Nutrient Requirements of Microorganisms
For its continual maintenance and its multiplication, microorganisms must draw from its environment the substances required for the synthesis of its cell material and the generation of energy. The substances microorganisms require are termed nutrients. The nutrient requirement of microorganisms varies with type of microorganism and can be very complex. Escherichia coli is very simple in its nutritional requirement, whereas Lactobacilus spp. are very demanding (fastidious). A culture medium must supply not only the nutrients a specific microorganism requires, but these must also be present in the appropriate concentration. A too high concentration of a nutrient e.g. amino acid, may inhibit the growth.
Composition of microorganisms-macromolecules
The solid matter of microorganisms contains in addition to hydrogen and oxygen (derivable from water) carbon, nitrogen, phosphorus and sulfurs. These six element account for 95 % of the cellular dry weight.
Microorganisms consists of water and macromolecules. Apart from lipids, the macromolecules are build from monomers. Monomers are the precursors of the macromolecules. Examples of macromolecules are:
Protein
Polysaccharides
Lipid
Lipopolysaccharide
DNA
RNA
Proteins are the most abundant class of macromolecules and consists of a polymers of the monomers amino acids. After protein ribonucleic acid (RNA) is the most abundant macromolecule. Ribonucleic acid is a polymer of nucleotides and occurs in ribosomes, messenger and transfer RNA’s, the key players in the protein synthesis. Lipids rank in abundance as third. Fatty acids are the main constituent of lipids. The simplest form of a lipid is a tri-glyceride and the more complex forms are phospholipid and glycolipid. Lipids are crucial for the membrane structure and serve also storage depots of excess of carbon.
The smaller fractions of cell constitute polysaccharide, lipopolysacharide and DNA. Polysaccharides are polymers of sugars and are primarily present in cell walls. They also serve as carbon and energy source (e.g. glycogen) Lipopolysachharides, such as glycolipid and glycoprotein, play an important role in cell membrane and cell surface receptor molecules. DNA is the other polymer of nucleotides and its contribution to the bacterial cell weight is small. Its function as the repistory of genetic information is, however, crucial to microorganisms.
Nutrient requirements
Microorganisms differ in the specific form under which carbon, nitrogen, sulfur and oxygen must be provided as nutrients. Nutritional studies have shown that microorganism that do not perform photosynthesis or bacteria that obtain energy from the oxidation of inorganic compounds obtain carbon simply from organic nutrients. These include i.e. amino acids, fatty acids, organic acids, sugars, nitrogen bases, aromatic compounds. The C source has a dual function and serves both as source of carbon and source of energy. Carbon is the major element in all classes of macromolecules. Some organism require a single organic compound whereas other can not grow with only one compound. Microorganism are extremely divers in the kind and the number of organic compounds they require as C source. After carbon nitrogen is the most important element and is found i.e. in proteins as amino acids and in nucleic acids.
The nitrogen (N) source for most organisms are inorganic compounds, that is ammonia, and nitrate or, organic compounds, that is, amino acids, nitrogen bases of nucleotides and many N-containing organic compounds. Nitrogen fixing microorganisms require nitrogen gas.
In addition to C and N source a microorganisms requires macronutrients such as phosphor, sulfur, potassium, magnesium, calcium, sodium and iron. Phosphor is required for the synthesis of nucleic acids and phospholipids. Sulfur is required in the amino acids cysteine and methionine and in vitamins such as thiamine, biotin, lipoic acid, and co-enzyme A. Most cell sulfur originates from inorganic source such as sulfate or sulfide. Potassium is required for the protein synthesis and plays an important role in the homoeostasis. Magnesium functions to stabilise ribosomes, cell and nucleic acids. It is also required for the activity of many enzymes. Calcium helps to stabilize the cell wall and plays a key role in the heat stability of endospores. Sodium plays a role in the homeostasis. Iron plays a major role in the cellular respiration and is a key component of cytochromes and iron-sulfur proteins involved in the electron transport.
Microelements or trace elements are elements such as cobalt, nickel, chromium, copper, manganese, selenium tungsten, vanadium and zinc. Many of the trace elements play a structural role in enzymes.
Any compound that a microorganism can not synthesise from simpler carbon sources must be provided as a nutrient. Such organic compounds are termed growth factors. These include vitamins, amino acids, purines and pyrimidines. Growth factors fulfill specific needs in biosynthesis and they are required in only small amounts. Vitamins function as co-enzymes. Lactic acid bacteria are renowned for their complex vitamin requirement.
