How Is Bacterial Growth Measured?
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Growth Rate
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Growth rates tell how quickly bacteria are multiplying. A rate is given as the difference in population for a given period of time. Bacteria go through different growth stages and lags depending on available nutrients and population density. Population counts are necessary to determine initial population size and final population size. Population counts can be made directly with optical microscopes, chemical measurements, turbidity analysis and plating techniques.
Using a Microscope
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Special microscope slides are designed with grid patterns that allow technicians to count the number of bacteria in a given area. This information is extrapolated from the sample size that was placed on the slide to give the total population size or to find a concentration of bacteria per milliliter of solution. Bacteria are usually dyed with a fluorescent marker to make them visible.
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Turbidity Measurement
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Bacteria absorb and refract light. As the population of bacteria becomes denser, this can have an observable optical effect on an otherwise clear liquid solution. This effect is called turbidity. Less light passes through the solution with a higher concentration of bacteria. Measurement of bacteria concentration by turbidity is performed on analytical instruments with precision optics and detection equipment.
Chemical Measurements
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Chemicals that are produced by bacteria can be analyzed to provide an idea of how many bacteria are present. Common measurements are for DNA, protein or nitrogen. The higher the concentration of the chemical, the larger the number of bacteria that are present.
Plating Technique
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A solution containing the bacteria population is serially diluted. This means that a given concentration, for instance 1 ml, is diluted ten-fold again and again. At each step, a small quantity is spread over the surface of a nutrient plate that will cause live bacteria to produce colonies that are visible with the naked eye. At high concentrations the bacteria will cover the entire plate. At a sufficiently low concentration, the bacteria will be spread far enough apart from each other to form single colonies that can be counted. The number of bacteria at that dilution is used to extrapolate the concentration in the undiluted solution.
Physical Counting and Sorting
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An instrument called a flow cytometer can produce droplets so tiny that they can only contain one bacterium. Droplets of the solution containing bacteria are shot through a space where it can be optically analyzed for the presence or absence of a bacterium. An electric field can be used to deflect droplets containing bacteria into a sorting chamber if desired. The number of bacteria present in the total number of droplets produced gives the concentration of bacteria in the initial solution.
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