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 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 how the bacteria react 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 stay purple, they are Gram-positive. If the bacteria turns pink or red, they are Gram-negative.