Ask Me Help Desk - Effect of temperature on hand wash!

Hi,
I've recently done an experiment in biology investigating the efficiency of handwash at different temperatures. I found out that the hand wash kills more bacteria at 25degrees celsius and gradually less when increasing the temperature up to 100degrees, which is fine, I hope. But now I need to write the experiment up, and explain why the temperature has this effect on the hand wash. I don't think there is enzymes which are responsible for it but I'm not sure...
Please help!
Thanks

Do you know what was in the hand wash you used?

Most of them are detergents, they work by disrupting the cell wall of the bacterium which kills them. However I believe that it is the detergents soluabilising fats and oils on the skin capturing the bacterium which is the main cleaning effect of handwash. I don't think there are many hand washes that contain enzymes, although many clothes powders do.

I'm surprised that the handwash got less effective as the temperature increased as many bacterium die at greater than 40 degrees Celsius.

Are your results statistically significant? Can you give me a little bit of the experimental detail? I'm sure you didn't wash you hands in 100 degree water! Maybe part of the method would help explain the result.

One thing I think would degrease the bacterialcidal effect form 25 to 37 degrees is that bacterium divide faster up to this point so the population might recover from the wash effect quicker.

These are surprising results to me I would definitely like to know more about what you did.

This paper
Wiley InterScience :: Session Cookies

Found that there was no increasing in handwashing efficacy at higher temperatures, but instead an increase in skin damage which increases the risk form pathogens.

Yeah, I've found that a couple of the ingredients, Sodium Hydroxymethylglycinate and Cocamidopropyl Betaine, have cleansing properties and I think the second one work by disrupting the cell membrane of the bacteria.

Quote:

Originally Posted by templelane

I'm surprised that the handwash got less effective as the temperature increased as many bacterium die at greater than 40 degrees Celsius.

So basically, it isn't the hand wash that is affecting the growth of the bacteria; it's the fact that bacteria die over 40degrees?

I didn't wash my hands, I set up lawn agar plates with M. luteus and put a paper disc covered in hand wash on the middle of the agar plate. The hand wash samples were kept in different temperatures for 2 hours before the experiment was carried out. After I set up the agar plates, I incubated them at 25degrees. I measured the diameters of the inhibition zones produced.

I carried out spearman rank correlation on my results and it showed a significant negative correlation between the temperature and the diameters of inhibition zones.

Thanks

Sorry its late and I've been at a science symosium all weekend (too much science!) can I clarify that as temperature increased the inhibition zones decreased, thus showing less bactericidal activity?

EDIT
By negative correlation (inhibition zone on the y axis, temperature on the x axis I would interprete the opposite result)

If this is the case then the only explanation I can think of is maybe the higher temperatures caused some of the key bacteriacidal ingredients to become unstable and degrade.

Unfortuanately I'm not a chemist so I don't know how stable the compounds in handwash are.

I haven't read this through carefully, I admit. But I think this experiment is NOT a measure of hand washing effectiveness. You are measuring the heat-mediated destruction of the bacteriacidal properties of the ingredients.

In actual hand washing, ordinary soap works well. Better with warm water. No one could wash their hands in 100 degree celsius water. This makes no sense.

Quote:

Originally Posted by asking

In actual hand washing, ordinary soap works well. Better with warm water. No one could wash their hands in 100 degree celsius water. This makes no sense.

I didn't actually wash my hands in 100degree celsius water, I am testing whether the storage temperature affects how well the hand wash works. There were no hands involved in the actual experiment! I kept the hand wash at different temperatures so I could test whether hand washes are kept at their optimum working temperature in hospitals and homes.

Got it. I thought you were a science fair student who was saying this was a proxy for measuring the effectiveness of handwashing with water and hand wash at different temperatures. (I've seen a lot like this.)

You should be careful in writing up to state: " I found out that the hand wash kills more bacteria IN VITRO WHEN STORED at 25degrees celsius and THAN WHEN STORED AT HIGHER TEMPERATURES, UP TO 100 DEGREES. [[gradually less when increasing the temperature up to 100degrees,]]

Also, did you let the stuff cool off before testing it? Was it all the same temperature when applied? Why else it would matter how bacteria respond to 40 degree temps. I'm still not clear what you did.

I will restate that I have read that the effectiveness of handwash is not primarily related to bacteriacidal properties. Bar soap contaminated with bacteria worked just as well as other forms of handwashing. It's how well and thoroughly people scrub their hands that matters most, not the product they use. As long as the product is still able to cut oils and, with warm water and vigorous scrubbing and rinsing, remove dirt, it will work.

So, as far as just killing bacteria in a dish, you need to figure out the chemistry of the active ingredients and what happens to them at different temperatures. I think Templelane is right: the ingredients are breaking down at the higher temperatures, which isn't surprising,and then not working. I can't imagine any of these products being boiled in most houses or institutions...

This is interesting!

Yeah sorry I didn't really explain it too well at the beginning,
I heated up the hand wash in waterbaths and dipped a piece of paper in the hand wash straight away after taking it out of the waterbaths, so they were at different temps when applied to the plates.

Okay, thanks for your help, I've just got to find out how to write it in a biological way now instead of chemistry,

Oh and I don't think they actually are boiled or heated to these temperatures, I just needed a range of temperatures to test! I'm glad you think it's interesting though ! :D

Thanks

If you applied hot, you have confounded your results with two different variables.