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In this article we will discuss about the primary and secondary screening of microorganisms. The economics of a fermentation process largely depends upon the type of microorganism used. If fermentation process is to yield a product at a cheaper price the chosen microorganism should give the desired product in a predictable and economically adequate quantity. The microorganism with a desired characters is generally isolated from natural substrates like soil etc. Such an organism is generally called as a producer strain.
A producer strain should possess the following characters:
1. It should be able to grow on relatively cheaper substrates.
2. It should grow well in an ambient temperature preferably at 30-40°C. This reduces the cooling costs.
3. It should yield high quantity of the end product.
4. It should possess minimum reaction time with the equipment used in a fermentation process.
5. It should possess stable biochemical characteristics.
6. It should yield only the desired substance without producing undesirable substances.
7. It should possess optimum growth rate so that it can be easily cultivated on a large scale.
Detection and isolation of a microorganism from a natural environment like soil containing large number of microbial population is called as screening. It is very time consuming and expensive process. For example, Eli Lilly & Co. Ltd discovered three species of antibiotic producing organisms in a span of 10 years and after screening 4,00,000 organisms.
Although there are many screening techniques, all of them are generally grouped into two broad categories.
They are:
1. Primary screening, and
2. Secondary screening.
1. Primary Screening of Microorganisms:
Primary screening may be defined as detection and isolation of the desired microorganism based on its qualitative ability to produce the desired product like antibiotic or amino acid or an enzyme etc. In this process desired microorganism is generally isolated from a natural environment like soil, which contains several different species. Sometimes the desired microorganism has to be isolated from a large population of different species of microorganisms.
The following are some of the important primary screening techniques:
(i) The crowded plate technique
(ii) Indicator dye technique
(iii) Enrichment culture technique
(iv) Auxanographic technique
(v) Technique of supplementing volatile and organic substrates.
(i) The Crowded Plate Technique:
This technique is primarily employed for detecting those microorganisms, which are capable of producing antibiotics. This technique starts with the selection of a natural substratum like soil or other source consisting of microorganisms. Progressive serial dilution of the source is made. Suitable aliquot of the serial dilution is chosen which is able to produce 300 to 400 individual colonies when plated on an agar plate, after incubation. Such a plate is called as crowded plate.
The antibiotic producing activity of a colony is indicated by no growth of any other bacterial colony in its vicinity. This region of no growth is indicated by the formation of a clear and colorless area around the antibiotic producing microorganism’s colony on the agar plate. This region is called as growth inhibitory zone.
Such a colony is isolated from the plate and purified either by making repeated sub-culturing or by streaking on a plate containing a suitable medium, before stock culture is made. The purified culture is then tested for its antibiotic spectrum.
However, the crowded plate technique has limited applications, as it will not give indication of antibiotic producing organism against a desired organism. Hence, this technique has been improved later on by employing a test organism to know the specific inhibitory activity of the antibiotic.
In this modified procedure, suitable serially diluted soil suspension is spread on the sterilized agar plate to allow the growth of isolated and individual microbial colonies (approximately 30 to 300 per plate) after incubation. Then the plates are flooded with a suspension of test organism and the plates are incubated further to allow the growth of the test organism. The formation of inhibitory zone of growth around certain colonies indicates the antibiotic activity against the test organism.
A rough estimation of the relative amounts of antibiotic produced by a microbial colony can be estimated by measuring the diameter of the zone of inhibited test organism’s growth. Antibiotic producing colonies are later on isolated from the plate and are purified before putting to further testing to confirm the antibiotic activity of a microorganism.
(ii) Indicator Dye Technique:
Microorganisms capable of producing acids or amines from natural sources can be detected using this method by incorporating certain pH indicator dyes such as neutral red or bromothymol blue into nutrient agar medium. The change in the color of a particular dye in the vicinity of a colony will indicate the ability of that colony to produce an organic acid or base.
Production of an organic acid can also be detected by an alternative method. In this method calcium carbonate is incorporated into the agar medium. The production of organic acid is indicated by the formation of a clear zone around those colonies which release organic acid into the medium.
