Mass Measurements
One method for measuring bacterial growth is to assay the mass of the bacteria. Bacteria are grown under appropriate conditions, which includes growth media for energy and nutrients as well as incubating them at a specific temperature (different bacteria grow better or worse at specific temperatures). Then, once the bacteria have been isolated (either by scraping them off the growing surface or, if they are growing in a liquid, by filtration), they can be weighed using a sensitive scale. In this way their weight (which will be proportional to their growth) can be measured over time.
Turbidity
Another method for directly measuring the growth of bacteria is turbidity. This type of measurement requires the bacteria to be suspended into a liquid (although they can be grown on a solid surface and then transferred into a liquid). Turbidity works by taking a small amount of this bacteria-filled liquid and putting it in a small, clear glass container called a cuvette. This cuvette is then placed in a device called a spectrophotometer, which shines a beam of light through the cuvette and then measures how much of the light passes through the cuvette and onto a light detector. The less light that gets through the cuvette, the more bacteria there are.
Counting
Bacterial growth can be measured by directly counting the bacteria. In this method, a small amount of a liquid bacteria suspension is taken and placed onto a small slide or piece of glass with grid lines. The grid with bacteria on it can then be placed under a microscope, and the bacteria can be directly counted. The amount of bacteria on the grid divided by the volume of liquid that was placed on the grid can be used to determine the concentration of the bacteria (how many bacteria are present per unit of volume). Sometimes before the bacteria suspension is placed on the grid, it must be diluted if there are too many bacteria to be counted.
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