You have a bacterial broth culture that contains a concentration of 2 x 10^7 cel

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Question

You have a bacterial broth culture that contains a concentration of 2 x 10^7 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 L of culture and add it to 900 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 a little peculiar at first, cells/mL is a concentration ratio just like gram/mL, moles/L, etc. In this case, it is often called the cell density.) Show work please.

Then the other question is from tube 2, you pipette 100mu L, in a new microcentrifuge labeled tube A. From tube 3, you pipette 1000 mu L into a centrifuge labeled tube B. What is the cells density in tubes A and B?

Explanation / Answer

Ans 1) The initial concentration is M1 = 2x 107 cells/ml

For Tube 1

Initial volume is V1 = 10 mu L

Final volume is V2 = 900 mu L

Therefore M1V1 = M2V2

2 x 107 x 10 = M2 x 900

M2 = 2.2 x105 cells/ml, concentration of Tube 1 after dilution.

For Tube 2

So here M1 =2.2 x105 cells/ml

V1 = 10 mu L

V2 = 900 mu L

M2 = ?

M1xV1 = M2xV2

M2 = 2.2 x 105 x10 /900 = 2.4 x 103 cells/ml, concentration of Tube 2

For Tube 3

M1 = 2.4 x 103 cells/ml

V1 = 100 mu L

M2 = ?

V2 = 900 mu L

M1V1 = M2V2

M2 = 2.6 x 102 cells/ml, concentration of Tube 3

Ans 2) Here M1 = 2.4 x 103 cells/ml, V1 = 100 mu L

since volume taken from tube 2 is 100 mu L into A (microcentrifuge), and the cell density is 2.4 x 103 cells/ml. So in 1 ml we have 2.4 x 103 cells/ml therefore in 100 mu L the cell density would be 2.4 x 102 cells/100 mu L.

Similarly if we take 1000 mu L from tube 3 into microcentrifuge tube B , the concentration of tube 3 is 2.6 x 102 cells/ml so 2.6 x 102 cells are present in 1 ml therefore in 1000 mu L cells present would be 2.6 x 102 cells means cell density in Tube B is 2.6 x 102 cells/ml.

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