Refrigeration is the most common method used to slow microbial growth and preserve cultures for short periods of time.  Cultures can also be preserved by deep-freezing at temperatures between -50 oC and -90 oC in liquid nitrogen.  Lyophyllization (freeze drying) involves the removal of water from a culture under low atmosphere and temperature conditions.  Lyophyllized cultures can be stored as a dry powder for long periods of time and restored by mixing with liquid media.