Gram Stain

A Gram stain is used to determine if bacteria are present in an area of the body that is normally sterile, such as spinal fluid. A sample from the infected area is smeared on a glass slide and allowed to dry. A series of stains and a decolorizer is applied. The stained slide is then examined under a microscope where bacteria appear either purple (gram positive) or pink (gram negative). The test is named for Dr. Christian Gram, who invented the process.

A Gram stain can predict the type of bacteria causing an infection, such as pneumococcal pneumonia or a staphylococcal abscess. Viruses cannot be seen with a Gram stain since they lack the cell wall, which takes up the stain.

Usually, samples are collected using sterile swabs to obtain cells or exudate at the site of suspected infection. Body fluids may be collected in sterile containers or by needle and syringe.