A gram stain is a type of microbiology or laboratory test that determines whether bacteria are present. It also determines whether bacteria are gram negative or gram positive. The difference between gram negative and gram positive bacteria can be important when determining appropriate treatment for an infection. Gram stains are performed on various types of specimens including blood, tissue, stool, and sputum.
In some instances, it isn’t clear whether an infection is bacterial, fungal, or viral. And these types of infections may be treated very differently. In addition, different types of bacteria may require different treatments. A gram stain lets physicians determine whether bacteria are causing an infection and what type of bacteria is present.
A gram stain has very few side effects. The types of side effects that can occur depend on the type of sample and method used for obtaining the sample. Collecting some types of samples, such as a stool sample, has no side effects. Gram stains that use a blood sample, meanwhile, can sometimes cause side effects due to the blood draw, such as bruising.
In order to perform a gram stain test, a sample of body fluid or tissue is needed. Collection methods vary depending on the type of sample. For sputum or mucus, a patient can cough up a sample into a specimen container. In other instances, a blood sample can be collected with a blood draw.
The main benefit of a gram stain is that it allows the doctor to determine the best type of treatment for an infection.
There are no known risks of performing a gram stain with a sample of urine, stool, or mucus. If a gram stain uses a blood or tissue sample, there are a few risks related to the blood draw or removal of the tissue, such as excess bleeding or infection (University of Rochester Medical Center).
There is often no special preparation needed to give a sample for a gram stain.
The prognosis for people who have a bacterial infection identified through a gram stain varies greatly. Gram stains that are performed early can help improve the ultimate outcome.
A negative gram stain test means that there are no bacteria present. A positive gram stain indicates the presence of bacteria.
Because of the staining technique, gram positive bacteria will be seen under the microscope as purple and gram negative bacteria will be viewed as pink. The shape, size, and quantity of bacteria present will also be evaluated.