MathBench > Population Dynamics

Bacterial Growth:
The E. coli ate my homework

Doubling time practice for fun and profit

Here is some practice in finding doubling times:

A strep culture is sampled at two points in time, 1 hour and 10 minutes apart. 3.2 x 106 cells/ml are found in the first sample, and 1.8 x 107 cells/ml in the second sample. What was the doubling time?

I think I have the answer: 70/2.49 = 28.1 minutes.


A beginning yogurt maker decides to test how consistently his yogurt cultures grow. He starts with 1 x 105 cells/ml in each of two cultures. After 4 hours, he has 16 x 105 cells/ml in the first culture, and 20 x 105 cells/ml in the second culture. Are the doubling times within 10 minutes of each other?


I think I have the answer: Yes, the times are 60 and 55.5 minutes.


A syphilis patient has tissue cultures taken one week apart. The first sample has 5000 CFUs per ???, while the second sample has 32000 CFUs. Four days later a third sample has 58000 CFUs. Is the population growing exponentially?


I think I have the answer: No -- Doubling time was faster for the first week (2.61 days) compared to the next four days (4.66 days).

A doctor receives a pathogen sample which she believes to be M. tuberculosis (doubling time 13 hours in culture). She cultures the sample, finding that it grows from 0.2x105 cells/ml to 8 x 106 cells/ ml in 24 hours. Assuming that the culture was growing exponentially, is the sample likely to be M. tuberculosis?


I think I have the answer: No -- the culture grew 400-fold in 24 hours-- a doubling time of a little under 3 hours.