## Review

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.

Using 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

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.*