Gram Stain

What is a Gram stain?

A Gram stain is a test that checks for bacteria at the site of a suspected infection such as the throat, lungs, genitals, or in skin wounds. Gram stains may also be used to check for bacteria in certain body fluids, such as blood or urine.

There are two main categories of bacterial infections: Gram-positive and Gram-negative. The categories are diagnosed based on the how the bacteria reacts to the Gram stain. A Gram stain is colored purple. When the stain combines with bacteria in a sample, the bacteria will either stay purple or turn pink or red. If the bacteria stays purple, they are Gram-positive. If the bacteria turns pink or red, they are Gram-negative. The two categories cause different types of infections:

• Gram-positive infections include methicillin-resistant Staphylococcus aureus (MRSA), strep infections, and toxic shock.
• Gram-negative infections include salmonella, pneumonia, urinary tract infections, and gonorrhea.

Knowing whether bacteria is Gram-positive or Gram-negative can help your provider identify the type of infection you have and which antibiotics will be most effective in treating it.

Other names: Gram's stain

What is it used for?

A Gram stain is most often used to find out if you have a bacterial infection. If you do, the test will show if your infection is Gram-positive or Gram-negative.

A Gram stain may also be used to diagnose fungal infections.

Why do I need a Gram stain?

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 in the body it is located.

What happens during a Gram stain?

Your health care provider will need to take a sample from the site of a suspected infection or from certain body fluids, depending on what type of infection you may have. The most common types of Gram stain tests are listed below.

Wound sample:

• A provider will use a special swab to collect a sample from the site of your wound.

Blood test:

• A provider will take a sample of blood from a vein in your arm.

Urine test:

• You will provide a sterile sample of urine in a cup, as instructed by your health care provider.

Throat culture:

• Your health care provider will insert a special swab into your mouth to take a sample from the back of the throat and tonsils.

Sputum culture. Sputum is a thick mucus that is coughed up from the lungs. It is different from spit or saliva.

• Your health care provider will ask you to cough up sputum into a special cup, or a special swab may be used to take a sample from your nose.

Will I need to do anything to prepare for the test?

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

Are there any risks to the test?

There is no risk to having a swab, sputum, or urine test.

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?

Your sample will be placed on a slide and treated with the Gram stain. A laboratory professional will examine the slide under a microscope. If no bacteria were found, it means you probably don't have a bacterial infection or there weren't enough bacteria in the sample.

If bacteria were found, it will have certain qualities may provide important information about your infection:

• If the bacteria was colored purple, it means you likely have a Gram-positive infection.
• If the bacteria was colored pink or red, it means you likely have a Gram-negative infection.

Your results will also include information about the shape of the bacteria in your sample. Most bacteria are either round (known as cocci) or rod-shaped (known as bacilli). The shape can provide more information about the type of infection you have.

Although your results may not identify the exact type of bacteria in your sample, they can help your provider get closer to finding out what's causing your illness and how best to treat it. You may need more tests, such as a bacteria culture, to confirm what type of bacteria it is.

Gram stain results may also show whether you have a fungal infection. The results may show what category of fungal infection you have: yeast or mold. But you may need more tests to find out which specific fungal infection you have.

If you have questions about your results, talk to your health care provider.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about a Gram stain?

If you are diagnosed with a bacterial infection, you will probably be prescribed antibiotics. It's important to take your medicine as prescribed, even if your symptoms are mild. This can prevent your infection from getting worse and causing serious complications.