15: Summary


The infective dose (ID) for cholera is about 1 million. It would take about 20 generations of doubling for a population starting from a single cell to reach 1 million cells.

Exponential growth follows the pattern

  • Initial Number –> 1
  • Number after 1 doubling –> 1×2
  • Number after 2 doublings –> 1×2×2
  • Number after 3 doublings –> 1×2×2×2

The (untransformed) graph is a line curved up. When the graph is log-transformed, it becomes a straight line (pointing up, assuming the population is growing). This happens because during exponential growth, the population always grows by the same multiplicative factor.

bacterial population growth phasesUsing knowledge about how the log-transformed graph appears, we can determine whether a bacterial population is in a lag, log, stability, or death phase.

We can determine doubling time roughly by reading it off a graph. To do this, pick a starting population (on the y axis) which falls within the exponential growth window. Find the doubled population and check that it also falls within the exponential growth window. Find the amount of time elapsed (on the x axis) between the two population readings. This is the approximate doubling time.

Another way to estimate doubling time would be to calculate it based on data about how long it takes to get to the infective dose, and how many generations that represents.

Learning Outcomes

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Now that you completed this set of modules you should be able to:

    • Explain what is meant by ‘exponential growth’ and generation (doubling) time
    • Explain why logarithms are used to plot bacterial growth curves
    • Describe the phases of growth in a bacterial culture

If you want a printer-friendly version of this module, you can find it here in a  PDF document. This printer-friendly version should be used only to review, as it does not contain any of the interactive material, and only a skeletal version of problems solved in the module.