chromogens

Chromogen is a molecule that can be converted into a colored product through a chemical reaction, most commonly oxidation, reduction, or enzymatic cleavage. The term is used across several disciplines, including chemistry, microbiology, and histology.

In microbiology, chromogenic substrates are linked to a color-producing dye. Bacteria expressing specific enzymes cleave the substrate, releasing a chromophore that produces a visible color, allowing rapid identification on chromogenic media. Common examples include X-Gal for beta-galactosidase (blue colonies) and various substrates for enzymes such as β-glucuronidase, phosphatase, or tryptophanase, yielding blue, pink, or yellow colonies depending on the substrate.

In histology and immunohistochemistry, chromogenic substrates are attached to enzymes such as horseradish peroxidase or alkaline phosphatase. The enzyme converts the substrate into a colored insoluble precipitate that marks the location of the target antigen. DAB, yielding brown, and AEC, yielding red, are widely used chromogenic substrates; newer chromogens offer a range of colors.

In chemical analysis and clinical chemistry, chromogens are substrates that produce a colored product in colorimetric assays. Examples include 3,3',5,5'-tetramethylbenzidine (TMB) for peroxidase assays, o-dianisidine, and ABTS, which produce blue, green, or brown colors depending on reaction conditions.

Limitations and considerations include color stability, sensitivity, substrate specificity, and safety concerns, as some chromogenic reagents are hazardous or require special handling. The term chromogen is sometimes used interchangeably with chromophore, though chromogen emphasizes the precursor form that becomes colored upon reaction.