Chromogenic Culture Media: Principle, Composition and Results

Chromogenic  Agar Media are culture medium used to isolate, identify, and differentiate specific microorganism from a heterogeneous population. The medium contains chromogenic substrate which is utilized by the microorganism to give colored colonies that is specific for each microorganism. Depending on the color of the result, the presence or absence of 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 a specific enzymatic activity of the microorganism) and a chromophore.

Principle of Chromogenic Agar Medium (source: chromagar.com)

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 colour. 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 solidifers (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.

Chromogenic Agar Media (Source:biomerieux)

Advantages of Use of Chromogenic Agar Media:

  1. Less Labor intensive and more economic:
    Though people perceive chromogenic media as expensive alternative, use of a single chromogenic medium rather two-three selective ones, reduces the  cost of sample processing. Chromogenic media may eliminate the need of subculture and further biochemical test 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 better outcome for the patients but also helps in prevention and spread of infections.
  3. Easy identification: On chromogenic agar medium, target colonies of specific microorganisms can be recognized by their colour 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

S.NTarget organismMedia Available
1.CandidaCHROMagar Candida, Albicans ID, CandiSelect , Candida ID, Pourmedia Vi Candida, Chromogenic Candida agar , Brilliance Candida Agar  and HiCrome Candida differential agar. 
2.Pseudomonas aeruginosaChrom ID Pseudomonas 
3.Urinary Tract pathogensCHROMagar Orientation, UriSelect medium, Rainbow Agar UTI medium , Chromogenic UTI medium , USA agar , Harlequin CLED and Urichrom agar 
4.Bacillus cereusBacillus cereus ChromoSelect Agar 
5.Escherichia coli serotype O157:H7 CHROMagar O157
6.Methicillin resistant Staphylococcus aureus (MRSA)CHROMagar MRSA II 
7.Carbapenem-resistant EnterobacteriaceaeBrilliance CRE Agar
About Nisha Rijal 18 Articles
I am working as Microbiologist in National Public Health Laboratory (NPHL), government national reference laboratory under the Department of health services (DoHS), Nepal. Key areas of my work lies in Bacteriology, especially in Antimicrobial resistance.

1 Comment

  1. Iam a medical laboratory Technician , working in national Public Health laboratory (NPHL)
    my question is , what are the most challenges in antibiotic resistance to Gram negative Bacteria ?

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