Gram stain: What is it? Mechanism, Indications, Administration, Results, Benefits and Forecasts

It is a laboratory technique that is routinely used in microbiological studies of bacteria.

Also called the Gram method , it is a staining method used to distinguish and classify bacterial species into two large groups (gram-positive and gram-negative). The name comes from the Danish bacteriologist Hans Christian Gram, who developed the technique.


Gram stain differentiates bacteria by the chemical and physical properties of their cell walls by detecting peptidoglycan, which is present in the cell wall of Gram-positive bacteria.

Gram-negative cells also contain peptidoglycan, but a very small layer dissolves when alcohol is added. This is the reason why the cell loses its initial color from the primary spot.

The Gram-positive bacteria retain the crystal violet dye and, therefore, are stained with violet, while the Gram-negative bacteria do not; after washing, a counterstain is added that will stain these Gram-negative bacteria of a pink color.

Both the Gram-positive and the Gram-negative bacteria take the contratinction. Contrast staining, however, is not seen in Gram-positive bacteria due to the darker crystalline violet spot.

Gram stain is almost always the first step in the preliminary identification of a bacterial organism.

While Gram stain is a valuable diagnostic tool in clinical and research settings, not all bacteria can be definitively classified by this technique. This results in groups gram-variable and gram-indeterminate.

If your doctor suspects that you have an infection, they may order a culture and gram stain to check for bacteria. If bacteria are present, this test can also help your doctor know if the bacteria are gram negative or gram positive.

Gram stains can be performed on several sample types, which include:

  • Blood.
  • Tissue.
  • Urine.
  • Sputum.
  • Stool


Your doctor may order a Gram stain if you have symptoms of an infection. They may not know if the infection is bacterial, viral, fungal, or parasitic. These types of infections are usually treated differently.

Your doctor may use a Gram stain to know if the bacteria are responsible for your symptoms and what types of bacteria are present. They may also request other tests to help diagnose their illness.

How is a Gram stain given?

To perform a Gram stain, your doctor must collect a sample of body fluid or tissue for analysis.

Collection methods will vary according to the type of sample they need:

  • To collect a sputum sample, you may be asked to cough a little into a sample container.
  • To collect a urine sample, you may be asked to urinate in a sample cup.
  • To collect a blood sample, they can perform a blood draw.

After they have collected the sample, they will send it to a laboratory for testing. A technician will use a special staining technique to make it easier to see the bacteria under a microscope.

What do the results of a Gram stain mean?

If the results of your gram stain are negative, it means that no bacteria were found in your sample. If they are positive, it means that there were bacteria present.

Due to the staining technique used, gram-positive bacteria will appear purple under the microscope and gram-negative bacteria will appear pink. The shape, size and number of bacteria present will also provide information about your infection.

What are the risks?

There are no known risks associated with performing a Gram stain on a sample of urine, feces or mucus.

If your doctor orders a Gram stain on a sample of blood or tissue, collecting the sample carries some risks. For example, you may experience bruising, bleeding or infection as a result of a blood draw.

What are the benefits of a Gram stain?

The main benefit of Gram staining is that it helps your doctor know if you have a bacterial infection and determines what type of bacteria is causing it. This can help your doctor determine an effective treatment plan.

Forecast after performing the test

Your treatment plan and perspective will vary based on your diagnosis.

If you are diagnosed with a bacterial infection, your doctor may prescribe antibiotics. If your gram stain is negative for bacteria that cause disease, your doctor may order other tests to find out what is causing your symptoms.