Bacterial growth curve

A bacterial growth curve tracks a population of bacteria after a few cells are placed in a fixed volume of fresh medium, a closed or batch culture in which no food is added and no waste removed. The number of living cells is plotted on a logarithmic scale against time, which turns steady doubling into a straight line, and the curve falls into four phases.

In the lag phase, numbers barely change while cells adjust to their new conditions: they repair damage, make the enzymes and ribosomes they need and grow larger. In the exponential, or log, phase, the population doubles at a steady pace, once every generation time, which for E. coli in rich medium can be about 20 minutes. Cells in this phase are most easily killed by antibiotics that attack growth, such as those blocking cell wall synthesis. As nutrients run out and wastes build up, growth slows into the stationary phase, where new cells roughly balance dying ones and many species switch on stress responses or form spores. In the death, or decline, phase, viable counts fall, though a few hardy cells may persist for a long time.

Students build such curves from optical density readings or plate counts, and the slope of the straight log section gives the growth rate.