Colors are powerful visual indicators for many aspects in science. It will be wonderful to know how do we add color to the microbial colonies. Microbial media that normally provide nutrient components for cultivation of microbes are suitably modified so that specific organisms generate enzymes that interact with the specific dyes. Here we merely use certain dyestuffs that are sensitive to these enzymes. Specific chromogenic substrates as they are called are integrated into the medium. Chromogenic substrates have two chemical groups, viz a site for action of an enzyme, and a chromophore group. Only at a minimal lethal level do they do their job without inhibiting the growth of the organism. Selection of the chromophore is critical in determining the desired color contrast against the substrate color tone.
If the cultured microbe has the ability to produce desired enzyme, the expected color of the colony reveals its signature. For convenience, this principle can be applied to solid media, liquid media, or even to paper discs or strips. The color becomes a potent tool in the hands of the microbiologist. The first chromogenic agar for detection of E.coli was invented and patented by Dr A Rambach in 1979. Later the chromogen technology turned to be promising tool for culturing and differentiation of organisms. Hence, the detection of microorganisms using chromogenic media is gradually increasing, despite the molecular biology techniques.
Over two decades, HiMedia and some other organizations have been developing a comprehensive range of chromogenic media for easy detection of various pathogens, to keep pace with the requirements of clinical diagnosis. Chromogenic media are available for detection of E.coli, Enterococci, Candida, Clostridium perfringens, Leuconostoc, Listeria, Bacillus, Staphylococcus aureus, MRSA, MRSE, VRE, ESBL, Salmonella, Vibrio, Pseudomonas, Cronobacter sakazakii, Klebsiella, ESBL producing Enterobacteriaceae, Carbapenem resistant Enterobacteriaceae, UTI pathogens, Acinetobacter, Group B Streptococci, Lactic acid bacteria, and Yeasts and molds .
Conventional culture media procedures, involves isolation on general media followed by selective medium. Isolated colonies are then picked for biochemical testing. Early isolation and differentiation is very critical in dealing with pathogenic microorganisms especially in case of food outbreak, clinical diagnosis, and epidemiology studies. Thus, chromogenic media are the solutions to overcome these limitation involved in conventional procedures. Major advantages include saves time and manpower; visual color differentiation makes easy identification; cost effective (as more types of media are required for conventional method); chromogen based media narrows down the identification to genus level making confirmation easier; versatility of its application for diverse types of microbes.