International A Level Biology / Microbiology

The bacterial growth curve revision notes

Edexcel International A Level Biology (YBI11), chapter 17: Microbiology, specification point 6.3. Free lesson.

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A bacterial growth curve shows how the number of living bacteria changes in a closed culture. Its four phases reflect changes in enzyme activity, nutrient supply and toxic waste, while a log scale makes very large population changes easier to plot and read.

What the four phases look like

A closed culture is sealed, so nothing is added or removed. The bacteria use only the nutrients present at the start. The graph shows time in hours along the bottom and the log₁₀ of the number of living bacteria per cm³ up the side. The lag phase is almost horizontal because the number of bacteria changes very little. The exponential (log) phase rises as a straight line on the log scale. The stationary phase is horizontal because the total number stays level. The death phase falls as the number of living bacteria decreases.

What causes each phase

The phases depend on enzymes, nutrients and waste. These affect how quickly bacteria can divide and how many cells die. In the lag phase, bacteria adjust to the new medium, take up water and switch on genes to make the enzymes they need. They grow but hardly divide. In the exponential phase, nutrients are plentiful and waste is low. Cells divide at a constant rate, so the population doubles each generation. A generation is the time one cell takes to divide. In the stationary phase, nutrients run short and toxic waste builds up. The rate of cell division equals the rate of cell death, so…

How to explain the early growth

A good explanation gives a reason for both the lag phase and the fast growth that follows. Saying only that the bacteria adapt does not explain why the population then grows quickly. During the lag phase, bacteria make the enzymes they need. Then, with plenty of nutrients and little toxic waste, cells divide at a constant rate and double each generation.

Why a log scale is used

The number of bacteria can range from thousands to tens of millions. An ordinary scale would squash the early values together, so a log₁₀ scale is used. The log₁₀ of a number is the power of 10 that gives that number. 100 = 10², so log₁₀(100) = 2. Similarly, 10 000 = 10⁴, so its log is 4. 1 000 000 = 10⁶, so its log is 6. Each increase of 1 on the log scale means ten times as many bacteria. 2 000 000 = 2 × 10⁶ has a log of about 6.3. 4 000 000 = 4 × 10⁶ has a log of about 6.6. Each doubling adds about 0.3 to the log. A straight line on a log graph means the population is multiplied by the…

How to read a log-axis value

A value read from a log axis is already a logarithm. Convert it back to the number of bacteria by using it as the power of 10. A reading of 7 means 10⁷ bacteria per cm³. Do not take the logarithm of 7 again. In exponential growth, the number doubles each generation. The log therefore rises steadily, even though the number of bacteria rises faster and faster.