From a bacterial suspension, you sequentially transfer 0 1,0 1,0 2,0 5, and 1 0

Question

From a bacterial suspension, you sequentially transfer 0 1,0 1,0 2,0 5, and 1 0 ml into 9.9,0.9, 1.8, 4.5, and 1 0 ml sterile diluent tubes From the final tube, you plate 0 1 ml. and get 50 colonies How many viable bacteria per ml are in the original? From an original suspension you accomplish sequential dilution transfers of 0.1 ml into sterile diluent tubes containing 4.9 ml 1.9 ml,0.9 ml, and 0.9 ml. From the last tube, you spread plate 0.1 ml and observe 25 colonies the next day How many viable bacteria per ml. did the original tube contain?

Explanation / Answer

The first question:

0.1 ml of culture was added to 9.9 ml of sterile water. Now the total volume is 10 ml. So it is 100 times dilution of original culture (10/0.1 is equal to 100)

Then 0.1 ml of that was added to 0.9ml of sterile water. Both together will be 1 ml. So the dilution factor in this step is = 1/0.1=10.

Next, 0.2 ml of that solution was added to 1.8ml of sterile water. Volume of these two together will be 2ml. So the dilution factor in this step is = 2/0.2=10.

Next, 0.5 ml of that solution was added to 4.5 ml of sterile water. Volume of these two together will be 5ml. So the dilution factor in this step is = 5/0.5=10.

Next, 1.0 ml of that solution was added to 1.0 ml of sterile water. Volume of these two together will be 2ml. So the dilution factor in this step is = 2/1=2.

0.1 ml of this solution gave 50 colonies. So, 1 ml of the same solution could give 10 x 50 = 500 colonies. So, colony forming units (CFU) in final diluted solution is 500 colonies/ml.

To calculate CFU in original solution, we have to multiply all dilution factors with CFU of final dilute solution.

So, CFU of original solution = 100x10x10x10x2x500 colonies/ml= 108 colonies/ml.

Second question:

0.1 ml of culture was added to 4.9 ml of sterile water. Now the total volume is 5 ml. So it is 50 times dilution of original culture (5/0.1 is equal to 50)

Then 0.1 ml of that was added to 1.9ml of sterile water. Both together will be 2 ml. So the dilution factor in this step is = 2/0.1=20.

Next, 0.1 ml of that solution was added to 0.9 ml of sterile water. Volume of these two together will be 1 ml. So the dilution factor in this step is = 1/0.1=10.

Next, 0.1 ml of that solution was added to 0.9 ml of sterile water. Volume of these two together will be 1 ml. So the dilution factor in this step is = 1/0.1=10.

From this solution, 0.1 ml was plated and it gave 25 colonies. So, 1 ml of the same solution could give 10 x 25 = 250 colonies. So, colony forming units (CFU) in final diluted solution is 250 colonies/ml.

To calculate CFU in original solution, we have to multiply all dilution factors with CFU of final dilute solution.

So, CFU of original solution = 50x20x10x10x250 colonies/ml= 2.5x107 colonies/ml.

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