Question

 

1. The student also took samples at different times during the growth to do dilution plating and get viable plate counts. Here are the results for the samples taken at hours 3 and 6:

3 hours	6 hours
10-3 dilution = too many to count	10-5 dilution = too many to count
10-4 dilution = 476 CFUs	10-6 dilution = 155 CFUs
10-5 dilution = 49 CFUs	10-7 dilution = 17 CFUs
10-6 dilution = 5 CFUs	10-8 dilution = 2 CFUs

Which dilution will give the best estimate for the 3 hour time point? Which will give the best estimate at 6 hours? Using these data, how many CFUs/ml were present in the original culture at times 3 and 6 hours? (Assume 1 ml inoculum for for each plate)

Answer:

For 3 hours, 10^-5dilution gave 49 CFUs. So 10^-5gave the best estimate at 3 hours.

=4.9x10⁶CFU/ml were present in the original culture at 3 hours.

For 6 hours, 10^-6 dilution gave 155 CFUs. So 10^-6gave the best estimate at 6 hours.

=1.55x10⁸ CFU/ml were present in the original culture at 6 hours.

QUESTION 2. Calculate the generation time (minutes/generation) of the student's culture using the CFUs/ml that you calculated in question 2.

Answer 1

2.

Viable plate count = 30 to 300 colonies

For 3 hours, best calculations can be done using 10^-5 dilution because the count is in the range of 30 to 300. Similarly, for 6 hours, we will use 10^-6 dilution.

CFU per mL = #colonies / (volume plated × dilution)

For 3 hours,

CFU per mL = 49 / (1 × 10^-5) = 49 × 10^5 = 4.9 × 10^6 CFU per mL

For 6 hours,

CFU per mL = 155 / (1 × 10^-6) = 155 × 10^6 CFU per mL

Original cell concentration = average of both the values given above = (4.9 × 10^6) + (155 × 10^6) / 2 = 79.95 × 10^6 = 7.995 × 10^7 CFU per mL

7.995 × 10^7 CFU per mL

Generation time = time taken by one generation to grow

Nt = No 2^n

​​​​​​155 × 10^6 = (4.9 × 10^6) 2^n

2^n = 31.63 = 32 (approx)

​​​​​​2^5 = 2^n

n = 5 (number of generations)

5 generations occured in 3 hours (180 minutes)

1 generation would have occured in 180/5 = 36 minutes

Generation time = 36 minutes

Please rate high.