COURSE: B.SC. (MICRO)SUBJECT: ELEMENTARY MICROBIOLOGY

UNIT 4.3

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When you have developed good strains of microorganisms, it must be preserved properly, so that it could be used in future also.

If the cultures have not properly been preserved, their characteristic features may decline after certain period.

Therefore, several methods have been developed for maintaining the organisms in viable conditions over a long period of time. These methods vary according to strains.

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Agar stabs or slants -prepared after autoclaving the medium.

inoculated carefully with individual culture and incubated overnight.

stored at 5 to -20°C.

sub-cultured at an interval of six months.

Then the cultures are covered with mineral oil to a depth of 1 cm above the slant. They are sub-cultured after the storage of about 1 year.

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Bacteria -salts for various metabolic activities.

But at high concentration, salts act as bacteriostatic (temporary growth inhibitor).

Hence, bacterial culture is preserved in 1% salt concentration in screw-capped tubes to prevent evaporation.

The tubes are stored at room temperature.

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The cultures - dried over CaCl2 in a vacuum, then stored in a refrigerator.

At such conditions, the organisms survive for longer period than the air dried cultures.

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microbial suspension is put in small ampoules and frozen through drying under vacuum.

Vacuum drying results in sublimation of cell water.

Then the ampoules are sealed off in vacuum.

The lyophilised cultures are stored at 4°C.

The cultures remain viable for about 10 years.

By this method a large number of cultures are maintained without variation in their characteristics.

Easy to transport

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The cultures are grown in culture tubes.

10-30% of a cryo-protective agent (e.g. Glycerol or dimethyl

sulfoxide) is added into it so that it could protect the culture from

cold sock.

The culture is dispensed into ampoules.

The ampoules are properly sealed and kept in liquid nitrogen at -176

to -196°C.

The liquid nitrogen method has been successful with many species

that cannot be preserved by Lyophilization and most species can

remain viable under these conditions for about 30 years or more

without undergoing change in their characteristics.

Expensive

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Whichever technique is used for the preservation, maintenance of industrially important organisms it is essential to check the quality of the preserved organisms stocks.

Each batch of newly preserved cultures should be routinely checked to ensure their quality.

A single colony is transferred into a shake-flask to ensure growth of particular kind of microorganism; further shake-flask subculture is used for the preparation of huge quantity of vials.

For the assessment of purity, viability and productivity of cultures few vials are tested. If samples fail any one of these tests the entire batch should be destroyed. Thus, by the use of such a quality-control system stock cultures many be retained, and used, with confidence.

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