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Gram Stain

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Also known as: Gram's Stain
Formal name: Gram Stain

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The Test Sample

What is being tested?

A Gram stain is a laboratory procedure used to detect the presence of bacteria and sometimes fungi in a sample taken from the site of a suspected infection. It gives relatively quick results as to whether bacteria or fungi are present and, if so, the general type(s).

The Gram stain involves applying a sample from the infected area onto a glass slide and allowing it to dry. The slide is then treated with a special stain and examined under a microscope by a trained laboratorian. Any bacteria that may be present are categorized by color and shape during the microscopic evaluation:

  • Color — typically bacteria may be either "Gram positive" (purple) or "Gram negative" (pink)
  • Shape — the most common shapes include round (cocci) or rod-shaped (bacilli)

Illustration of the differences between a positive and negative gram stain

Additional information may be obtained by observing the groupings of the bacteria on the slide, such as cocci that are present singly, in pairs, in groups of four, in clusters or in chains, or bacilli that are thick, thin, short, long, or have enlarged spores on one end. Any bacteria that are present within white blood cells (intracellular) are also noted.

The Gram stain color and the bacterial shape give clues as to what bacteria might be causing the infection. One example of gram-positive cocci is Staphylococcus aureus, the bacteria associated with staph infections. An example of gram-negative bacteria is Escherichia coli, the cause of many urinary tract infections.

Fungi (in the form of yeasts or molds) can also be initially recognized with the Gram stain, but viruses cannot be seen with a Gram stain.

Though Gram stains are useful as initial tests for detecting and identifying general types of bacteria or fungi, results are usually considered preliminary. Results of a culture and/or other tests such as antigen, antibody, or molecular testing for particular types of bacteria are necessary to confirm a diagnosis. Sometimes, susceptibility testing is necessary to determine which antibiotic will be most effective in treating the infection.

How is the sample collected for testing?

Several different types of samples may be collected for Gram stains. Some samples are collected using sterile swabs to obtain cells or exudate at the site of a suspected infection. Other samples, such as urine or sputum, may be collected in a sterile container. Some body fluids may be collected by needle and syringe. A swab may be used to collect a sample of bacteria grown and isolated in a culture.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.