Chromogenic

Chromogenic describes substances or processes that produce color. The term is used across chemistry, biochemistry, and microbiology to refer to compounds or reactions that generate a visible color, typically through enzymatic action that releases a chromophore.

In microbiology, chromogenic media contain substrates that release colored products when cleaved by specific bacterial enzymes. Colonies appear in distinct colors that help differentiate organisms, enabling rapid presumptive identification and reducing the need for additional testing.

A classic example is X-gal, a chromogenic substrate for beta-galactosidase. When the enzyme acts on X-gal, a blue indigo dye is produced, yielding blue colonies on selective media. Similar substrates yield other colors for different enzymes, enabling multiplex identification on a single plate.

Chromogenic substrates also underpin many enzyme assays in biochemistry and clinical chemistry. For instance, p-nitrophenyl phosphate is colorimetric; hydrolysis releases p-nitrophenol, which turns yellow in alkaline solutions, allowing quantification by absorbance measurements.

In addition to microbiology, chromogenic reagents are used in histology and diagnostic tests, where colored products indicate the presence of a target molecule or enzyme. Limitations include color interpretation challenges, substrate specificity, and potential interference from media color or lighting.

See also: chromogen, chromogenic medium, chromogenic substrate, colorimetric assay.