What are chromogenic media?

Chromogenic culture media provide a rapid and accurate method for isolating and enumerating target microorganisms based on the detection of specific enzyme activities. These activities cause a distinctive colour change to the target organism which allows for quick and easy identification without the need for advanced laboratory instruments or skill. Chromogenic media were originally formulated for clinical use, however, today it is widely used in food, water and environmental applications.

In this article, we will explore the inner workings of Neogen’s Harlequin® Chromogenic Media and also look at how it can be of benefit to your laboratory’s efficiency and costs.

How does it work?

The word ‘chromogenic’ can be broken down into two parts i.e. ‘chromo’ which means colour and ‘genic’ meaning producing or causing.  This term could not be more fitting as the chromogenic reaction is essentially a ‘colour producing’ reaction.

Let’s look at this in a little more detail:

Just like classic selective and differential media, chromogenic media are comprised of nutrients such as peptones, vitamins, minerals as well as inhibitors like antibiotics. The major difference is that the Harlequin® formulation incorporates a substrate that is bonded to a chromogen. The substrate selected would be specific to an enzyme that is produced by the target organism. Growth on the plate will remain colourless when the organism (and its enzyme) is absent. However, the presence of the relevant enzyme will result in the cleavage (splitting) of the substrate-chromogen bond. The splitting and subsequent release of the chromogen, resulting in a colour change, is what allows the target colonies to be clearly distinguishable from other colonies.


Does it remind you of differential media?

While being somewhat similar in terms of how the results are interpreted, it is important to note that chromogenic media involves a completely different mechanism compared to differential media which is primarily pH-based. Differential media utilises indicator dyes such as phenol red, neutral red and bromothymol blue to distinguish between groups of microorganisms based on their ability to metabolise certain ingredients. The breakdown of these ingredients by the growing microorganisms causes a pH shift in the media which results in a colour change. Although useful, differential media is still less specific than chromogenic media since there is a larger pool of microorganisms that metabolise the same ingredients, compared to the tiny pool that produce the same substrate-specific enzymes.

Is it really all it’s made out to be? Let’s have a look at the key benefits:

  • Quick and easy: interpretation by means of a visual check with the naked eye
  • Highly specific: only one confirmatory test is necessary i.e. biochemical or serological
  • Rapid and efficient: results within 24 hours due to the elimination of the sub-culturing steps, saving you time
  • Single solution: because it is selective, differential and chromogenic, colonies other than the target organism can also be identified on a single plate
  • Affordable: cost per gram is slightly higher than conventional media, however, since there are less steps involved in processing a sample, less resources are used overall

Final thoughts

From the above discussion, it’s clear that the benefits and flexibility offered by chromogenic media are quite significant. It’s difficult to come up with a reason for not using this technology if you currently test, or plan to test for the organisms that are targeted by Neogen’s Harlequin® range. Whether your lab requires a quicker, more sensitive method, or maybe just a little a bit of colour, we recommend this range as the quintessential solution to optimising your microorganism identification processes. 

Is your tested organism included in those targeted by Neogen's Harlequin range?