting themselves organised so that you sort of go around almost like a maze to get out. But it means you don't have to physically open external doors to get out and touch surfaces because that's how these bacteria spread - it's touching surfaces. So to answer your question, you will pick up bugs when you touch surfaces including the soap dispenser and this is why in many places where medicine is done, doctors' surgeries, nurses' rooms and so on, you'll see that the taps have these long wings on them. This is so that you can actually close them off with your elbows, rather than actually having to physically touch them with the thing you've just washed. So sorry Helen, despite your best ministrations to your hands, you're probably actually picking up bugs by using that grotty bit of soap, I'm sorry to say.

Helen - I'll just have to keep my elbows clean by the sound of it.


can the same thing happen to a soap made with ginger

I am doing a sience fair project on this and I would like to know what good sites I could go on do you suggest any.

WRONG. Bacteria do not have 'cell walls'. Do better next time.

Bacteria have an outer cell wall and there are two main cell wall types differentiated by their "Gram" staining characteristics: Gram positive and Gram negative. Some classes of antibiotic, like penicillins, work by targeting components of the cell wall. If bacteria lacked a cell wall, as you claim, then penicillin would be ineffective. 

Please take a look at a textbook, or Wikipedia.

I noticed a white layer on my soap
Just cuz it wasn't there last night I scraped it out
But as soon as I did there were these spiral white thread like structure wrapped around which weren't probably worms but a growth
As it was a clear soap I could see it
What can it possibly be?

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