This presentation highlights the importance of screening for the isolation and selection of microbial cultures at an industrial scale.

1. Screening of microbial
cultures
Presented By:
Dr. Shikha Thakur
Assistant Professor
TCSC, Mumbai, Maharashtra

2. Screening – It refers to the isolation of only those microorganisms
which are of interest from among a large population of microbes.
They are mainly of two types:
 Primary and Secondary Screening
 Primary Screening : The effective screening must in one or few steps which
allow the discarding of many valueless microorganisms, while at the same
time allowing the easy detection of the small percentage of useful
microorganisms that are present in the population.
 Secondary Screening : As primary screening allows detection and isolation of
microorganisms that possess potentially interesting industrial applications.
This is usually followed by a secondary screening to further test the
capabilities of and gain information about these organisms.

3. Types of Primary Screening
 Crowded-plate technique – Natural microbial source such as soil is diluted to
provide a cell concentration such that aliquots spread, sprayed or applied in
some manner to the surfaces of agar plates will yield colonies not touching
neighbouring colonies.
 An alternative procedure for detecting organic acid production involves
incorporation of calcium carbonate in medim so tht organic acid production is
indicated by cleared zone of dissolved calcium carbonate around the colony.
 Screening approach has also been employed extensively in search for the
microrganisms capable of producing useful antibiotics. The simplest screening
technique for antibiotic procedures is the “crowded plate” procedure.
Colonies producing antibiotic activity are indicated by an area of agar around
the colony that is free of growth of other colonies. Such a colony is
subcultured to a similar medium and purified by streaking before making
stock cultures.

5. Improvement of crowded technique
 Antibiotic screening is improved, therefore, by incorporation into the procedure of
“test organism”, that is an organism us as an indicator for the presence of specific
antibiotic activity.
 A suspension of test organism is then sprayed or applied in some manner to the
surface of the agar, and then plates are further incubated to allow growth of the
test organism.
 When volatile substrates such as hydrocarbons, low molecular weight alcohols, and
similar carbon sources are being considered. The dilutions from a microbial
sources are applied to plates of agar media containing all nutrients but the
specific substrate and the specific substrate is placed in the lid of petriplate after
inversion of the plate.
 Enough vapours from the volatile substrate rise to the surface of the agar within
this closed atmospheric to provide the specific nutrient for the microorganism.

6. Secondary Screening
 Secondary screening allows the further sorting out of those microrganisms
that have real value for industrial processes and discarding of those lacking
this potential.
 Secondary screening is conducted on agar plates, in flasks or small fermenters
containing liquid media, or combination of these approaches.
 Liquid culture provides a much better picture of the nutritional physical, and
production of responses of an organism to actual fermentation production
conditions.

7. Secondary screening can be qualitative
and quantitative in its approach
 Qualitative approach – eg. Spectrum or range of microorganisms which is sensitive
to a newly discovered antibiotic.
 Quantitative Approach – This tells us about the yields of antibiotic which can be
expected when the microorganism is grown in various differing media.
 Secondary screening should yield types of information which are needed in order
to evaluate the true potential of a microorganism for industrial usage.
 Secondary screening should determine whether a more economical process is
possible.
 To determine whether a product actually is newly discovered sompound, we can
utilize paper, thin layer or other chromatographic procedures to compare the
product with known compounds.
 Secondary screening should detect real differences in product yiels potentials
among various isolates.

8.  Secondary screening should reveal whether there are pH, aeration, or other
critical requirements associated with particular microorganisms, both for the
growth of the organism and for the formation of chemical products.
 Screening should show whether certain medium constituents are missing or
possibly are toxic to growth of the organism, or its ability to accumulate
fermentation products.
 It should show something of chemical stability of the product and product
solubility.
 It should determine whether the product has simple, complex, or even a
macromolecular structure.
 It should show whether the product possesses physical properties such as U.V.
light absorption or fluorescence, or chemical properties that can be employed
to detect compound.

9.  Certain kinds of fermentation products, determinations should be made as to
whether gross animal, plant or human toxicity can be attributed to the
fermentation product, particularly if it is to be utilized in disease treatment.
 Secondary screening should reveal whether a product resulting from a
microbial fermentation occurs in the culture broth in more than one chemical
form, and whether it is an optically or biologically active material.
 Secondary screening should reveal whether microorganisms are able to
chemically alter or even destroy their own fermentation products. Thus, a
microorganism might produce a “racemase” enzyme that will change the L-
configuration of an amino acid product to a mixture of the D and L- isomer of
being little biological value.

10. Thank You