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DM Agar

Sometimes we want to plate colonies on a defined minimal agar. For that, we generally use the same basic composition as for DM liquid medium, except that we: add agar (of course), increase the sugar concentration (so that colonies are robust), and sometimes use arabinose (instead of glucose).

We use glucose when we want to examine the colonies on the standard minimal medium, and in that case we call the medium MG agar. But we use arabinose when we want to obtain a new Ara+ mutant from an Ara- parent (or check Ara phenotypes), in which case we call the medium MA agar.

Depending on the genotype, it may be necessary to incubate MG and MA agar plates for two days at 37C before colonies are apparent. (Sometimes one will see micro-colonies produced by Ara- genotypes on MA plates that are incubated for two days. We think this represents growth on other carbon sources in the agar or the arabinose. With appropriate Ara- and Ara+ controls, one can avoid confusing these with real colonies.)

The following recipe is for 1 liter of MG agar, which should make about 45 plates.

Potassium phosphate (dibasic trihydrate) 7 g
Potassium phosphate (monobasic anhydrous) 2 g
Ammonium sulfate 1 g
Sodium citrate 0.5 g

dH20 1000 ml

Agar 16 g
Antifoam (5%) 1 ml

Glucose 4 g

Split the water into three parts: autoclave the salts, agar, and sugar separately. Combine the three parts after autoclaving; then add from stock solutions:

Magnesium sulfate (10%) 1 ml
Thiamine (= vitamin B1: 0.2%) 1 ml

For MA agar, simply replace the Glucose with am equal amount of L(+)Arabinose.