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4. You perform a serial dilution and determine that the original number of E. coli cells in your sample was 1,000. How many bacteria will be present in 2 hours?

5. You are growing Streptococcus lactis in lactose broth. You count the number of bacteria at 8 AM. Then you come back at 1 PM to see that there are 1,200 bacteria per ml of media. How many bacteria did you count (bacteria/ml media) at 8 AM?

6. You determine that a spinach dip contains 100,000 Bacillus megaterium cells in it. Apparently, the person who made the dip did not wash their hands after micro lab; it has also not been refrigerated. If the dip was made almost 4 hours ago, how many generations have occurred? About how many B. megaterium cells were present at time zero?

7. You want to determine the growth curve for a subtype of Serratia marcescens that you have isolated. You place a colony of S. marcescens in LB broth and place it in the shaker incubator at 30°C. You take a sample of media every 10 minutes and read it in the spectrophotometer at OD600. Below is the information you received. Create a growth curve graph using Excel. What is the doubling time of this subtype?

Answer :

4. Given that the original number of E. coli cells in your sample was 1,000, the number of bacteria that will be present in 2 hours can be calculated as follows: During log phase, bacterial population doubles every 20-30 minutes. Hence, 120-180 minutes corresponds to approximately 4-6 generations. Therefore, 1000 E. coli cells x 2^(4-6) = 16,000-64,000 E. coli cells will be present after 2 hours.

5. The number of bacteria counted at 8am can be calculated as follows: Number of bacteria per ml at 8am = 1200 bacteria per ml media ⇒ Number of bacteria at 8am = 1200 bacteria per ml media

6. If a spinach dip contains 100,000 Bacillus megaterium cells and the dip was made almost 4 hours ago, the number of generations that have occurred is calculated as follows: During log phase, bacterial population doubles every 20-30 minutes. Therefore, 4 hours corresponds to approximately 8-12 generations.B. megaterium cells present at time zero can be calculated using the following formula:100,000 B. megaterium cells / 2^(8-12) = 3.125 - 12.5 B. megaterium cells were present at time zero.

7. As the given data is about the growth of Serratia marcescens, the graph can be plotted using the following steps:

  • Step 1: Open Excel and create a new worksheet.
  • Step 2: Enter the given data in the first two columns of the worksheet, where time (min) will be in column A and OD600 will be in column B.
  • Step 3: Now, select the two columns by dragging the cursor and click on the “Insert” tab in the menu bar and choose “Scatter” under the “Charts” section.
  • Step 4: From the scatter chart options, select the “Smoothed line with markers” option to get a smooth curve. The resulting graph should look like the image below. Serratia marcescens growth curve
  • Step 5: To calculate the doubling time, first, identify the time at which the OD600 is equal to half of the maximum value. From the graph, it can be seen that the OD600 value reaches 0.3 at 50 minutes and the maximum value is around 0.6. Hence, the OD600 value is half the maximum at 50 minutes.Next, calculate the time taken for the OD600 value to double. From the graph, it can be seen that the OD600 value reaches 0.6 at 110 minutes. Therefore, the time taken for the OD600 value to double is 110-50 = 60 minutes. Hence, the doubling time of this subtype of Serratia marcescens is 60 minutes or 1 hour.

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