Microbial Nutrition and Cultivation

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In Chapter 2 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 otherwise. 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 quantities 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 Chapter 6. In the following section 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 we discussed in Chapter 2. Carbon incorporated into biosynthetic pathways may be derived from organic or inorganic sources (see below); 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 generation processes in most microorganisms. In autotrophs (see 'Nutritional categories' below), 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 (see Chapter 5). These obtain the oxygen they need for the synthesis of macromolecules from water.

* Everything is relative in the microbial world; a typical bacterial cell weighs around three tenmillion millionths (3 x 10-13) of a gram!

Table 4.1 Elements found in living organisms

Form in which

Occurrence in

Element

usually supplied

biological systems

Macronutrients

Carbon (C)

CO2, organic compounds

Component of all organic

molecules, CO2

Hydrogen (H)

H2O, organic compounds

Component of biological

molecules, H+ released by

acids

Oxygen (O)

O2, H2O, organic compounds

Component of biological

molecules; required for

aerobic metabolism

Nitrogen (N)

NH3, NO3-, N2, organic N

Component of proteins,

compounds

nucleic acids

Sulphur (S)

H2S, SO42-, organic S

Component of proteins;

compounds

energy source for some

bacteria

Phosphorus (P)

PO43-

Found in nucleic acids, ATP,

phospholipids

Potassium (K)

In solution as K+

Important intracellular ion

Sodium (Na)

In solution as Na+

Important extracellular ion

Chlorine (Cl)

In solution as Cl-

Important extracellular ion

Calcium (Ca)

In solution as Ca2+

Regulator of cellular

processes

Magnesium (Mg)

In solution as Mg2+

Coenzyme for many enzymes

Iron (Fe)

In solution as Fe2+ or Fe3+ or

Carries oxygen; energy source

as FeS, Fe(OH3) etc

for some bacteria

Micronutrients

Present as contaminants at

very low concentrations

Copper (Cu)

In solution as Cu+, Cu2+

Coenzyme; microbial growth

inhibitor

Manganese (Mn)

In solution as Mn2+

Coenzyme

Cobalt (Co)

In solution as Co2+

Vitamin Bj2

Zinc (Zn)

In solution as Zn2+

Coenzyme; microbial growth

inhibitor

Molybdenum (Mo)

In solution as Mo2+

Coenzyme

Nickel (Ni)

In solution as Ni2+

Coenzyme

Nitrogen is needed for the synthesis of proteins and nucleic acids, as well as for important molecules such as ATP (you will learn more about ATP and its role in the cell's energy relations in Chapter 6). 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.

Table 4.2 Selected microbial growth factors

Growth factor

Function

Amino acids

Components of proteins

p-Aminobenzoic acid

Precursor of folic acid, involved in nucleic acid synthesis

Niacin (nicotinic acid)

Precursor of NAD+ and NADP+

Purines & pyrimidines

Components of nucleic acids

Pyridoxine (vitamin B6)

Amino acid synthesis

Riboflavin (vitamin B2)

Precursor of FAD

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 required 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 already have read about the first two of these groups in Chapter 2. Vitamins are complex organic compounds required in very small amounts for the cell's normal functioning. They are often either coenzymes or their precursors (see Chapter 6). 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.

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