Why is it allowable to use the same pipette tip??

[angelyn1985]

I wonder why it is allowable to use the same pipette tip for bacterial count on plates if we start with the lowest dilution and then the next lowest dilution??

[Cat]

If you start with the lowest concentration and go to the highest, carryover amount is negligible.

[angelyn1985]

I see! Thanks!!

[canalon]

Simply because the amount of bacteria from the diluted sample is negligible compared to the amount in the next tube. With 10 fold dilution, even if you were to plate the totality of the lower dilution there would be less than 10% error. Since it is likely that you will in fact empty closer to 99%of the volume in your tip, that makes an error closer to 0.1%. This is lower than the accuracy of most pipettes. Ergo, save yourself some work, and some tips.

[MrMistery]

Lol... This thread reminds me... I once worked with 24 flasks using the same pipette. Talk about saving....