Nutrients are required –
To obtain energy and construct new cellular components.
For biosynthesis and energy production.
For microbial growth. Catabolism breaks molecular structures down, releasing energy in the process Anabolism uses energy to build larger molecules from smaller ones.
Collectively, these reactions are called metabolism.
Different organisms need different complements of nutrients, and not all nutrients are required in the same amounts. To understand the nutritional requirements it is necessary to understand the chemical composition of the microbial cell.
Elemental composition of microbial cells
1) Element – Carbon
% of cell dry weight – 50
Physiological Function – Constituent of organic cell materials
2) Element – Oxygen
% of cell dry weight – 20
Physiological Function – Constituent of cellular water, organic cell materials, electron acceptor in respiration
3) Element – Nitrogen
% of cell dry weight – 14
Physiological Function – Constituent of proteins, nucleic acids, coenzymes
4) Element – Hydrogen
% of cell dry weight – 8
Physiological Function – Constituent of cellular water, organic cell materials
5) Element – Phosphorus
% of cell dry weight – 3
Physiological Function – Constituent of proteins (as amino acids cysteine and methionine), coenzymes
Chemical Formula of a cell
- An approximate chemical formula for a cell is CH2O0.5N0.5 indicating that C, H, O, and N constitute the bulk of a living organism.
- Besides water, which makes up 80-90% of the wet weight of a microbial cell (a single cell of Escherichia coli weighs just 10-12 g), cells consist primarily of macromolecules—proteins, nucleic acids, lipids, and polysaccharides.
- The essential elements make up the building blocks (monomers) of these macromolecules, the amino acids, nucleotides, fatty acids, and sugars.
- Proteins dominate the macromolecular composition of a cell, making up 55% of total cell dry weight.
There are some microorganisms with unique nutritional requirements
In such condition , the basic composition of the growth medium needs to change.
Thiobacillus spp. chemolithotrophs need a high content of sulfur and ferrous sulfate. For enrichment of this bacteria , medium is totally free of organic matter.
Diatoms synthesize cell walls which are heavily impregnated with silica. Hence the medium for growth is supplemented with sillicates.
Carbon and Nitrogen
All cells require carbon, and most prokaryotes require organic (carbon-containing compounds as their source of carbon.
Heterotrophic bacteria assimilate organic compounds and use them to make new cell material.
Amino acids, fatty acids, organic acids, sugars, nitrogen bases, aromatic compounds, and countless other organic compounds can be transported and catabolized by one or another bacterium.
Autotrophic microorganisms build their cellular structures from carbon dioxide (CO2) with energy obtained from light or inorganic chemicals.
A bacterial cell is about 14% nitrogen, which is present in proteins, nucleic acids, and several other cell constituents.
The bulk of nitrogen available in nature is in inorganic form as ammonia (NH3), nitrate (NO3-), or nitrogen gas (N2).
Virtually all prokaryotes can use NH3 as their nitrogen source, and many can also use NO3
By contrast, N, can only be used by nitrogen-fixing prokaryotes.
Nitrogen in organic compounds, for example, in amino acids, may also be available to microorganisms can immediately enter the monomer pool for biosynthesis or be catabolized as an energy source.
Hydrogen and Oxygen
Water is main source, metabolic reactions take place i.e. ionization
- They are directly incorporated into a molecular reaction (hydrolysis) or help in transport of a molecule(osmosis).
- These molecules are required in molecular form.
- Molecular hydrogen is used by some species such as Hydrogenomonas as a source of energy.
- Oxygen is required in aerobic respiration where it acts as a terminal electron acceptor / oxidizing agent.
On the basis of oxygen requirements, organisms are classified as –
Obligate aerobes – Organisms that require oxygen to live .e.g. Pseudomonas spp.
Facultative anaerobes – can use oxygen when it is present but are able to continue growth by using fermentation or anaerobic respiration when oxygen is not available.
These are able to substitute other electron acceptors, such as nitrate ions, for oxygen, which is something humans are unable to do.
Obligate anaerobes – are bacteria that are unable to use molecular oxygen for energy-yielding reactions. In fact, most are harmed by it. e.g. Clostridium spp.
Oxygen is toxic to obligate anaerobes.
Most bacteria produce hydrogen peroxide in the presence of free oxygen.
Its destruction is important as it is toxic to living cells.
Microaerophilic organisms – Grow best at a partial pressure of oxygen. (below 0.2 atmospheres, much below that present in the air.)
The accumulation of hydrogen peroxide in cultures is controlled by 2 factors
- The production of catalase.
- Degree of sensitivity of the organism to hydrogen peroxide.
Catalase is capable of decomposing hydrogen peroxide to water and molecular oxygen.
2H2O2 + Catalase = 2H2O + O2 The enzyme catalase is present in larger amounts in strictly aerobic bacteria and absent in Obligately anaerobic bacteria.
It is an organic compound that is required by an organism as a precursor or constituent of its organic cell material but which cannot be synthesized from simpler carbon sources.
It must be provided in the medium in small amounts.
It includes amino acids, purines. Pyrimidines and vitamins.
Nutritional classification of microorganisms Microorganisms are classified broadly into 2 categories –
1. Phototrophs – Depend on light/radiant energy as a source of energy.
2. Chemotrophs – Use chemical compounds as a source of energy.
Autotrophs – use carbon dioxide/ inorganic carbon as a carbon source.
Phototrophs – use organic compounds as a source of carbon.