In microbiology, bacteria are classified as either "Gram-positive" or "Gram-negative" depending on their appearance at the microscopic level following a particular staining procedure called the Gram stain. Originally developed by Hans Christian Gram, the Gram stain is always written with an upper case "G" as it refers to the name of a person.

Gram-positive bacteria appear dark blue or violet due to the crystal violet stain following the Gram stain procedure; Gram-negative bacteria, which cannot retain the crystal violet stain, appear red or pink due to the counterstain (usually safranin).

The reason bacteria are either Gram-positive or Gram-negative is due to the structure of their cell envelope. (The cell envelope is defined as the cell membrane and cell wall plus an outer membrane, if one is present.) Gram-positive bacteria, for example, retain the crystal violet due to the amount of peptidoglycan in the cell wall. It can be said therefore that the Gram-stain procedure separates bacteria into two broad categories based on structural differences in the cell envelope.

Further reading: Microbiology

Labels: Gram stain, Gram-negative bacteria, Gram-positive bacteria