There are six kinds of nutrients in microorganisms, namely water, carbon source, nitrogen source, inorganic salt, growth factor and energy.

1. water

Water is an important part of microorganisms and plays an important role in metabolism. There are two forms of water in cells: bound water and free water. Bound water combines with solutes or other molecules, so it is difficult to use. Free water (or unbound water) can be used by microorganisms.

2. Carbon source

Carbon accounts for about 50% of cell dry matter, so microorganisms have the greatest demand for carbon. All nutrients or metabolites that are the source of carbon skeleton in microbial cell structure are called carbon sources.

There are many kinds of carbon source substances as microbial nutrition, ranging from simple inorganic substances (CO2 and carbonate) to complex organic carbon-containing compounds (sugar, sugar derivatives, lipids, alcohols, organic acids, aromatic compounds and various carbon-containing compounds, etc. However, different microorganisms have different abilities to use carbon sources. Pseudomonas can use more than 90 kinds of carbon sources, methane-oxidizing bacteria can only use two kinds of organic substances: methane and methanol, and some cellulose-decomposing bacteria can only use cellulose.

Most microorganisms are heterotrophic and use organic compounds as carbon sources. There are many kinds of carbon sources that can be used, among which sugar is the best.

Heterotrophic microorganisms use carbon sources to form cellular substances or provide energy for physiological activities through a series of complex chemical reactions in vivo. Therefore, carbon source is often also an energy substance.

Autotrophic bacteria use CO2 and carbonate as the sole or main carbon source. CO2 is a completely oxidized substance, and its transformation into cellular components is a reduction process. Therefore, these microorganisms need to obtain energy from light or other inorganic oxidation at the same time. The carbon source and energy source of these microorganisms belong to different substances respectively.

3. Nitrogen source

Any nutrient substance derived from nitrogen in substances or metabolites that constitute microbial cells is called nitrogen source. The content of nitrogen in cell dry matter is second only to carbon and oxygen. Nitrogen is an important element of nucleic acid and protein, and plays an important role in the growth and development of microorganisms. From molecular N2 to complex nitrogen-containing compounds, they can be used by different microorganisms, and the nitrogen sources available to different types of microorganisms are very different.

Nitrogen-fixing microorganisms can use molecular N2 to synthesize amino acids and protein, as well as inorganic nitrogen and organic nitride, but in this case, they lose their nitrogen-fixing ability. In addition, some photosynthetic bacteria, cyanobacteria and fungi also have nitrogen fixation.

Many saprophytic bacteria and pathogenic bacteria of animals and plants cannot fix nitrogen, so ammonium salts or other nitrogen-containing salts are generally used as nitrogen sources. Nitrate must be reduced to NH+4 before it can be used in biosynthesis. Microorganisms with inorganic nitrides as the sole nitrogen source can use ammonium salts, but not all microorganisms can use nitrates.

Organic nitrogen sources include peptone, beef paste, yeast extract, corn syrup, etc. Bean cake powder, peanut cake powder and fish meal can be used as nitrogen sources in industry. Nitrogen in organic nitrogen sources is often protein or its degradation products.

Nitrogen sources generally only provide raw materials for the synthesis of cytoplasm and other structures in cells, and are not used as energy sources. Only a few bacteria, such as nitrifying bacteria, use ammonium salts and nitrates as nitrogen sources and energy sources.

4. Inorganic salts

Inorganic salts are also indispensable nutrients for microbial growth. Their main functions are:

(1) cell composition;

② As an enzyme component;

③ Keep the activity of enzyme;

④ Regulating the osmotic pressure, hydrogen ion concentration and redox potential of cells;

⑤ As an energy source for some autotrophic bacteria.

Salts such as phosphorus, sulfur, potassium, sodium, calcium and magnesium participate in the composition of cell structure and are related to the functions of energy transfer and cell permeability regulation. Microorganisms have a great demand for them (10-4 ~ 10-3 mol/L), which are called "macro elements". Without them, microorganisms cannot grow. Iron and manganese. The demand is small (10-8 ~ 10-6 mol/L), so it is called "trace elements". Different microorganisms have different requirements for the above elements. Iron is between macro elements and trace elements.

When preparing culture medium, a large number of elements can be supplemented by adding relevant chemical reagents, among which K2HPO4 and MgSO4 are preferred, which can provide a large number of elements: K, P, S and Mg. Trace elements exist as impurities in some chemical reagents, natural water and natural culture medium components, and also exist in a small amount in experimental supplies such as glassware, so it is not necessary to add them separately.

5. Growth factors

Some heterotrophic microorganisms can't grow or grow badly in the medium of common carbon source, nitrogen source and inorganic salt. When some tissues (or cells) are added to the culture medium, these microorganisms grow well, indicating that these tissues or cells contain nutritional factors necessary for the growth of these microorganisms, which are called growth factors.

Growth factors can be defined as organic substances that cannot be synthesized by ordinary carbon and nitrogen sources, but need to be added in small amounts to meet their needs, including amino acids, vitamins, purines, pyrimidines and their derivatives, and sometimes some fatty acids and other membrane components% a.