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Understanding antibiotic resistance
Understanding antibiotic resistance

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1.1 Bacterial growth

Bacteria reproduce by a straightforward process in which each cell splits into two identical, new cells. This process is called binary fission and you will learn more about it in Week 4.

Individual bacterial cells can divide, and the bacterial population can double, very quickly – in as little as 20 minutes in some species. Watch the following video to see a speeded-up film of binary fission.

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Transcript: Video 2 Speeded-up film of bacterial growth.

NARRATOR:
These rod-shaped bacteria are about to replicate. There are eight in this culture, but each bacterial cell is about to divide into two. Now there are 16 bacteria, and soon there will be 32.
Under ideal conditions, some bacteria can divide in as little as 20 minutes. A bacterial infection can take hold so quickly because bacteria can double their numbers in such a short time unless something keeps them in check. Handwashing with soap, vaccination to boost the immune system, and anti-bacterial drugs may halt this replication cycle. But left unchecked, in less than four days, this bacterial culture would weigh as much as the Earth.
End transcript: Video 2 Speeded-up film of bacterial growth.
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Video 2 Speeded-up film of bacterial growth.
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However, bacteria do not continue growing at such a rapid rate indefinitely. This is because factors such as the availability of nutrients and rising toxin levels start to have an effect. You can explore this in the first activity. Don’t worry if you don’t understand all the terms, as they will be explained later.

Activity 1 Bacterial growth phases

Timing: Allow about 10 minutes

In nature, bacterial growth follows a typical pattern shown in Figure 1. The growth curve is comprised of four phases.

  • Lag phase: during the lag phase, the bacteria are adapting to their environment; nutrients are plentiful and the cells grow in size without dividing. Cell number remains constant.
  • Exponential phase: the exponential phase marks a big increase in cell number. Maximum growth rate is achieved with a constant doubling of the bacterial population. Growth then slows as nutrients become depleted and bacterial waste products build up to toxic levels.
  • Stationary phase: the bacteria enter the stationary phase when the number of new cells equals the number of cells dying. The total number of cells in the population remains constant.
  • Death phase: unless nutrients are replenished and waste products removed, the bacteria progress to the death phase. More cells die than are produced and the number of cells in the population declines.

(a) Using drag-and-drop, match the descriptions above to the correct phase of the growth curve. Don’t worry if you are not sure of the correct answer at this stage. You can check your answers by clicking on the buttons below Figure 1.

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Figure 1 Graph of bacterial growth showing how the number of cells changes with time in a culture in which the bacteria are reproducing by binary fission.
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This figure is a simple graph in which the horizontal axis is labelled time and the vertical axis is labelled number of cells. The four phases of the growth curve are distinguished using background shading of different colours. In the lag phase (yellow), cell number increases very slowly with time; cell number increases progressively rapidly during the exponential phase (red), levels off to a constant value in the stationary phase (blue), then falls during the death phase (grey).

Figure 1 Graph of bacterial growth showing how the number of cells changes with time in a culture in which the bacteria are reproducing ...
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Answer

  • During the lag phase, the bacteria are adapting to their environment; nutrients are plentiful and the cells grow in size.
  • The exponential phase marks a big increase in cell number. Maximum growth rate is achieved with a constant doubling of the bacterial population. Growth then slows as nutrients become depleted and bacterial waste products build up to toxic levels.
  • The bacteria enter the stationary phase when the number of new cells equals the number of cells dying.
  • Unless nutrients are replenished and waste products removed, the bacteria progress to the death phase. More cells die than are produced and the population declines.

Because they can divide so rapidly, bacteria adapt quickly to changes in their surroundings. Advantageous characteristics which allow the bacteria to flourish in the new conditions are passed on to successive generations and the species evolves rapidly. In Week 4 you will learn about the genetic mechanisms underlying this process.