A Gram stain is produced when a scientist colors a microorganism with a dye to emphasize certain structures within the organism. The Gram stain is the most widely used stain in microbiology to identify bacteria. Developed in 1884 by Danish physician Hans Christian Gram (1853–1938), Gram stain results can be combined with other information on a bacterium’s cellular structure and biochemical characteristics, generally allowing scientists to identify an unknown type of bacteria.

The Gram stain categorizes bacteria into a gram-positive or gram-negative group (often referred to as the “Gram reaction”). The most important structural difference between gram-negative and gram-positive bacteria is that gram-negative bacteria are enclosed by two layers—called a cytoplasmic membrane and an outer membrane. Between the two membranes is a thin layer that is linked to the outer membrane. In contrast, gram-positive bacteria have a thick layer, with its cell wall two to eight times as thick as the cell wall of gram-negative bacteria. (For more about stains, see the chapter “Biology in the Laboratory.”)