Bacterial Growth & Doubling Time Activity
Goal: Model bacterial growth by binary fission with N = N₀ × 2ⁿ, calculate generation time from two counts, and explain why real cultures stop growing exponentially.
What to do
- On Find the final population, press “E. coli · 20 min”. Record the number of generations, the final cells and the time to one million cells.
- Press “Vibrio natriegens · 10 min” and then “M. tuberculosis · 20 h”, and record the same readouts.
- Switch to Find the generation time and press “Measured: 100 → 6,400 in 2 h” to check your answer to question 4.
Record your results
| Culture (1,000 cells to start) | Generations n | Final cells N | Time to one million cells |
|---|---|---|---|
| E. coli · 20 min | |||
| Vibrio natriegens · 10 min | |||
| M. tuberculosis · 20 h |
Questions
E. coli divides every 20 minutes. How many generations fit into 4 hours, and how many cells do 1,000 become? Show the calculation.
Vibrio natriegens divides twice as fast, every 10 minutes. Does it make twice as many cells in 4 hours? Explain.
M. tuberculosis divides only about every 20 hours. How long do 1,000 cells take to reach a million? How does this help explain why tuberculosis develops slowly?
A culture grew from 100 to 6,400 cells in 2 hours. Find the generation time.
Why does a real culture in a flask stop growing exponentially? Name the phase that follows the log phase.
Challenge: on the log-scale graph, exponential growth is a straight line. Why?
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