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I'm curious how much damage is potentially inflicted by shear stress by pipetting. I know that with syringes for stem cell injection cause a lot of damage. However, to what extend does this happen with P20 and P200 pipet tips? Understandably the shear modulus of bacterial cells are significantly different from cancer cells which will be different from stem cells.

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Anecdotally I have not observed any cell death upon pipetting of E. coli DH5alpha or TOP10, however as competent cells, mixing by pipetting up and down is discouraged due to the compromised cell wall.

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It's an easy experiment to do. Take your cells aliquot them into 10 microfuge tubes, and pipette each suspension increasing amount of times, stain with trypan blue and count.

The most important factors will be which pipette-type you use; I would expect a p1000 to cause more damage then a p200 then a p20 due to velocity of the fluid. Also the most important factor will be the skill of the scientist, if you pipette slowly it should decrease stress as opposed to pipetting quickly.

In my experience it depends on the pipette-type and the skill of the operator. The only way to answer this for you is to try it empirically.

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Well, then that will heavily be based on operator error. Anecdotally, I've heard that it is the other way; a P1000 would causes less damage than a P20. –  bobthejoe Jun 13 '12 at 6:42
    
Artem is right: the only way to say is empirically. The P1000 will influence the cell damage likely because of the velocity. In contrast, the P200 and P20 could give some damage due to the narrow tip hole. –  Gianpaolo R Jun 13 '12 at 18:36
    
Give it a try and post the results. I do believe that operator skill is the single most important factor. –  Artem Jun 18 '12 at 21:50
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