Friday, June 8, 2012

Won't bacteria die when you clap your hands or rub them together?

I'm in the habit of pestering my seatmates when I'm on the verge of sleepiness. Yesterday the lecture was on preventing hospital acquired infections. As the discussion veered towards the superiority of clorhexidine over povidone iodine as an antiseptic, I asked Marvyn Chan, "Do you kill germs when you clap or rub your hands together?"

"Seriously, Lance?" He looked at me with insulting, condescending dagger eyes. I was amused. Perhaps he expected too much from me because I have a molecular biology degree and I don't know squat.

"I'm serious."

"Why would you ask that?"

"Won't bacteria die when you clap your hands or rub them together?" I explained my hypothesis: that the trauma generated by clapping may be—for all I know—sufficient to induce mechanical destruction and therefore death.

He ignored me and went back to sleep.

This morning I searched PubMed, but it generated no useful studies. I chanced upon this online forum where the same question was asked. I don't know how reliable it is, but the answers given in the thread are interesting:
—"Not from the shock of clapping your hands together. The force that you exert by clapping hands is so spread out in terms of surface area that on the bacterium's scale, it doesn't matter much. An example would be dropping a piece of paper the size of a city block onto a neighborhood. Sure, it weighs 1 ton: but the mass is distributed evenly and the pressure it exerts on any one house is relatively inconsequential. Sure, you could come up with situations where there was an unusually large bacterium, the force was just right, the bacteria happened to be sandwiched perfectly between your palms, etc., but it's not likely."

—"I haven't studied how sonication works formally, but from what I understand, the high frequency oscillation of an ultrasound probe causes rapid cavitation and explosion of bubbles in a liquid (cell suspension) which creates huge changes in pressure and temperature, causing cells to lyse. Wikipedia says that the ultrasound frequencies are around 20-50 kHz. Off the topic, but I think this is the mechanism behind sonochemistry as well - it's not so much that the sound waves directly interact with molecular bonds, but that bubble cavitation and implosion generates enormous amounts of energy. I don't doubt that some bacteria get killed when you clap your hands. The mechanism may well be from transient ultrasonic vibrations, but I just think it's unlikely that pressure from clapping your hands could squash the bugs."

—"I suppose bacteria which are dying or have weaker membranes may be destroyed by clapping your hands, and it's possible that even healthy microbes could be. But I doubt it would be any significant number. Sonication for lysis still requires more time than a single clap of the hands."
Someone even did an experiment to prove a point:
"OK, so I did the clapping experiment. I pushed my hands onto Todd-Hewitt agar plates. Then I clapped them as hard as I could and pushed them onto new TH plates. I grew them overnight at 37C (human body temp) as anything that will grow on the skin theoretically will grow at this temperature as your body is the same. Here are my results: 
"No significant difference in growth on either set. The left hand had more bacteria than the right hand. After the clap, my left hand had fewer organisms and my right hand had a bit more than it had before, but not much. It actually seemed that the clapping may have served to even out the number on each of my hands, if very slightly. Yes, there is a difference, but not a significant one. Clapping your hands will not reduce the number of organisms on your hands . . . ."
Here's another thread from Mythbusters.

What do you think? Finding the answer may win you the Nobel.

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