Nutritional Requirements of microorganisms
Learning objectives
At the end of this lecture, the student will be able to:
• List the nutritional requirements of microorganisms
• Classify the different types of culture media
• Explain the significance and application of different culture
media
• Classify nutritional types of bacteria
Introduction
• Nutrients are substances used in biosynthesis and energy production and therefore are required for microbial growth
• Environmental factors such as temperature, oxygen levels,
and the osmotic concentration of the medium are critical in the successful cultivation of microorganisms
Common nutritional requirements
Macroelements
• Required by microorganisms in relatively large amounts
• 95% of cell dry weight is made up of a few major elements:
carbon, oxygen, hydrogen, nitrogen, sulphur, phosphorus, potassium, calcium, magnesium, and iron.
|
Element |
% |
Source |
Function |
|
Carbon |
50 |
Organic compounds or CO2 |
Main constituent of cellular material |
|
Oxygen |
20 |
H2O, organic compounds, CO2 and O2 |
Constituent of cell material and cell water, O2 is electron acceptor |
|
Nitrogen |
14 |
NH3, NO3, organic compounds, N2 |
Constituent of amino acids, nucleic acids nucleotides, and |
|
Hydrogen |
8 |
H2O, organic compounds, H2 |
Main constituent of organic compounds and cell water |
|
Phosphorus |
3 |
inorganic phosphates (PO4) |
Constituent of nucleic acids, nucleotides, phospholipids, LPS, |
|
Potassium |
1 |
Potassium salts |
Main cellular inorganic cation and cofactor for certain enzymes |
|
Magnesium |
0.5 |
Magnesium salts |
Inorganic cellular cation, cofactor for certain enzymatic reactions |
|
Sulphur |
1 |
Organic sulphur compounds |
Constituent of cysteine, methionine, glutathione, several coenzymes |
|
Calcium |
0.5 |
Calcium salts |
Inorganic cellular cation, cofactor for certain enzymes and a |
|
Iron |
0.2 |
Iron salts |
Component of cytochromes and certain non-heme iron-proteins and a |
The micronutrients
• Manganese, zinc, cobalt, molybdenum, nickel, and
copper—are needed by most cells.
• Contaminants in water, glassware, and regular media
components often are adequate for growth
• Micronutrients are normally a part of enzymes and
cofactors, and they aid in the catalysis of reactions and maintenance of protein structure
• Zinc is present at the active site of some enzymes
• Manganese aids many enzymes catalyzing the transfer of
phosphate
• Groups Molybdenum is required for nitrogen fixation,
cobalt is a component of vitamin B12.
Nutritional Types of Microorganisms
Growth factors
There are three major classes of growth factors:
• Amino acids
Needed for protein synthesis
• Purines and pyrimidines
Needed for nucleic acid synthesis
• Vitamins
Needed as coenzymes and functional groups of certain enzymes
• Some bacteria (e.g. E. coli) do not require any growth
factors: they can synthesize all essential purines, pyrimidines, amino acids and vitamins, starting with their carbon source, as part of their own intermediary metabolism.
• Certain other bacteria (e.g. Lactobacillus) require
purines, pyrimidines, vitamins and several amino acids in order to grow. These compounds must be added in advance to culture media that are used to grow these bacteria.
• Mutant strains of bacteria that require some growth factor
not needed by the wild type (parent) strain are referred to as auxotrophs.
• Thus, a strain of E. coli that requires the amino acid
tryptophan in order to grow would be called a tryptophan auxotroph and would be designated E. coli trp-
Culture media
• A culture medium is a solid or liquid preparation used to
grow, transport, and store microorganisms
• Specialized media are essential in the isolation and identification of microorganisms, the testing of antibiotic sensitivities, water and food analysis, industrial microbiology, and other activities
• Knowledge of a microorganism’s normal habitat often is
useful in selecting an appropriate culture medium
Defined and complex media
Synthetic or Defined Media
• A medium in which all components are known is a defined
medium or synthetic medium.
• Many chemoorganotrophic heterotrophs also can be grown in defined media with glucose as a carbon source and an ammonium salt as a nitrogen source.
• Defined media are used widely in research, as it is often
desirable to know what the experimental microorganism is metabolizing
Complex Media
• Contain some ingredients of unknown chemical composition
• May be sufficiently rich and complete to meet the nutritional requirements of many different microorganisms contain undefined components like peptones, meat extract, and yeast extract
• Three commonly used complex media are
(1) Nutrient broth,
(2) Tryptic soy broth,
(3) MacConkey agar
Ingredients of complex media
|
Peptones |
Protein hydrolysates prepared by partial proteolytic digestion of |
Sources of carbon, energy, and nitrogen
|
|
Meat extract, Beef extract |
Aqueous extracts of lean beef |
Beef extract contains amino acids, peptides, nucleotides, organic |
|
Yeast extract |
Aqueous extracts of brewer’s yeast |
Yeast extract is an excellent source of B vitamins as well as |
Agar in media
• Liquid media can be solidified with the addition of 1.0 to
2.0% agar
• Agar is extracted from red algae
• After it has been melted in boiling water, it can be
cooled to about 40 to 42°C before hardening and will not melt again until the temperature rises to about 80 to 90°C
• Agar is also an excellent hardening agent because most
microorganisms cannot degrade it.
Types of media
• General purpose media – tryptic soy broth and tryptic soy agar are called general purpose media because they support the growth of many microorganisms
• Enriched media – Blood and other special nutrients may be added to general purpose media to encourage the growth of fastidious heterotrophs. These specially fortified media (e.g., blood agar) are called enriched media.
Enrichment media
• If the sample contains more than one bacteria, the undesired bacterial growth can be reduced by addition of certain media components
• The desired organism is facilitated to grow
Selective media
• Favor the growth of particular microorganisms
• Bile salts or dyes like basic fuchsin and crystal violet favor the growth of gram-negative bacteria by inhibiting the growth of gram-positive bacteria without affecting gram-negative organisms.
• Examples: Endo agar, eosin methylene blue agar, and
MacConkey agar
• A medium containing only cellulose as a carbon and energy source is quite effective in the isolation of cellulose-digesting
• Endo agar – Inhibition of gram-positive microorganisms was achieved by the sodium sulfite and basic fuchsin
• EMB agar – EMB contains dyes that are toxic for Gram positive bacteria and bile salt which is toxic for Gram negative bacteria other than coliforms.
MacConkey agar – The crystal violet and bile salts inhibit the growth of gram-positive organisms which allows for the selection and isolation of gram-negative bacteria
Differential media
• Media that distinguish between different groups of
bacteria
• Permit tentative identification of microorganisms based on
their biological characteristics
Examples –
• Blood agar is both a differential medium and an enriched one
• MacConkey agar is both differential (crystal violet & bile salts) and selective (lactose & neutral red dye)
Summary
• Nutrients are substances used in biosynthesis and energy production and therefore are required for microbial growth
• Major elements required for growth – carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, potassium, calcium, magnesium, and iron.
• Minor elements – manganese, zinc, cobalt, molybdenum, nickel, and copper
• Nutritional types of bactera – Autotrophs, Heterotrophs, Phototrophs, Chemotrophs, Lithotrophs, Organotrophs
• Growth factors – Amino acids, Purines and pyrimidines, Vitamins
• A culture medium is a solid or liquid preparation used to grow, transport, and store microorganisms
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