microorganisms

Fermentation is a very important process in the food industry, and the success of fermentation depends on the isolation of different microorganisms from the sample. Microorganisms are isolated via different culture techniques. 

Methods to isolate microorganisms

One of the most prominent techniques to isolate microorganisms is the crowded plate technique. In this technique, first the soil is diluted. Then the diluted soil is kept over the nutrient agar plates. These plates are incubated and left for observation.

Enrichment isolation: this process allows certain microbes to grow while hindering the growth of other non-targeted microbes. The growth is accelerated by providing a carbon source. 

There are over a million microorganisms living in nature, and only a little less than one percent of the total microbes have been studied. Some microorganisms are commercially very important. Some bacteria produce acetic acid, glutamic acid, some fungi produce citric acid, penicillin, ethanol, steroids, etc. Some other common techniques to isolate microorganisms include direct sponge of soil, soil dilution, gradient plate method, aerosol dilution, flotation, centrifugation, etc. 

At first, the soil is diluted in sterile water, and an emulsifying agent called Tween is added. Then the sample is thoroughly mixed and kept at room temperature. Various culture mediums are mixed with diluted samples and incubated. Colonies that grow on the plate are isolated and are observed and identified. The pure strains are preserved in this case. 

Strains of microorganisms: Scientists often tend to isolate microorganisms from very extreme and unusual environments. These unusual environments include cold habitats, high altitudes, deserts, deep oceans, petroleum fields, etc. This is done in the expectation that they will be very useful and beneficial for commercial purposes. As mentioned earlier, the agar plates can be used for isolation. Some microorganisms are commercially very important, so they need to be screened off. 

The process of screening includes isolation, detection, and separation of microorganisms from a mixed sample. In the food industry, the sources of microorganisms are milk, cheese, etc. While screening, samples are taken from soil, water, air, milk, compost, etc. As mentioned earlier, nutrients are supplied to ensure the growth of the desired microorganisms, and as mentioned earlier, proper isolation and detection of desired microorganisms are also very important. 

Screening is divided into two parts, which are primary screening and secondary screening. Primary screening is used in organic acid production, antibiotic production, etc. Primary screening detects which microorganisms are able to produce a compound. But it doesn’t provide any rough outline or speculation about the yield or potential production of the desired microorganisms. It isolates microorganisms that have some commercial value. The pH-indicating dyes are useful in detecting microorganisms that are capable of producing organic acid. The dyes show a change of colour according to their pH. Some dyes, such as Neutral Red, Bromothymol Blue, etc., are added to the agar media. This testing needs to be done repeatedly as not only organic acids but also bases are the metabolic products of microbial growth. The calcium carbonate is also incorporated to isolate organic acid producing microbes. 

In the case of antibiotic producing microbes, the crowded plate technique is used. It doesn’t provide any information on microbes’ sensitivity towards other microorganisms. At first, the soil is diluted with sterile water and then is poured over, which produces 300 to 400 colonies per plate. Colonies showing antibiotic activity are separated and observed. Such colonies are again sub-cultured and purified by streak before being turned into stocks.

In the case of extracellular metabolite producing microbes, the auxanography technique is applied. This technique detects the microorganisms that are able to produce vitamins, amino acids, etc. First, a strip of filter paper is taken and placed at the bottom of the petri dish. Agar is prepared and is poured into that same dish. Then the mixture is kept aside to solidify. Then the soil sample is diluted. In this way, the first plate is prepared. Then, for the second plate, a medium lacking growth factor is prepared and incorporated with the test organism. Then the mixed medium is poured into the petri dish and it is kept aside for some time to set. Thus, the second plate is completed. The agar in the first plate is then lifted and transferred to the second plate. The growth factors present in the agar can diffuse into the lower layer of the set up. 

The enrichment culture technique was introduced by Beijerinick to isolate the desired microorganisms from a heterogeneous mixture. In this case, nutrients are added along with the microorganisms and the source material is incubated. A small portion of this is placed on the solid medium and, in this way, isolated colonies are acquired. These colonies are sub-cultured on fresh media and are further tested. 

Conclusion 

These methods to isolate microorganisms are heavily used by biotechnologists, and these concepts are also very important for aspirants.