You have a bacterial broth culture that contains 1 times 10^11 cells/ml. From th

Question

You have a bacterial broth culture that contains 1 times 10^11 cells/ml. From this stock culture you take 10 mu l and add it to 990 mu l of sterile broth. You label this tube 1. From tube 1, you take 10 mu of culture and add it to 990 mu l of sterile broth. This is tube 2. From tube 2, you take 100 mu l of culture and add it to 900 mu l of sterile broth. This is tube 3. What is the concentration of cells in each of the three dilutions? (Although this may seem peculiar, cells/ml is a concentration ratio just like grams/ml and moles In this case, cell concentration is called cell density.)

Explanation / Answer

C1 = 10^11 cell/mL

For Tube 1:

Cnew * Vnew= Coriginal * Voriginal

Cnew = (10^11) * 10 / (990+10) = 0.01 * (10^11) (a 1% dilution)

Cnew = 10^9 cell/mL

Tube 2:

Cnew = 0.01 * (10^11) * 10 / (990+19) =0.0001 * (10^11)

Cnew = 10000000 cells/mL

Tubre 3.

V = 100 microL --> V = 900 microl

Cnew = (0.0001 * (10^11) ) * 100 / (900+100) = 0.1(0.0001 * (10^11) ) = 10^-5 * 10^11 = 10^6 cells/mL

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