Lugol solution

Lugol′s solution is a ready-to-use solution that comprises 6% iodine in 4% potassium iodide. It is commonly used as a mordant for Gram staining in bacteriology. It is used for staining bacterial target structures for diagnostic purposes by fixing, staining, counterstaining, and mounting bacteriological specimen materials.

Iodine diffuses rapidly through hydrophilic and hydrophobic domains of tissues and loosely binds to proteins, staining them brown. It reacts irreversibly with unsaturated lipid linkages and with tyrosine and histidine aromatic rings.

In bacteriology, Gram staining allows rapid differentiation between Gram-positive and Gram-negative bacteria. The mureine structure of the bacterial cell wall is the basis of the color affinity. Bacteria are first stained with the primary stain, crystal violet. Lugol solution is added thereafter as the mordant, forming crystal violet-iodine complexes. During the decolorization, these complexes stay entrapped in the multilayer mureine structure of the cell wall of Gram-positive bacteria which appear blue-violet. Gram-negative bacteria have a single-layered cell wall that does not retain the dye-iodine complexes and are counterstained with safranine solution, appearing pink.