A Bacterial Growth Curve Illustrates the Dynamics of Growth in Culture Medium

In growing bacteria, a standard growth curve can be observed; it illustrates the events that occur over time within the population of bacteria. There are four distinct phases that take place: the lag phase, the logarithmic phase, the stationary phase and the decline phase.

In the lag phase cover the first few hours of the curve, no cell divisions occur, because bacteria are adapting to the new environment. In cases of infection in the respiratory tract, white blood cells may swallow up the bacteria, in culture medium; they may die from the inability to adapt to the new environment. The actual length of the lag phase depends on the metabolic activity of the remaining bacteria which were able to adapt. Then the preparation for binary fission begins, they grow in size, store nutrients, and synthesize enzyme. Then the population now enters the active stage called the logarithmic phase or log phase. This is the exponential growth for E.coli. During the log phase, all cells undergo binary fission; the length of the lag phase depends on the species and the environmental conditions. As each generation time passes, the number of bacteria doubles and the graph rises in a straight line on a logarithmic scale.

In human infection, symptoms develop during the lag phase. Bacteria and their metabolic wastes cause tissue damage. Coughing or fever may occur, and fluid may enter the lungs if air sacs are damaged. However, this is also the stage where bacteria are very susceptible antibiotics. In liquid medium preparations, bacterial growth is implies bacterial growth. If prepared in solid medium, growth seen through the emergence of colonies, a single colony may consist of millions of organisms.

After some time: days in cases of infection and hours in culture preparations, the dynamism of the population changes, as the reproductive and death rate equalizes, the population enters a plateau. This is called the stationary phase. In an infection, antibodies, and possibly antibiotics too are now attacking the bacteria. In an in vitro environment, available nutrients become scarce and waste products accumulate.  If the bacterium is a species that secretes antibiotics, they are produced during the stationary phase, if the organism is a species of Bacillus or Clostridium, all vegetative cells will produce endospores.

If such conditions persist, the environment reaches its limit and the population declines thus entering the exponential death phase or decline phase. At this point, the number of dying cells exceeds the number of new cells formed. A bacterial glycocalyx may hinder death by acting as buffer to the environment. For many species, though, the history of the population ends with death of the last cell.

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