Gram Stain

A Gram stain is a test that checks to see if you have a bacterial infection. A sample is taken from the site of a suspected infection, such as your throat, lungs, genitals, or a skin wound. Gram stains may also be used to check for bacteria in certain body fluids, such as blood or urine.

If you have a bacterial infection, your sample will have bacteria in it. A Gram stain will group the bacteria by color and shape to help determine which kind of bacteria is causing your infection. There are two main categories of bacterial infections: Gram-positive and Gram-negative.

The categories are determined by how the bacteria react to the Gram stain test. After the sample is taken, the Gram stain is colored purple. Then, a solvent is added to the stain.

When the solvent combines with bacteria in a sample, the bacteria will either stay purple or turn pink or red. If the bacteria remain purple, they are Gram-positive. If the bacteria turn pink or red, they are Gram-negative.

The two categories cause different types of infections: Gram-positive infections include MRSA (methicillin-resistant Staphylococcus aureus) infections, strep infections, and toxic shock syndrome.; Gram-negative infections include salmonella infections, pneumonia, urinary tract infections, bacterial meningitis, and gonorrhea.; Knowing whether bacteria is Gram-positive or Gram-negative can help your health care provider identify the type of infection you have and which antibiotics will be most effective in treating it.

Why this test is done

You may need this test if you have symptoms of a bacterial infection. Pain, fever, and fatigue are common symptoms of many bacterial infections. Other symptoms will depend on the type of infection you have and where it's located in your body.

How to prepare

You don't need any special preparations for a Gram stain.

Sample type

Blood Urine Tissue/Biopsy

Other names for this test

Gram's stain