Mordant

In microbiology, mordant is defined as a substance having a strong chemical affinity for the dye and the substrate i.e. microorganism. It basically helps the dye to fix on or in a cell. They are frequently used in staining process as they increases the interaction between the dye and cell, as a result of which, cells can be stained more strongly and can be easily visualized under microscope. Some commonly used mordants are – potassium alum, iodine, tannic acid etc.

In Gram staining procedure, iodine, used as a mordant, is an oxidizing agent. It causes the substance to oxidize more acid character thereby increases their affinity towards basic dye (like crystal violet). Iodine (I) along with crystal violet (CV) forms an insoluble complex CV-I which results in a dark purple color.