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This question deals with dilutions. It has four parts.

A) If the dilution of a sample is 1/8, what is the dilution factor (D.F.)? I got my answer to be 8.

B) Suppose you want to make a 1/18 dilution of a sample.
If you use 7 ml of the sample, how many ml of diluent do you need to use?
(NOTE: You do not need to put units in your answer, the units in this problem are milliliters.) I got my answer to be 119.

C) In a three step serial dilution:
the first dilution is 1/6
the second dilution is 1/2
the third dilution is 1/10

What is the total dilution factor of this three step serial dilution procedure?
(HINT: Remember that D.F. = 1/dil. Also remember that the term for D.F has no units.) I got my answer to be 720.

D) To determine the concentration of bacteria in a liquid sample, you prepare a serial dilution of the culture and spread 0.10 ml aliquots of several of the dilutions on plates of a rich agar medium.
After incubating, you see 200 colonies on the spread plate that was prepared from the 10-2 dilution.
What was the concentration of viable bacteria in the liquid culture?
(Units for the answer to this problem are CFU/ml but do not enter the units in your response.) I got my answer to be 2e5.

Can someone please check these for me? This is online homework, and I have to submit everything at the same time, so if I get one wrong, the entire problem is wrong!

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Lelia Lubowitz
Lelia LubowitzLv2
28 Sep 2019
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Related questions

Viable cell counts are often performed on exponentially growing cells in order to relate the absorbance reading (AU) with actual cell number. For this purpose, a sample of the culture is removed during the exponential phase after the absorbance reading is recorded. Spread plates are prepared in order to perform a viable cell count. The goal is to have a spread plate containing between 30 and 300 cells (colonies) when plating a volume of 0.1 ml (100 μl). To determine the correct dilution, you will need to estimate the number of cells/ml present from the AU (absorbance units) value of your sample. For this calculation, we will assume that 1 AU is equivalent to 107 cells /ml. So:

AU of sample x [(107 cells/ml)/1 AU] x 0.1 ml plated = total # of cells expected


You want to calculate two different dilutions that will give you between 30 and 300 colonies (cells). In order to determine the maximum dilution factor, divide the total # of expected cells by 30. In order to determine the minimum dilution factor, divide the total # of expected cells by 300. For example if the AU was 0.3, the total number of expected cells would be 3.0 x 105. In this case the maximum dilution factor would be 104 (3.0 x 105/30) or a dilution of 10-4 and should produce approximately 30 colonies. The minimum dilution factor would be 103 (3.0 x 105/300) or a dilution of 10-3, which should produce ~300 colonies. Since performing a single dilution of 10-4 or 10-3 would be highly inaccurate, serial dilutions are performed. Typically, the spread plates are prepared in duplicate for greater accuracy.

QUESTION 1:

You have removed a volume of a culture of E. coli during exponential growth in order to measure the viable cell count. If the absorbance reading was 0.3, estimate the maximum dilution factor you would need in order to obtain between 30 and 300 colonies on a plate spread with 0.1 ml of the dilution. Assume that there are 107 cells/ml in one absorbance unit.

QUESTION 2:

You have removed a volume of a culture of E. coli during exponential growth in order to measure the viable cell count. If the absorbance reading was 0.3, estimate the minimum dilution factor you would need in order to obtain between 30 and 300 colonies on a plate spread with 0.1 ml of the dilution. Assume that there are 107 cells/ml in one absorbance unit. ​

Write the equations used to calculate tube dilution, total dilution and plate dilution. Define what is meant by diluate vs. diluent in the equation.

2. Draw out a dilution scheme showing how you would perform a 1/350 dilution. Making sure that no more than 10ml is in each tube. Please show both tube and total dilutions.

3. A hamburger sample was diluted 10-1 and then 0.1ml of the 10-1 hamburger suspension was plated on nutrient agar. Incubating for 48 hours at 370 C, 279 colonies appeared. • What is the original concentration? • How many CFU per 100uL

Determining Dilution Scheme




Viable cell counts are often performed on exponentially growing cells in order to relate the absorbance reading (AU) with actual cell number. For this purpose, a sample of the culture is removed during exponential phase after the absorbance reading is recorded. Spread plates are prepared in order to perform a viable cell count. The goal is to have a spread plate containing between 30 and 300 cells (colonies) when plating a volume of 0.1 ml (100 ?l). To determine the correct dilution, you will need to estimate the number of cells/ml present from the AU (absorbance units) value of your sample. For this calculation, we will assume that 1 AU is equivalent to 107 cells /ml. So:


AU of sample x [(107 cells/ml)/1 AU] x 0.1 ml plated = total # of cells expected





You want to calculate two different dilutions that will give you between 30 and 300 colonies (cells). In order to determine the maximum dilution factor, divide the total # of expected cells by 30. In order to determine the minimum dilution factor, divide the total # of expected cells by 300. For example if the AU was 0.3, the total number of expected cells would be 3.0 x 105. In this case the maximum dilution factor would be 104 (3.0 x 105/30) or a dilution of 10-4 and should produce approximately 30 colonies. The minimum dilution factor would be 103 (3.0 x 105/300) or a dilution of 10-3, which should produce ~300 colonies. Since performing a single dilution of 10-4 or 10-3 would be highly inaccurate, serial dilutions are performed. Typically, the spread plates are prepared in duplicate for greater accuracy.



(NOTE: Do not put units in your answers, if you do the computer will grade the answer as wrong and you will lose the points. Answers should be written in numerical form or if you want to write as scientific notation you can write as 1.22E8 or 1.22E-8. The system will only take up to 15 significant figures.) (Another NOTE: Your answer must be within 5% of the correct value to get credit for this problem.)



You have removed a volume of a culture of E. coli during exponential growth in order to measure the viable cell count. If the absorbance reading was 0.8, estimate the maximum dilution factor you would need in order to obtain between 30 and 300 colonies on a plate spread with 0.1 ml of the dilution. Assume that there are 107 cells/ml in one absorbance unit. Answer:



You have removed a volume of a culture of E. coli during exponential growth in order to measure the viable cell count. If the absorbance reading was 0.8, estimate the minimum dilution factor you would need in order to obtain between 30 and 300 colonies on a plate spread with 0.1 ml of the dilution. Assume that there are 107 cells/ml in one absorbance unit. Answer:

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