Chromogens are enzyme substrates that release a colored dye when hydrolysed, coloring the target organism as a result and allowing it to be differentiated from other commensal flora present in food. There are a wide range of chromogenic substrates now available for a range of enzymes, including glycosidases, esterases and virulence enzymes like phospholipases to detect many food pathogens such as Cronobacter sakazakii, E. coli and Salmonella.
With conventional culture media, the identification of food pathogens can take several days. This can be due to difficulty differentiating bacteria based on morphology and poor biochemical indicators such as pH dyes resulting in prolonged testing of false positives. The arrival of chromogens has allowed the development of rapid culture methods: for example, the Thermo Scientific Oxoid Salmonella Precis* method allows the identification of Salmonella up to 3 days faster than standard methods. This method involves an 18 hour enrichment which can be plated straight onto Brilliance Salmonella agar and identified in 24 hours; the presence of two chromogens colors Salmonella purple and other Enterobacteriacae appear blue or colorless, making differentiation much easier and reducing the time to result.
There are many drawbacks associated with rapid immunological and molecular methods such as inhibitory food matrices and a need for enrichment steps, which makes culture media a preferred method for food testing in many instances. With the development of novel chromogens targeting unexploited enzymes in the future, the range of chromogenic media is set to expand and replace some of the substandard and dated culture media that we still rely on for some organisms, such as Legionella spp.
* Also known as the Salmonella Rapid Culture Method
- Thermo Scientific Oxoid Salmonella Precis datasheet
Do you use culture media in your work? If you’d like to know more about it, or about the Precis method, then get in touch!