The purpose of this lab is to learn the principles of determining how many bacteria exist in a sample. This is a very important part of microbiology because in order to develop new drugs, it is crucial to understand the effect of the drug on the survival of the organism being tested. Knowing how many bacteria exist in a sample is used in many facets, and in this lab, we will perform experiments to learn about this concept. In this experiment, we will use the optical density readings to correctly calculate the number of bacteria present, as well as the growth rate of the sample. By the end of the lab we will be able to successfully be able to determine how many bacteria exist in a sample. The amount of bacteria will be dependent on different parameters, such as oxygen, temperature, ph, salinity, radiation, light, nutrients available, pressure and others.