In the majority of cases, when we want to embark on the process of identifying an
organism, we find that the organism of interest is usually mixed with many other organisms. For
example, to identify the pathogen that is causing a disease in a patient, we find that there are
many different species of bacteria present in the stool or tissue sample that we have obtained
from the patient. So, if we know that the organism is not in pure form, our first step would be to
separate it from all the others. This is accomplished in two stages.