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The Forbes.com article Bacteria Become Art Tools In Annual Agar Art Competition says:

...People’s choice winner Zita Pöstényi, a microbiologist at SYNLAB Hungary Ltd, used three agar plates and four different bacteria to create a pattern inspired by Hungarian folk art.

[...]

Pöstényi used another trick to create her colorful plates. “I used a special media for this drawing,” she says, “It contains a chromogenic mixture which enables the detection of activities of specific enzymes.” Thanks to this special mixture, the four bacteria species she used all showed up in different colors, which allowed her to create the traditional folk art pattern.

Usually, these unique colors tells researchers something interesting about the bacteria, but for the ASM Agar Art competition, these colorful features have become the medium.

Question: How does a "chromogenic mixture" result in different bacteria showing up in different colors in such a vivid way?

enter image description here

American Society for Microbiology Agar Art 2019 Contest, People's Choice. "Hungarian Folk Art," Zita ... [+]ASM / ZITA PÖSTÉNYI

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She is referring to a mixture of chromogenic substrates: molecules which are more or less colourless until cleaved by a specific enzyme, releasing a coloured product.

For example X-gal is colourless until the enzyme beta-galactosidase hydrolyzes it, releasing a blue coloured product. If you spread a mixture of bacteria onto a plate, with some expressing beta-galactosidase and some not, you will see both blue and colourless colonies, as seen here:

enter image description here [source]

Chromogenic substrates are often used to create differential media that allow the differentiation of different species of bacteria. This plate, for example, contains several different chromogenic substrates (a mixture, if you will) for identifying the bacteria commonly found in urinary tract infections:

enter image description here

[source]

If you wanted to make a picture with this plate, you could “paint” with Enterobacter spp. for blue, Staphylococcus aureus for yellow and S. saprophyticus for pink. Each species produces a specific enzyme that acts on a specific chromogenic substrate in the plate, changing its colour.

Alternatively, you could express the desired enzymes recombinantly in one species.

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    $\begingroup$ Thanks for the authoritative, well-sourced and colorful answer! With a CMY mixture and bacteria with variable expression rates of three enzymes one could make quite a palate. $\endgroup$ – uhoh Dec 26 '19 at 23:01

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