Agar is a mixture used in laboratories to culture microbes. It contains a mixture of nutrients including agar, yeast or beef extract, salt and peptides. The nutrient rich mixture allows microbes to grow quickly and in isolation, aiding in the identification and classification of micro-organisms and the diagnosis of infections. Different formulations of nutrient agar exist, each tailored toward specific types of microbes.
In the laboratory, a scientist fills round, shallow plates with the nutrient agar mixture to prepare an isolated, sterile environment for microorganisms to breed. He then places a sample of bacteria into the plate, often by swirling a swab through the agar mixture. This transfers bacteria cultures into agar, allowing them to quickly eat their fill and begin to reproduce. In time, distinct colonies form inside the plate, and the form these colonies take serves as an important tool for identifying the microorganism involved. For instance, E. coli bacteria tend to form small round colonies while staphylococcus bacteria may form oblong, rectangular colonies with rounded ends.
In addition to identifying microorganisms, agar colonies determine the population of bacteria in a given sample. A sample rich in microbes shows colony growth much quicker than a sample with only a few bacteria.