Nutritional requirements and raw materials used for culture media
Introduction:
Nutrition is substances used in biosynthesis and energy production and therefore are required for all living things.
Bacteria, like all living cells, require energy and nutrients to build proteins and structural membranes and drive biochemical processes.
Bacteria require sources of carbon, nitrogen, phosphorus, iron and a large number of other molecules.
Carbon, nitrogen, and water are used in the highest quantities.
The nutritional requirements for bacteria can be grouped according to the carbon source and the energy source.
Some types of bacteria must consume preformed organic molecules to obtain energy, while other bacteria can generate their own energy from inorganic sources.
Important Terminologies:
CULTURE - the growth or crop of microorganisms obtained in a culture medium after its incubation period.
CULTURE MEDIUM - any material where microorganisms may thrive for their nourishment and reproduction.
INCUBATION PERIOD – is the time needed to let previously inoculated culture media to show a distinct colony or colonies within a desired temperature.
It is also the time needed for the microorganisms to adapt, grow and multiply in its new environment.
COLONY - a group of microorganisms growing together characteristically in a culture medium.
INOCULUM – the fished-out colony hanging on the wire loop, cotton swabs, etc. that is ready for transfer to another culture medium for cultivation and/or for further isolation.
Bacteriological Media:
Bacteriological media or culture media are an artificially prepared mixture of various nutrients for the growth , multiplication, differentiation and transportation of microorganisms.
A culture media must supply suitable carbon, nitrogen, energy sources and other nutrients.
A lot of culture media are available but there is no single media suitable for growth of all microorganisms.
Common ingredients used in bacteriological media :
Water:
Microbes contain almost 70 to 80% water in their Protoplasm.
Water is used for preparation of culture media by dissolving various organic and inorganic compounds.
Copper distilled water should not be used for preparation of culture media as copper is known to inhibit bacterial growth.
Peptone:
Peptone Is a partially digested protein obtained from partial hydrolysis of lean meat.
The important constituents of peptone are amino acids, proteases, certain inorganic salt and vitamins.
Peptone acts as a nitrogen source and as a buffer.
It must be stored in a tightly closed container as it is highly hygroscopic and becomes very sticky on contact with air.
Yeast Extract:
It is prepared from Baker's Yeast i.e. Saccharomyces.
It contains carbohydrates, amino acids, inorganic salts and vitamins from the B complex group.
It is mainly used as a source of vitamins and can be substituted for meat extract.
Meat Extract:
It is prepared by hot water extraction of fresh lean meat.
Contents are gelatin peptones, proteases amino acids, mineral salt and certain vitamins.
Agar:
is long chain polysaccharide obtained from extraction of seaweeds called Agarophytes.
Agar contains a mixture of two polysaccharides Agarose and Agaropectin along with certain inorganic salts, a protein like substance and many inorganic ions.
important properties of agar are as follows
Acts as a good solidifying agent (2%).
Has no nutritional value in the media.
Bacteriologically inert.
Resistant to the action of all microorganisms.
Stable at different temperatures used for incubation.
Melts @ 95 to 98 ℃ and remain liquid @ 40-45 ℃.
Gets solidified below 40℃ temp.
Economical and easily available.
Types of Culture Media:
They are classified in different way as follows;
Depending on Physical State (Consistency):
Solid Media (1.5 to 2.5 % Agar): e.g. Nutrient Agar.
Semisolid Media (0.2 to 0.5 % Agar): e.g. Nutrient broth containing 0.5% Agar.
Liquid Media (Absence of Agar): Fluid Thioglycollate broth.
Depending on Oxygen Requirement:
Aerobic Media: e.g. Mackonkey’s Broth.
Anaerobic Media: e.g. Robertson’s Cooked Meat medium.
Depending on Chemical Composition:
Simple or Basal Media.
Synthetic or Defined Media.
Non-synthetic or Undefined or Complex Media.
Special Media:
Enriched Media.
Enrichment Media.
Selective Media.
Indicator Media.
Differential Media.
Sugar Media.
Transport Media.
Assay Media.
Storage Media.
Simple or Basal Media:
Basal media are basically simple media that supports most non-fastidious bacteria.
Peptone-water, nutrient broth, and nutrient agar (NA) are considered as basal medium.
These media are generally used for the primary isolation of microorganisms.
Synthetic or Defined Media:
These are prepared from pure chemicals and hence exact chemical composition is known.
Used for research purposes and to study metabolic activities of bacterias.
Non-synthetic or Undefined or Complex Media:
They contain complex chemical materials from biological origin whose exact composition is unknown.
These biological products provide growth nutrients required by bacteria.
These culture mediums may contain added nutritional materials.
Special Media:
Enriched Media.
These media contain added nutritional substances for specific bacterias (Fastidious Bacterias).
E.g. Blood Agar (blood is added for streptococcus bacteria), Chocolate agar for Haemophilus sp., etc.
Enrichment Media:
These mediums contain added substance that inhibit growth of unwanted bacteria and favors growth of a specific bateria.
It is a liquid medium.
E.g. Tetrathionate broth which inhibits growth of E. coli and favors growth of Salmonella sp. In feces.
Selective Media:
It is just like enrichment media but is in solid form.
It also contains a substance that prevents growth of unwanted microbes and promotes growth of a specific microbe.
e.g. Lowenstein - Johnson medium for Mycobacterium tuberculosis which contain Penicillin, nalidixic acid and Malachite green that inhibit growth of other unwanted microbes.
Indicator Media:
These media contain an indicator that changes color on growth of a specific microbe.
e.g. Wilson and Blair Media which contains sulphite, Salmonella typhi reduces this sulphite to sulphide in presence of glucose and produces colonic having black color with metallic shine.
Differential Media:
A differential medium is used to distinguish colonies of different bacterias.
e.g. MacCocnkey’s Agar which contains lactose, peptone, neutral red and differentiates lactose fermentation in red colored colonies from non lactose fermentation in pale or white colonies.
Sugar Media:
In this 1% sugar is added to peptone water along with a suitable indicator.
Contains a small tube called Durham’s tube kept inverted for detection of production of gas by bacterias.
Transport Media:
Clinical specimens must be transported to the laboratory immediately after collection to prevent overgrowth of contaminating organisms.
This can be achieved by using transport media.
Such media prevent drying (desiccation) of a specimen, maintain the pathogen viable, and inhibit the overgrowth of unwanted bacteria.
Some of these media (Stuart’s & Amie’s) are semi-solid in consistency.
Addition of charcoal serves to neutralize inhibitory factors.
e.g. Cary Blair transport medium and Venkatraman Ramakrishnan (VR) medium are used to transport feces from suspected cholera patients.
e.g. Pike’s medium is used to transport streptococci from throat specimens.
Assay Media:
These media are used for the assay of vitamins, amino acids, and antibiotics.
e.g. antibiotic assay media are used for determining antibiotic potency by the microbiological assay technique.
Storage Media:
These help in preservation and storage of bacteria for a longer duration.
Dorset’s Egg Medium, Nutrient agar stabs, Robertsons cooked meat medium etc.
Labels: Microbiology
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