Your professor handed you a tube with an E. coli culture and asked you to prepar
ID: 196849 • Letter: Y
Question
Your professor handed you a tube with an E. coli culture and asked you to prepare a 10-3 dilution using 100 ! of culture. How would you do this? 1. 2. Your professor handed you a tube with 1 ml of an E. coli culture and asked you to prepare a 101, 102 and 103 dilutions using the full 1 ml volume. Explain how you would do this. what would be the dilution if you add 50 1 to 0.950 ml of diluent? Suppose you were instructed to add 10 1 to 0.99 ml of diluent but instead you added 0.1 ml. what was the 3, 4. intended dilution, and what was the actual dilution? 5. You plate 0.1 ml of a sample diluted by a factor of 1 x 103 and count 100 colonies. Calculate CFU/ml in the undiluted sample? Plating 200 1 of a sample diluted by a factor of 1 x 10" and count 100 colonies. Calculate CFU/ml in the undiluted sample? 6. You have 0.05 ml of an undiluted culture at 4.2 x 105 CFU/ml. You then add 4.95 ml of sterile diluent. What is the dilution and what is the final cell density (CFU/ml)? 7. You have 15 1 of a sample and want to dilute the sample 100-fold. How much diluent do you have to add to the 15 l sample? 8, 9, If the sample has 5.5 x 107 CFU/ml, how many colonies should be counted if you plate 100 ! from the 10. The original concentration in a sample is 1.5 x 10 CFU/ml. What dilution should you prepare to obtain 11.You plate 200 1 of a sample diluted by a factor of 105 on a nutrient agar plate. After incubation you count 72 dilution 1 x 10 countable colonies after plating 100 !? colonies. Calculate the CFU/ml in the undiluted sample. 12. How many colonies would you count if you plate 100 1 of the sample in Question 11 diluted by a factor of 1 x 103? 13. A student plated 0.2 ml and counted 40 colonies in the 103 dilution. He calculated that the original broth had 8 x 10* CFU/ml. Is his calculation correct? Give the correct response if you think the calculation is incorrect.Explanation / Answer
(1)
Dilution required = 10^-3
Tube 1= 100 uL of culture + 900 uL of media = 1/10 dilution
Tube 2 = 100 uL from the tube 1 + 9900 uL of media = 1/10 x 1/100 = 1/1000 = 10^-3 dilution
(2)
Tube 1= 1 mL E Coli culture + 9 mL media = 10^-1 dilution
Tube 2 = 1 mL from Tube 1 + 9 mL of media = 10^-2 dilution
Tube 3 = 1 mL from Tube 2 + 9 mL of media = 10^-3 dilution
(3)
Dilution factor= (aliquot volume + diluent volume)/aliquot volume
Dilution factor= (50 uL + 950 uL)/50 uL = 20 times
Dilution is 20 times or 1/20
(4)
Original CFU/ml = ?
Dilution factor = 10^-3
Number of colonies= 100
Volume of sample =0.1 mL to the plate
cfu/ml = (no. of colonies)/( dilution factor x volume of culture plate)
cfu/ml = (100)/ (10^-3 x 0.1)
cfu/ml = 1 x 10^6 cfu/mL
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