Culture Media Contains Nutrients for Culture Growth

Since Pasteur and Koch, microbiologists have tried to mimic the natural environment of bacteria in order to grow them in a laboratory. Beef broth is one medium used for cultivation of bacteria, or nutrient broth which contains water, beef extract and peptine, a nitrogen preparation from plant or animal sources. Agar is used to solidify the medium, upon the addition of this, the product is called nutrient agar.

"Agar is a polysaccharide derived from marine red algae. Introduced by the school of bacteriology of Robert Koch, agar is a unique colloid that remains liquid until cooled top below approximately 36˚ C. This allows for mixing blood with culture media for determination of haemolytic reactions. The solidified medium can be used to cultivate bacteria, isolate pure cultures, or accomplish other tasks, such as a medium for measuring bacterial growth. Once solidified, agar will remain solid at room temperature. It will not melt until it reaches a temperature of 85˚C, making the material excellent for growing thermophilic bacteria on nutrient agar. Agar adds no macronutrients to the nutrient medium. Sometimes it is valuable to use semi-solid medium, such as when testing bacterial motility. In this case, a lower concentration of agar is added to the medium to make it stiff but not as solid as nutrient agar"

  Nutrient broth and nutrient agar are examples of chemically undefined medium or complex medium.  We cannot be certain of the exact components or the quantity thus it is called complex. We do not precisely know what carbon and energy sources are present nor what other factors that facilitates growth are present. Complex media are commonly used in the teaching laboratory. Because you simply want to grow bacteria and are not concerned what specific nutrients are needed to accomplish this.

Another type of medium is a chemically defined medium or synthetic medium. In this type of medium, the components are known. This is commonly used when understanding the growth conditions necessary for bacterial growth.