You are given a yeast culture of concentration 1.8 x 1010 cfu/ml. You are asked

Question

You are given a yeast culture of concentration 1.8 x 1010 cfu/ml. You are asked to make a 1.8 x 10-8 dilution from this stock culture. The volumes in your tube(s) should not exceed 10 ml and the volumes you pipette should not be less than 0.1 ml.

(i) What is the CDF for this dilution?

(ii) Given your volume constraints, can you do this dilution in a single step? If yes, show me how. If no, answer questions (iii) and (iv) below.

(iii) If no, demonstrate (draw or grid out) how you would set up the dilution. For each step, indicate in an orderly fashion the following information: V2, V1, diluent volume, DF for each dilution step, CDF up to that step, yeast concentration at that step.

(iv) Is this a serial dilution or a dilution series?

(v) What volume should you plate from the 1.8 x 10-8 dilution to get 81 colonies

Explanation / Answer

i. CDF = 1 / 1.8 x 10-8 cfu/ml = 0.55 x 108 fold.

ii. To do this in one step, you will need in 10 ml volume, 10^-7 ml of the stock to get the given dilution, which is not possible with your max 100uL pipette constraint.

iii. I will take 0.1ml of stock and dilute it with 9.9ml Culture medium - CFU at start =1.8 x 10^10 cfu/ml; at end=1.8 x 10^8 cfu/ml

I will take 0.1ml of above medium and dilute it with 9.9ml Culture medium - CFU at start =1.8 x 10^6 cfu/ml; at end=1.8 x 10^4 cfu/ml

I will take 0.1ml of above medium and dilute it with 9.9ml Culture medium - CFU at start =1.8 x 10^4 cfu/ml; at end=1.8 x 10^2 cfu/ml

I will take 0.1ml of above medium and dilute it with 9.9ml Culture medium - CFU at start =1.8 x 10^4 cfu/ml; at end=1.8 x 10^2 cfu/ml

I will take 0.55ml of above medium and dilute it with 0.45ml Culture medium - CFU at start =1.8 x 10^2 cfu/ml; at end=1 x 10^2 cfu/ml . This will give the dilution necessary.

IV THIS IS A SERIAL DILUTION .

V = 1.8*1010* 1.8*10-8= 324cfu/ml
now you need 81 colonies so....
324 colonies pe 1 ml , so for 81 colonies you need--

1/324 *81 =0.25 ml

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