Microbial Nutrition and Cultivation

Microbial Nutrition and Cultivation

We introduced the major groups of macromolecules found in living cells; the raw materials from which these are synthesised are ultimately derived from the organism’s environment in the form of nutrients (Table 4.1). These can be conveniently divided into those required in large quantities* (macronutrients) and those which are needed only in trace amounts (micronutrients or trace elements).

You will recall that carbon forms the central component of proteins, carbohydrates, nucleic acids and lipids; indeed, the living world is based on carbon, so it should come as no surprise that this is the most abundant element in all living cells, microbial or other-wise. Of the other macronutrients, nitrogen, oxygen, hydrogen, sulphur and phosphorus are also constituents of biological macromolecules, while the remainder (magnesium, potassium, sodium, calcium and iron in their ionised forms) are required in lesser quan-tities for a range of functions that will be described in due course. Micronutrients are all metal ions, and frequently serve as cofactors for enzymes.

All microorganisms must have a supply of the nutrients described above, but they show great versatility in the means they use to satisfy these requirements.

The metabolic processes by which microorganisms assimilate nutrients to make cellular material and derive energy will be reviewed. In the following sec-tion we briefly describe the role of each element, and the form in which it may be acquired.

Carbon is the central component of the biological macromolecules. Carbon incorporated into biosynthetic pathways may be derived from organic or inorganic sources ; some organisms can derive it from CO₂, while others require their carbon in ‘ready-made’, organic form.

Hydrogen is also a key component of macromolecules, and participates in energy gen-eration processes in most microorganisms. In autotrophs (see ‘Nutritional categories’ be-low), hydrogen is required to reduce carbon dioxide in the synthesis of macromolecules.

Oxygen is of central importance to the respiration of many microorganisms, but in its molecular form (O₂), it can be toxic to some forms. These obtain the oxygen they need for the synthesis of macromolecules from water.

Nitrogen is needed for the synthesis of proteins and nucleic acids, as well as for important molecules such as ATP. Microorganisms range in their demands for nitrogen from those that are able to assimilate (‘fix’) gaseous nitrogen (N₂) to those that require all 20 amino acids to be provided preformed. Between these two extremes come species that are able to assimilate nitrogen from an inorganic source such as nitrate, and those that utilise ammonium salts or urea as a nitrogen source.

Sulphur is required for the synthesis of proteins and vitamins, and in some types is involved in cellular respiration and photosynthesis. It may be derived from sulphur-containing amino acids (methionine, cysteine), sulphates and sulphides.

Phosphorus is taken up as inorganic phosphate, and is incorporated in this form into nucleic acids and phospholipids, as well as other molecules such as ATP.

Metals such as copper, iron and magnesium are re-quired as cofactors in enzyme reactions.

Many microorganisms are unable to synthesise certain organic compounds necessary for growth and must therefore be provided with them in their growth medium. These are termed growth factors (Table 4.2), of which three main groups can be identified: amino acids,purines and pyrimidines (required for nucleic acid synthesis) and vitamins. You will al-ready have read about the first two of these groups.. Vitamins are complex organic compounds required in very small amounts for the cell’s normal functioning. They are often either coenzymes or their precursors. Microorganisms vary greatly in their vitamin requirements. Many bacteria are completely self-sufficient, while protozoans, for example, generally need to be supplied with a wide range of these dietary supplements. A vitamin requirement may be absolute or partial; an organism may be able, for example, to synthesise enough of a vitamin to survive, but grow more vigorously if an additional supply is made available to it.