The identified colonies are isolated and purified either by repeated sub-culturing or by streaking methods and a stock culture is made which may be used for further qualitative or quantitative screening tests.
(iii) Enrichment Culture Technique:
This technique is generally employed to isolate those microorganisms that are very less in number in a soil sample and possess specific nutrient requirement and are important industrially. They can be isolated if the nutrients required by them is incorporated into the medium or by adjusting the incubation conditions.
(iv) Auxanotrophic Technique:
This technique is employed for the detection and isolation of microorganisms capable of producing certain extracellular substances such as growth stimulating factors like amino acids, vitamins etc. A test organism with a definite growth requirement for the particular metabolite is used in this method.
For this purpose, spread a suitable aliquot on the surface of a sterilized agar plate and allow the growth of isolated colonies, after incubation. A suspension of test organism with growth requirement for the particular metabolite is flooded on the above plate containing isolated colonies, which are subjected to further incubation.
The production of the particular metabolite required by the test organism is indicated by its increased growth adjacent to colonies that have produced the required metabolite. Such colonies are isolated, purified and stock cultures are prepared which are used for further screening process.
(v) Technique of Supplementing Volatile Organic Substances:
This technique is employed for the detection and isolation of microorganisms capable of utilizing carbon source from volatile substrates like hydrocarbons, low molecular weight alcohols and similar carbon sources. Suitable dilution of a microbial source like soil suspension are spread on to the surface of sterile agar medium containing all the nutrients except the one mentioned above.
The required volatile substrate is applied on to the lid of the petri plates, which are incubated by placing them in an inverted position. Enough vapors from the volatile substrate spread to the surface of agar within the closed atmosphere to provide the required specific nutrient to the microorganism, which grows and form colonies by absorbing the supplemented nutrient. The colonies are isolated, purified and stock cultures are made which may be utilized for further screening tests.
2. Secondary Screening of Microorganisms:
Primary screening helps in the detection and isolation of microorganisms from the natural substrates that can be used for industrial fermentations for the production of compounds of human utility, but it cannot give the details of production potential or yield of the organism. Such details can be ascertained by further experimentation.
This is known as secondary screening, which can provide broad range of information pertaining to the:
i. Ability or potentiality of the organism to produce metabolite that can be used as an industrial organism.
ii. The quality of the yield product.
iii. The type of fermentation process that is able to perform.
iv. Elimination of the organisms, which are not industrially important.
To evaluate the true potential of the isolated microorganisms both qualitative and quantitative analysis are generally conducted. The sensitivity of the test organism towards a newly discovered antibiotic is generally analysed during qualitative analysis, while the quantum yield of newly discovered antibiotic is estimated by the quantitative analysis.
Evaluation of Potentialities of Microorganisms:
Microorganisms isolated in the primary screening are critically evaluated in the secondary screening so that industrially important and viable potentialities can be assessed.
They include:
1. To determine the product produced by an organism is a new compound or not.
2. A determination should be made about the yield potentialities of various isolated microorganisms that are detected in primary screening for that new compound.
3. It should determine about the various requirements of the microorganism such as pH, aeration, temperature etc.
4. It should detect whether the isolated organism is genetically stable or not.
5. It should reveal whether the isolated organism is able to destroy or alter chemically their own fermentative product by producing adaptive enzymes if they accumulate in higher quantities.
6. It should reveal the suitability of the medium or its constituent chemicals for the growth of a microorganism and its yield potentialities.
7. It should determine the chemical stability of the product.
8. It should reveal the physical properties of the product.
9. It should determine whether the product produced by a microorganism in a fermentative process is toxic or not.
10. Secondary screening should reveal that whether the product produced in fermentation process exists in more than one chemical form. If so, the amount of formation of each chemical formation of these additional products is particularly important since their recovery and sale as byproducts can greatly improve the economic status of the fermentation industry.
11. The new organism should be identified to the species level. This will help in making a comparison of growth pattern, yield potentialities and other requirements of test organism with those already described in the scientific and patent literature, as being able to synthesize products of commercial value.
