Microbiology tests are used to find out what type of bacteria or fungus is causing an infection. When the type of bacteria has been cultured (grown), it is checked to see how it reacts against the presence of various antibiotics. If certain antibiotics slow down the growth or kill most of the bacteria then it is likely that this type will be selected to fight your infection. If the bacterial growth is unchanged then another more sensitive antibiotic will be selected. This explains why doctors will change some antibiotic treatments.
All human secretions such as blood, urine, stools, phleghm, ooze from wounds, etc. can be cultured. Some bacteria can take up to a week to be cultured for identification. This is why the test results may not be immediately available.