Identify a common method to enumerate bacteria in a sample.

The method of plate counts and serial dilutions is a widely used technique for enumerating bacteria in a sample due to its reliability and ability to provide viable cell counts. This method involves diluting the sample in a series of steps, allowing researchers to reduce the concentration of bacteria to a manageable level where individual colonies can be isolated on an agar plate.

When bacteria are incubated on an agar medium, each viable cell can grow into a distinct colony, which can be counted after an appropriate incubation period. This technique not only quantifies the number of living cells but also provides valuable information about the type of organisms present, since different bacteria may form colonies of varying sizes, shapes, and colors.

In contrast, while microscopy counting allows the observation and counting of bacterial cells, it does not differentiate between live and dead cells effectively. Colorimetric assays are generally used to measure certain biochemical activities or concentrations and do not specifically enumerate bacterial cells. PCR amplification is primarily utilized to detect and quantify specific DNA sequences, rather than directly counting viable bacteria. Therefore, plate counts and serial dilutions remain a standard practice for determining bacterial populations.