Question

FOR #4 A & B I don't know if I did it correctly.....Please check & the other questions I'm having a heard time on. Thanks!

A. You have been given a tube of E. coli. You are asked to make 1 mL total volume of 10^-1 dilution of the bacterial culture. Explain how you would do this. Show all necessary calculations.

____ ml cells   +    _____ ml water    = 1 ml    (total volume)

V1 D1 X V2 D2
1 mL X 1 = V2 X 10^-1
V2 = 1 mL/10^-1 so V2 = 10 mL.
But because V1 is part of V2. The answer is: 10 mL-1 mL= 9 mL of diluent must be added.

V1 D1 X V2 D2
1 mL X 1 = V2 X 10^-2
V2 = 1 mL/10^-2 so V2 = 100 mL.
But because V1 is part of V2. The answer is: 100 mL-1 mL= 99 mL of diluent must be added.

You have bacteria at a concentration of 5 x 10⁸ CFU/mL. You spread 1 mL of this sample on an agar plate to obtain isolated colonies. How many colonies do you expect to find the next day after incubation at 37⁰C? Can you count these colonies? Can you use this plate for determining concentration?

A. You have bacteria at a concentration of 1 x 10³ CFU/mL (in real life – you don’t know this, but we are just working on math skills here). You transfer 1 mL of this sample into 9 mL of water and then spread 1 mL on a plate of agar. How many colonies do you expect to find the next day after incubation at 37⁰C? Can you count these colonies? Can you use this plate for determining concentration?

You take 0.05 mL of a culture of bacteria at a concentration of 4 x 10⁷ CFU/mL, and add 4.95 mL of water to it. What is the dilution that you have performed? What is the concentration of bacteria (CFU/mL) in the diluted culture?

A. You have diluted a sample by 1000 fold (1/1000) and plated 1 mL on an agar plate. You observe 55 colonies. What was the concentration of the original sample in CFU/mL?

A. A bacterial sample has a concentration of 3 x10⁷CFU/mL. You make serial dilutions of 10^-3 followed by 10^-2 and 10^-1 dilutions. You finally plate 1 mL of the last dilution on an agar plate and incubate it at 37⁰C.   What is the total dilution? How many colonies do you expect to see on the plate?

Total dilution:

# of colonies expected on plate:

C. This time, you see 10 times fewer colonies than you had expected to see.What could have gone wrong? How will you fix this problem?

  A bacterial sample has a concentration of 2 x10⁶ CFU/mL. Show a scheme of dilutions to obtain 30-300 colonies on a plate. Your scheme should contain the volume of diluent (sterile water), volume of sample transferred each time, the concentration of bacteria (CFU/mL) in each dilution. You can assume you are plating 1 mL of the dilution on plates of nutrient agar. See Figure 1 in protocol.

These are the results of the experiment described in the protocol. From the data shown in the results section of Standard Plate Count protocol, calculate the concentration of bacteria (CFU/mL) in the original sample of E. coli.   Show your calculations. Hint: remember that we don’t use plates that have less than 30 or more than 300 colonies to do these calculations.

Answer 1

A. 0.1 ml cells + 0.9 ml water = 1 ml total volume of 10^-1 dilution of the bacterial culture

A. You have bacteria at a concentration of 5 x 108 CFU/mL. You spread 1 mL of this sample on an agar plate to obtain isolated colonies. 5 x 108 colonies is expected to be found the next day after incubation at 37⁰C. Yes, these colonies can be counted. Yes, we can use this plate for determining concentration.

A. You have bacteria at a concentration of 1 x 103 CFU/mL You transfer 1 mL of this sample into 9 mL of water and then spread 1 mL on a plate of agar. 10.3 colonies is expected to be found the next day after incubation at 37⁰C. Yes, these colonies can be counted. Yes, we use this plate for determining concentration.

A. You take 0.05 mL of a culture of bacteria at a concentration of 4 x 107 CFU/mL, and add 4.95 mL of water to it.

10^-2 dilution have been performed. 4 x 1.07 CFU/mL concentration of bacteria is the diluted culture.

A. You have diluted a sample by 1000 fold (1/1000) and plated 1 mL on an agar plate. You observe 55 colonies. The concentration of the original sample is 55000 CFU/mL.

A. A bacterial sample has a concentration of 3 x107CFU/mL. You make serial dilutions of 10-³ followed by 10^-2 and 10^-1 dilutions. You finally plate 1 mL of the last dilution on an agar plate and incubate it at 37⁰C. The total dilution is 10^-6. 3 x 0.000107 colonies we expect to see on the plate.

C. This time, you see 10 times fewer colonies than you had expected to see.10^-1 dilution has been done to the original sample. This problem can be fixed by adding 10 x clonies.

A bacterial sample has a concentration of 2 x106 CFU/mL.  30-300 colonies on a plate dilutions means 10^-2 dilution. For this take 1 mL of bacteria sample and add 99 mL of diluen (sterile water). The concentration of bacteria is 2 x 1.06 CFU/mL.

For the last question more informatio is needed. Figure 1 is not there in the question.