Standard Growth Curve

When bacteria are grown in a closed culture (nothing added and nothing removed after the start), the population follows a standard growth curve with four phases. Numbers are usually plotted on a log scale, because they rise so fast.1. Lag phase: little increase in numbers. The bacteria are adapting to the new conditions: taking up water, and synthesising the enzymes and other molecules they need before they can divide.2. Log (exponential) phase: the population doubles at a constant rate. Nutrients are plentiful and there is little waste, so growth is limited only by how fast the cells can divide.3. Stationary phase: the rate of cell division equals the rate of cell death, so numbers stay roughly constant. Nutrients are running out and toxic waste products (and sometimes low oxygen) are building up.4. Death (decline) phase: nutrients are exhausted and waste reaches toxic levels, so cells die faster than they are produced and numbers fall.Calculating numbers in the log phaseN = N₀ × 2ⁿwhere N₀ is the starting number and n is the number of generations (divisions).Worked example: a culture starts with 100 bacteria that divide every 20 minutes. In 2 hours there are 120 ÷ 20 = 6 generations.
N = 100 × 2⁶ = 100 × 64 = 6400 bacteria.