What methods are used for long-term preservation of microorganisms?

Long-term preservation methods include: cryopreservation (storage at -80C or in liquid nitrogen at -196C with cryoprotectants like glycerol or DMSO - most common for cell banks), lyophilization/freeze-drying (removal of water under vacuum from frozen state - excellent for stable storage at room temperature), and preservation in inert atmospheres. Short-term methods include refrigeration (weeks to months) and periodic subculturing (risk of genetic drift). Cryopreservation maintains viability and genetic stability for decades. Critical factors: cooling rate, cryoprotectant concentration, storage temperature stability, and thawing procedure. Recovery testing ensures method validity.