BACTERIAL DOUBLING TIME

Growth of cells is one of nature's most fundamental processes. Growth rates determine how life procedes, and some living things reach great age while others expire after a brief existance. Microorganisms are everywhere and are competing with one another. Their relative growth rates determine which ones win out and achieve predominance. The onset of disease depends on how fast pathogenic organisms reproduce in their hosts; if they multiply slowly, the natural body defenses may keep them under control. There are many industrial uses of microorganisms. Because time is related to costs, it is usually much more economic to conduct your process more rapidly, and this means faster growth.

This exercise takes you through some experiments on the doubling time of bacteria. Although bacteria can be spheres, elongated spheres, rods with rounded ends, or spirals, our computer program shows them as circles that stand for spheres. A real bacterium enlarges while accumulating cellular resources until it reaches a size twice that of a cell at birth. Shortly before reaching this size, an enlarged cell necks down and constriction continues until the cell divides into two equal cells. Our computer program shows cells enlarging. We do not show constriction - a cell just becomes two smaller cells suddenly. This process is known as binary fission.

There is some variation in how long it takes for a cell to go from birth to division. For a large number of cells, there will be an average doubling time that will be fairly close to the actual doubling time of most cells. Of course, the doubling time will never be zero because many cellular events must take place and much cellular material such as cell walls, membranes, enzymes, and genes must accumulate before there is enough to divide between the two new cells. Nevertheless, some cells will divide earlier than the expected average doubling time and some will divide at longer times. The distribution is not at all normal (a normal distribution is bell shaped), and the distribution for doubling times of bacteria is heavily skewed. Relatively few cells divide early and many divide late. In fact, some cells never divide and others seem to take forever.

The first research on doubling time used a microscope to focus on individual cells and used stop watches to time their division. This sounds easy, but you will see that keeping track of circles on the computer screen is difficult. The real experiments with the microscope had this same problem in following each cell, but a cell could slip out of focus or could leave the field of view. Moving the slide to keep a cell in view and refocussing frequently made the real experiment much more difficult than the computer experiment.

Now let's start the exercise. Your task is to use computer time and to see how long it takes for a cell that has just been born to reach its division time. You should expect this to be difficult and should time only one or two cells for your first run. To get good data for a statistical analysis, the real experiments required counting hundreds of cells. Points to keep in mind are:

1. Our method of timing gives only whole seconds. The real method with stopwatches gave finer resolution.

2. Our circles can get confused with overlapping circles but never disappear by going out of focus as would real cells with a microscope.

3. You should try to operate ten or twelve hypothetical stopwatches for your later runs if you can. Otherwise it would take many runs to collect sufficient data.

You probably realized very quickly that timing cells is tedious, difficult, and frustrating. It is cheating to request that the computer time the cells for you, but smart people don't perform miserable jobs that computers do better. The people that conducted the real experiments had no such easy way out and had to spend many weary hours looking through a microscope. In fact, they took photographs and made some other quite interesting observations. They recorded cell volume during the lifetimes of different bacteria and found that rate of enlargement is not at all constant.