The Results! Part 1

I've been busily rubbing all sorts of things on bacterial growth plates after all the great suggestions I got yesterday. I want to present the data from the first big experiment suggested by JerryM, who wondered what kind of bacteria would be present on my hands immediately after washing them and then every thirty minutes after that until the next washing. My hypothesis was that there would be a small number of colonies on the plate I touched right after washing, and then steadily growing in number over time. I thought this would especially be true considering that I spend most of my time touching bacteria and silkworms anyway, and yesterday I was also spending a lot of time touching dirty things on purpose to plate them for the experiment.

Well, boy was I wrong:

i-363c805bb41ef4fe910c271a5f9060c6-handwashingplates-thumb-510x333-48536.pngHere's the raw data, images of the plates I touched at thirty minute intervals between 0 and 120 minutes.

i-2488a81cf213bcf61df102de55fd8dc6-handwashinggraph-thumb-510x319-48538.pngAnd here is the graph of the same data. Surprisingly the highest number of colonies wasn't after two hours of not washing, but only after 30 minutes. Even more surprising, the smallest number of colonies wasn't right after washing, but at the end of the experiment after two hours!

This result is totally fascinating, and I think warrants more investigation. How does the ecosystem of bacteria on the skin change over time? How do harsh soaps affect this ecosystem? Does the skin ecology naturally maintain a lower number of the bacteria that can form colonies on LB plates and washing throws it off? Or is this result just a total fluke? Stay tuned for more experiments, and keep the suggestions coming!

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I wonder if your hands were still moist at 30 minutes which allowed for bacterial growth but dried out after that?

Or if you introduced bacteria during the washing or drying (probably more likely to be from drying and not washing).

One of the major components of the natural microbiome is ammonia oxidizing bacteria. However they are autotrophic bacteria and so don't show up using heterotrophic culture media like you are using, so virtually everyone doesn't recognize them as important.

I think supplying substrate (sweat containing ammonia) to them is the reason why the paws and feet of all mammals have the highest density of sweat glands. The ammonia gets converted to nitrite which is a broad-spectrum anti-microbial.…

Means if you want clean hands 2 hours from now.. wash them now.. and then touch a lot of stuff and wait 2 hours :/

One good experiment is of more value than the ingenuity of a brain like Newton - Daavy

keep at it

By Isaac newton (not verified) on 07 May 2010 #permalink

Repeat with Purell? Wash hands then 0, 30, 60, 90, 120 min plates; then Purell then 0, 30, 60, 90, 120 min plates?

What kind of soap did you use? I wonder how the results would change with different types of soap.

I wonder if the oxidizing theory is true?

You know I was just watching Life's Series on PBS about primates, and these bonobos literally use their hands to grab anything and everything - I know of no other species who has used soap.. and life in some form has survived without caring for bacteria on their limbs for 3.5 billion years.

There could some mystery there.
An Agapakis secret God hid at be only discovered in the year 2010.

By isaac newton (not verified) on 07 May 2010 #permalink

Nice :) You could wash only one of your hands and use the other as a control to see what the effect of the soap is. I would love to see the same experiment done with alcohol.


Awesome. Interesting pilot study :-)

We need more data now of course. As Jeff Goldblum said in this weeks great Law & Order: Criminal Intent episode: "In true science, repeatability is everything"

As an armchair scientist, and one with slightly more than a hint of OCD, I'd just go crazy with this, trying to account for all sorts of contaminations...
After washing, do you dry your hands with a fresh paper towel, or a cloth towel that hangs there.
Do you wipe your hands regularly off your cloths? Is this during labwork, or normal household time? How would a lunch in between affect the results...

Of course, all this is pretty meaningless without knowing what kind of bacteria were present.

How does this tie in with the news recently that "scientists" managed to match individuals to the bacteria they left behind on objects, possibly for forensic purposes?…

An experiment we did in microbiology class was to touch agar plates, then wash hands, then touch other agar plates. It was pretty consistent that we got more colonies growing on the second set, perhaps because hands were damp. OTOH, comparing hands half an hour after washing against a week after washing would probably favour soap.

fun w/agar plates? the REAL experiment would be to do this with your antibiotic plates and see how many colonies form... what do you have lurking around in your lab and how resistant are they to soap??

and maybe i'm taking this too seriously but...methods? is this 6 separate agar touchings post washing or 6 consecutive ones? if you really wanted to control for things i'd do the former, w/o touching anything. but that's boring/hard to justify doing. my first instinct would be that that this would be dominated by what you touched and how much.

anyway, good times. =)

By andrew giessel (not verified) on 10 May 2010 #permalink