This test is reasonable fast but tends to indicate only the bacteria which grow rapidly at higher temperatures (30-40oC) in two days (these are common times and temperatures that many laboratories use). These bacteria are more likely to have originated from warm blooded animals (including humans) rather than from the environment where temperatures are commonly at less than 300C. The test originally used to be a pour plate technique and it is still used sometimes. In this test, the water is diluted and then mixed with molten agar at >45oC so that colonies are mixed into the molten agar before it sets. This means that they can grow throughout the agar and be easily counted. To many bacteria growing naturally in the environment, this exposure to the 45+oC temperatures can create a shock which can be lethal. As a result, many of these shocked bacteria will not grow in two days when the counting of the colonies is often done. All colonies are counted and weighted by the dilution factor to give the bacterial population as colony forming units (cfu) per ml.
Each colony is presumed to have been formed from one bacterial unit (which can consist of one or more cells in a clump, possibly of diverse but compatible types) to form a colony forming unit which grows distinctly within or upon the agar. To eliminate the temperature shock effect, another technique can be used whereby the diluted water is streaked across the surface of the agar. The food medium commonly used to grow bacteria for this test is the standard plate count agar medium which also tends to be too rich in nutrients for many of the environmental bacteria! The total plate count was introduced to obtain a better handle on the bacterial population.