Chromogenic Culture Media: Principle, Composition, Results

Chromogenic culture media are used to isolate, identify, and differentiate specific microorganisms from a heterogeneous population. The medium contains chromogenic substrate which is utilized by the microorganisms to give colored colonies that is specific for each microorganism. Depending on the color of the result, the presence or absence of the target organism is determined and also accurately differentiated from others.

Classical culture media are based on the principle of change in color of the pH indicator whereas Chromogenic media are based on enzymatic utilization of chromogenic substrates.

Principle of Chromogenic Media

Chromogenic media contains soluble colorless molecules called chromogens. Chromogens are composed of two parts: a substrate (which is the target of specific enzymatic activity of the microorganism) and a chromophore.

When the bond between the substrate and chromophore is split by a specific enzyme produced by the target microorganism, chromophore is released. In its unconjugated form, the chromophore shows distinctive color. Due to reduced solubility, chromophore forms a precipitate that imparts unique color to the colony.

Composition of Chromogenic Agar Media

Like traditional culture media, chromogenic media also contains nutrients such as peptones, amino acids, yeast extract, minerals, vitamins, and solidifiers (agar). Depending on the purpose chromogenic media may also contain inhibitors. Unlike traditional media, they contain chromogenic substrates or chromogens. These chromogenic substrates such as ONPG, X-Gal, or X-Glu, together with a specified selectivity of the medium, is the simple principle behind chromogenic media.

Advantages of Use of Chromogenic Agar Media:

1. Less Labor intensive and more economic:
   Though people perceive chromogenic media as an expensive alternative, the use of a single chromogenic medium rather two-three selective ones reduces the cost of sample processing. Chromogenic media may eliminate the need for subculture and further biochemical tests for the identification of the isolates. 
2. Less time consuming:  As chromogenic media eliminate various steps of sample processing (e.g. subculturing, biochemical testing) results are available within 24 hours as compared to 48 hours or more by conventional methods.  Timely diagnosis not only ensures a better outcome for the patients but also helps in the prevention and spread of infections.
3. Easy identification: On chromogenic agar medium, target colonies of specific microorganisms can be recognized by their color at a glance. No specialized equipment needed.

Examples of Chromogenic Agar Medium

Most of the commercially available chromogenic media utilize substrates like indoxyl and its halogenated derivatives, metal chelators like Esculin, 8‐Hydroxyquinoline, substrates for β‐glucosidase enzyme etc. Based on these a number of companies have formulated several variations depending on need and usage.

Following are some of the examples

References

1. Perry J. D. (2017). A Decade of Development of Chromogenic Culture Media for Clinical Microbiology in an Era of Molecular Diagnostics. Clinical microbiology reviews, 30(2), 449–479. https://doi.org/10.1128/CMR.00097-16 
2. Garcia, B. L. N., Fidelis, C. E., Freu, G., Granja, B. M., & Dos Santos, M. V. (2021). Evaluation of Chromogenic Culture Media for Rapid Identification of Gram-Positive Bacteria Causing Mastitis. Frontiers in veterinary science, 8, 662201. https://doi.org/10.3389/fvets.2021.662201