Nutrient broth consists of peptone and meat extract dissolved in distilled water, according to Bio Lab Protocols. Once the media is mixed, the scientist adjusts the pH to 7.0 and sterilizes the broth using the heat and pressure of an autoclave or filters that remove bacteria.
Microbiologists use two primary types of media for growing cultures of bacteria, according to Science Prof Online. Liquid media are broths. They provide many advantages for the microbiologist. Nutrient broth provides the essential nutrients for bacteria to grow and remain healthy. However, broths have a disadvantage in that bacteria do not grow in cultures in the liquid. In broth, as the parent cell divides, the offspring become a part of the mix in the tube, making it difficult to determine if the culture is contaminated.
Microbiologists also use plates to grow cultures. Nutrient agar provides the same food for bacteria, but also includes agar, a gelatin like substance. Scientists pour the agar into plates before sterilization and spread the sample across the surface of the agar using a process they call streaking. Bacteria in the sample form colonies. Each colony grows from one parent cell, and all the bacteria in the colony are genetically identical. The scientist is able to count the number of cultures that grow and identify cultures that differ from the rest, according to Science Prof Online.