General purpose medium in microbiology is a substance that fosters the growth of most types of bacteria. According to the Michigan State University chemistry department, it is usually comprised of a mixture of protein digests and inorganic salts that is hardened by agar.
Tryptic soy agar and nutrient agar are both examples of general purpose medium. Sometimes the medium is infused with blood or serum. The purpose of the medium is to grow bacteria for culturing and identification purposes. Some media are considered selective since they only promote the growth of certain types of bacteria. The way in which a certain organism thrives and behaves in a particular medium helps the researcher or student in the microbiology laboratory identify the specific type of bacteria present.