Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 26

To count the bacteria in a culture or to isolate a pure culture, it is often necessary to obtain discrete colonies of the bacteria. However, because bacteria are so small, a concentrated culture contains billions of bacteria per milliliter. If such a culture is plated directly on a petri plate, the bacteria all grow together, and discrete colonies do not form. Serial dilutions offer a practical method for diluting solutions of bacteria before plating to obtain a measurable number of discrete colonies. The principle is that if dilutions are repeated in succession, they can be multiplied to produce the total dilution. For example, if a solution is diluted in three steps by adding 1 ml of the solution to 99 ml of water (one in a hundred), followed by adding 1 ml of this dilution to another 99 ml of water, and finally, by adding 1 ml of the second dilution to another 99 ml of water, the final dilution is 10^–2 x 10^–2 x 10^–2 = 10^–6, or one in a million. To achieve the same dilution in a single step, 1 ml of the original solution would have to be added to 1,000 liters (about 250 gallons) of water. Obviously, it is more convenient to handle three solutions of 100 ml each than to handle a solution of 250 gallons, which weighs about 2,000 lb!