What microbes need

To grow microbes, we must give them what they need: carbon, energy, a supply of electrons, other elements, and water.

Carbon. Heterotrophs take carbon from organic compounds: all fungi and protozoa, and most bacteria. Autotrophs make their own organic compounds from CO₂: many bacteria and almost all algae.

Energy. Phototrophs get energy from light; chemotrophs from chemical reactions. Combining the two ideas:

• Photoautotrophs use light and CO₂ (algae, cyanobacteria).
• Chemoautotrophs oxidise inorganic molecules for energy and use CO₂.
• Chemoheterotrophs use organic molecules for both: most heterotrophs.
• Photoheterotrophs use light, but organic carbon (the green and purple non-sulphur bacteria).

Electrons. Lithotrophs take electrons from inorganic molecules (water, H₂S, ammonia); organotrophs from organic ones.

Elements.

• Macronutrients are needed in large amounts: C, O, H, N, S, P, which build the cell's big molecules (sulphur, for example, is part of some amino acids), and K, Ca, Mg, Fe, which help enzymes work.
• Micronutrients are needed only in traces: Mn, Zn, Co, Mo, Ni, Cu. They are usually present already, as impurities in the other ingredients and in water.

So a culture medium supplies:

• a carbon source: glucose, starch, molasses;
• a nitrogen source: ammonium salts, urea, plant or animal extracts;
• phosphorus (as phosphate) and sulphur (as sulphate);
• growth factors: amino acids, purines, pyrimidines and vitamins that a microbe cannot make for itself;
• water.

When a medium is stirred, it may foam. Anti-foams such as olive oil, sunflower oil or silicones are added to stop this; they are not nutrients. Knowing where a microbe lives in nature helps in choosing its medium.