番红O，也称碱性红2，是一种碱性、阳离子和变色偶氮染料，用作生物染色剂。它特有的偶氮发色团包含吩嗪，这是一种平面三环二苯并吡嗪，由两个非平面苯环通过两个氮原子连接而成。发色团系统包括离域的正电荷、芳香和醌型结构的交替以及可重氮化的胺基。c-甲基化使该染料具有适中的整体尺寸。 番红的三甲基阳离子形式是亲脂性的，而二甲基形式是弱亲水性的。该染料市售形式为氯化物。

In microbiology, Gram staining allows fast differentiation of bacteria as Gram-positive or Gram-negative. The mureine structure of the bacteria cell wall is the basis of the color affinity. In the first step, bacteria will be stained with crystal violet, an aniline dye. After the treatment with iodine solution (Lugol’s solution), a dye-iodine complex will form. During the decolorizing step, this complex stays in the multilayer mureine structure of the cell wall of Gram-positive bacteria and appears blue-violet. Gram-negative bacteria, by contrast, have a cell wall consisting of a single-layered murein structure, and correspondingly re-release the staining dye after treatment with the decoloring solution. Gram-negative bacteria will be counterstained with safranin solution and will then appear pink to red.