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)

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.