Question

Why does a lyophilized culture of microbes remain viable for several years?

Answer 1

A pure culture of an organism is valuable and hence needed to be preserved for future use. The cultures which are of daily use, are normally grown in agar and kept in the refrigerator. The underlying principle for various methods of long-term preservation is to slow down the metabolic rate to an extreme minimum level.

Complete answer:
Long term preservation can be achieved by exclusion of water which is essential for optimal functioning of life processes, or by lowering the temperature to a level where normal enzyme functions are practically nil. In case of aerobic organisms, the exclusion of oxygen is also helpful in slowing down their metabolic activities.
For long time preservation of bacterial culture, one of the best methods is immobilization which involves freeze-drying of a small volume of the suspension of an organism taken in a small ampoule. The suspension of an organism is frozen by rotating it in a super cooled bath of alcohol or acetone (-80°is as a thin film. Several suspensions are then evacuated until the contents become dry and powdery.
The neck of the ampoules is sealed under vacuum and then preserved in the refrigerator (4°C). The viability of the lyophilized cultures varies from species to species and therefore requires testing at reasonable intervals. For reviving the lyophilized culture, the ampoule is broken and its content is aseptically transferred to a fresh medium and incubated until growth occurs.
For lyophilization, sophisticated equipment is necessary.
In lyophilization, the culture is dried by the exclusion of water and then refrigerated at a very low temperature. Because of this, optimal functioning of life processes and metabolic rate slow down and the culture of microbes remains viable for several years.

Note:
Another effective means of long-term preservation is cryopreservation in which culture is preserved at the temperature of liquid nitrogen (-196°C) where all enzyme-catalyzed biochemical reactions are practically shunted off. The organism remains in a state of suspended animation and they retain their original characters for a long time.