A Gram stain may help identify the type of bacteria that is causing an infection. For a lung infection, your doctor will take a sample of mucus and stain it. This makes bacteria easier to see when viewed through a microscope. The shape and color of the stained bacteria may help identify the type of bacteria that is causing the infection.

The sample of mucus then may be cultured in the lab to see if the bacteria grow. If bacteria from the mucus grow, other tests can be used to predict which antibiotics will work the best to treat the infection. This is called sensitivity testing and is done to help prevent bacteria from becoming drug-resistant.

These tests may be done when a doctor needs to identify the organism that is causing the infection. By doing so, the doctor can choose the antibiotic that is most likely to be effective. In most cases, unless there is a clear need to identify the organism, these tests will not be done.