Gram Stain

A Gram stain is a laboratory procedure used to detect the presence of microorganisms, especially bacteria, in a sample taken from the site of a suspected infection. It gives relatively quick results as to the general type of bacteria that may be present. 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 the patient's white blood cell (intracellular) are also noted. The Gram stain color and the bacterial shape give clues as to what microorganism might be causing the infection. Examples of gram-positive cocci include Staphylococcus aureus, the bacterium associated with staph infections. An example of gram-negative bacterium is Escherichia coli, the cause of many urinary tract infections. Fungi (in the form of yeasts or molds) can also be initially identified with the Gram stain, but viruses cannot be seen with a Gram stain.

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 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.

No test preparation is needed.