Coloured agar plates

by Moritz Hertel, Labtimes 05/2009

Stains agar plates with food colour: Moritz Hertel

Colours are used in nature and daily life as fast signals pointing at something without any further lengthy explanations. Just think of red and green traffic lights or biological colour signals. So why not use colours in the lab to distinguish agar plates containing different antibiotics?

After pouring, agar plates containing specific antibiotics are often marked with a ‘bar code’ by simply streaking up and down the plate stack with a water resistant pencil. One bar may stand e.g. for canamycin treated LB plates, while two may decode plates containing ampicillin. This fast and simple system is used by many groups; however, it is prone to errors. Quite often, only the lid is marked and the plates may be easily mixed up during cloning and bacterial selection with others containing the wrong antibiotics.

To unambiguously distinguish my agar plates, I stain canamycin plates with blue food colour and leave the ampicillin plates, that I use for blue/white screening, unstained. The blue food colour gives the canamycin plates a nice green colour that is recognisable at first glance. Usually, I add 300 microgrammes of blue food colour to one litre of agar solution, before or after autoclaving, to get green-coloured plates. Since canamycin plates are rarely used for blue/white screening, the green colour does not interfere with colony picking. Actually, colonies are equally well or even better visible on green plates than on unstained plates.

Many groups mark their agar plates with a ‘bar code’. If however the lids get mixed up you are in big trouble.

You may use any food colour available in your local food store or supermarket for staining your gels. So far, I have not encountered any problems when applying food colours from different manufacturers.

Agar plates stained with blue food colour are clearly to be distinguished from unstained plates.

(Moritz Hertel, Max Planck Institute for Ornithology, Seewiesen, Germany)

Last Changed: 10.11.2012