Microbiology

What are the four Phases of Bacterial Growth



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Anyone who has left sour cream or cottage cheese in the refrigerator too long can attest to the speed with which bacteria can grow. It seems that a colony of green, fuzzy gunk can appear overnight, spoiling your attempt to make your favorite dip at the Saturday afternoon football game. Bacteria can certainly grow quickly, but did you know that they have a distinct pattern of growth. Because they are single celled organisms, bacteria must literally split themselves in half to replicate. There are four phases to this growth cycle. Let's take a look at each of the four phases of bacterial growth.

The first of the four phases of bacterial growth is called the "lag phase". This is the longest of the four phases. During this phase, there is little or no cell growth. Instead of growing, the bacterial cells are busy replicating various proteins and DNA in preparation for the next phase. The cells are not shut down during the lag phase. They are very metabolically active, but are not getting any bigger.

The lag phase can go on for several hours, or many days, depending on the particular bacteria, and the conditions it is living in.

The second phase of bacterial growth is called the "log phase". In this phase the bacteria become extremely active and begin the process of dividing. Every chemical in the cell is being replicated in anticipation of the cell dividing. The log phase gets its name from "logarithmic", which roughly means that there is massive growth (this is not an exact definition, just what is implied).

The log phase is generally quite rapid and puts the bacterial cells in a vulnerable position. During the log phase, the size of the bacterial colony increases dramatically. The log phase is what's happening when your sour cream goes from white to green overnight.

The third phase of bacterial cell growth is called the "stationary phase". As you can probably guess, this phase involves a slow-down in the growth of the cells and the colony. As resources are used, the rate of cell death begins to match the rate of cell division. Thus, the entire colony slows it's growth. There's only so much sour cream to go around, right?

The stationary phase is often referred to as a being in a state of equilibrium. This simply means that the colony of bacteria is not getting any bigger or smaller, it is simply living. In addition to limited resources, the build-up of bacterial waste products (yes, bacteria have virtual "poop") can limit the growth of a bacterial colony.

When conditions around the bacteria get really bad, the colony can enter the fourth and final phase. This is called the "death phase". I'll give you one guess as to what's happening during this phase.

Good guess.

During the death phase, the number of dead or dying bacterial cells begins to outnumber the new ones. Again, this can be because of limited resources (food), because of waste production, or because of other changes in the environment around the bacteria. Cells in the bacterial colony will die off until there are few enough of them to survive on what's nearby, at which point they will start to cycle back to the first phase and begin the process all over again.

The four phases of bacterial growth may seem rather obvious and not important. In reality, scientists and doctors use the basic knowledge of the metabolic events which are taking place in these phases to develop ways to combat bacterial growth. For example, because bacterial cells are more vulnerable during the log phase, many antibiotics target metabolic processes in that phase in an attempt to stop the bacteria from growing.

The next time you open an old jar of food which is growing fuzzy colonies of goo, you'll have an idea what phases of growth they took to get there. Yummy thought, eh?

More about this author: Erich Rosenberger M.D.

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