12. It should select industrially important microorganisms and discard others, which are not useful for fermentation industry.
13. It should determine the economic status of a fermentation process undertaken by employing newly isolated microorganism.
Methods of Secondary Screening:
Secondary screening gives very useful information pertaining to the newly isolated microorganisms that can be employed in fermentation processes of commercial value. These screening tests are conducted by using petri dish containing solid media or by using flasks or small fermenters containing liquid media. Each method has some advantages and disadvantages. Sometimes both the methods are employed simultaneously.
Liquid media method is more sensitive than agar plate method because it provides more useful information about the nutritional, physical and production responses of an organism to actual fermentation production conditions. Erlenmeyer flasks with baffles containing highly nutritive liquid media are used for this method. Flasks are fully aerated with glass baffles and continuously shaken on a mechanical shaker in order to have optimum product yield.
There are several techniques and procedures that can be employed for secondary screening. However, only a specific example of estimation of antibiotic substance produced by species of Streptomyces, is described in the following paragraph. Similar methods could be used for the detection and isolation of microorganisms capable of producing other industrial products.
(i) Giant Colony Technique:
This technique is used for isolation and detection of those antibiotics, which diffuse through solid medium. Species of Streptomyces, is capable of producing antibiotics during primary screening. The isolated Streptomyces culture is inoculated into the central area of a sterilized petri plates containing nutrient agar medium and are selected. The plates are incubated until sufficient microbial growth takes place.
Cultures of test organism, whose antibiotic sensitivity is to be measured are streaked from the edges of plate’s upto but not touching the growth of Streptomyces and are further incubated to allow the growth of the test organisms. Then the distance over which the growth of different test organisms is inhibited by the antibiotic secreted Streptomyces is measured in millimeters.
The relative inhibition of growth of different test organisms by the antibiotic is called inhibition spectrum. Those organisms whose growth is inhibited to a considerable distance are considered more sensitive to the antibiotic than those organisms, which can grow close to the antibiotic. Such species of Streptomyces, which have potentiality of inhibiting microorganisms is preserved for further testing.
(ii) Filtration Method:
This method is employed for testing those antibiotics which are poorly soluble in water or do not diffuse through the solid medium. The Streptomyces is grown in a broth and its mycelium is separated by filtration to get culture filtrate. Various dilutions of antibiotic filtrates are prepared and added to molten agar plating medium and allowed to solidify.
Later on cultures of various test organisms are streaked on parallel lines on the solidified medium and such plates are incubated. The inhibitory effect of antibiotic against the test organisms is measured by their degree of growth in different antibiotic dilutions.
(iii) Liquid Medium Method:
This method is generally employed for further screening to determine the exact amount of antibiotic produced by a microorganism like Streptomyces.
Erlenmeyer conical flasks containing highly nutritive medium are inoculated with Streptomyces and incubated at room temperature. They are also aerated by shaking continuously and vigorously during incubation period to allow Streptomyces to produce the antibiotic in an optimum quantity.
Samples of culture fluids are periodically withdrawn aseptically for undertaking the following routine checks:
1. To check the suitability of different media for maximum antibiotic production.
2. To determine the value of pH at which there will be maximum growth of the microorganism and antibiotic production.
3. To check for contamination.
4. To determine whether the antibiotic produced is new or not.
5. To check the stability of the antibiotic at various pH levels and temperatures.
6. To determine the solubility of the antibiotic in various organic solvents.
7. To check about the toxicity of the antibiotic against the experimental animals.
After carrying out the above mentioned routine tests further studies are also conducted to know the following additional information:
1. Effect of incubation temperature and antifoaming agents on fermentation.
2. Rate of resistance developed among the test organisms.
3. Checking the antibiotic for its bacteriostatic or bactericidal properties. Its ability to precipitate serum proteins to cause hemolysis of blood or to harm phagocytes.
4. Checking for possibility of inclusion of precursor chemical of the antibiotic production in the medium.
5. Suitability of the organism for mutation and other genetic studies.