A student in Biology 328 performed a serial dilution series on his dirt sample to determine the starting concentration of bacterial cells. He performed 5 dilutions by adding 1 mL of the previous culture to the next culture (sterile water) and plated the last dilution as follows: He plated the bacteria by pipetting 1 mL and 0.1 mL (Plates A and B respectively) and spreading the culture evenly over a R2A plate. Please answer the questions on the following page regarding this dilution series. 5 mLs (1 gram dirt) Questions: 1. What is the dilution factor for each dilution? (Added amount/total amount = dilution factor) 2. What is the TOTAL dilution factor for Plate A? What makes the total dilution factor for Plate B different from Plate A? (Total dilution factor = individual dilution factors multiplied by each other) 3. After the plates were incubated at 30°C for 24 hrs, 273 colonies formed on plate A and 30 colonies formed on plate B. What is the concentration, in c.f.u./mL, of the original diluted dirt sample? 4. As shown, the student has 5 mLs of the original culture. How many c.f.u/g dirt are there in the sample? 5. Say the student changed dilution 5; adding 1 mL of the previous culture to 4 mLs of sterile PBS. A. How does this change the dilution factor of dilution 5? B. How does this change the TOTAL dilution factor? C. What would be the new original dirt sample concentration, in c.f.u./g dirt, if Plate A and Plate B yielded the same amount of colonies as indicated in question 3?
A student in Biology 328 performed a serial dilution series on his dirt sample to determine the starting concentration of bacterial cells. He performed 5 dilutions by adding 1 mL of the previous culture to the next culture (sterile water) and plated the last dilution as follows: He plated the bacteria by pipetting 1 mL and 0.1 mL (Plates A and B respectively) and spreading the culture evenly over a R2A plate. Please answer the questions on the following page regarding this dilution series. 5 mLs (1 gram dirt) Questions: 1. What is the dilution factor for each dilution? (Added amount/total amount = dilution factor) 2. What is the TOTAL dilution factor for Plate A? What makes the total dilution factor for Plate B different from Plate A? (Total dilution factor = individual dilution factors multiplied by each other) 3. After the plates were incubated at 30°C for 24 hrs, 273 colonies formed on plate A and 30 colonies formed on plate B. What is the concentration, in c.f.u./mL, of the original diluted dirt sample? 4. As shown, the student has 5 mLs of the original culture. How many c.f.u/g dirt are there in the sample? 5. Say the student changed dilution 5; adding 1 mL of the previous culture to 4 mLs of sterile PBS. A. How does this change the dilution factor of dilution 5? B. How does this change the TOTAL dilution factor? C. What would be the new original dirt sample concentration, in c.f.u./g dirt, if Plate A and Plate B yielded the same amount of colonies as indicated in question 3?
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A student grows a culture of E. coli in 50 ml of liquid medium contained in a 250 ml Erlenmeyer flask and decides to pour plate samples of the culture onto LB agar plates. (LB stands for Luria Broth. Itâs simply a type of broth on which E. coli grow well.) Before plating, the student sets up several dilutions of the culture (indicated in the table) and plates 0.2 ml of each by pour plating. S/He does this in duplicate for each dilution. S/He incubates all of her LB agar plates in a 37 ºC incubator and counts the colonies on each plate later. Her/his results are as follows
| Total dilution factor | Plate A | Plate B |
| 10-6 | 756 | 349 |
| 10-7 | 73 | 76 |
| 10-8 | 7 | 8 |
a) Determine the average density in cfu/ml of the culture based on the relevant results. Give your answer in scientific notation with three significant figures.
b) Significantly fewer colonies grew on Plate B at the 10-6 dilution as compared to Plate A at the same dilution. Which plate (Plate A or Plate B) is more likely to be a true indication of the number cells that were present in the culture at the 10-6 dilution? Explain your reasoning.
