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.
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.
