Culture preservation — Which storage method is best suited for long-term preservation of microbial samples when performed correctly?

Introduction / Context:
Maintaining culture viability and genetic stability over months to years requires appropriate preservation strategies. Temperature strongly influences metabolic activity, mutation rates, and viability loss. This question contrasts common but suboptimal practices with best-practice long-term storage.

Given Data / Assumptions:

• “Long-term” implies many months to years.
• Viability is enhanced by deep freezing (often with cryoprotectants like glycerol).
• Short-term working stocks can be kept on slants or plates but degrade over time.

Concept / Approach:
Ultra-low temperature storage (≈ −70°C to −80°C) dramatically slows biochemical reactions and preserves cells. Many labs use 15–25% glycerol stocks in cryovials at −80°C. Alternatives include lyophilization (freeze-drying) for certain organisms, but among the provided options, ultra-low freezers are best for general long-term use.

Step-by-Step Solution:

Verification / Alternative check:
Microbiology methods manuals recommend −80°C glycerol stocks or lyophilization for long-term repositories; many culture collections rely on these approaches for stability and reproducibility.

Why Other Options Are Wrong:

• −10°C: ice crystal damage and residual metabolism impair longevity.
• Refrigerator slant: weeks to a few months at best.
• Room-temperature plate: unsuitable for preservation.

Common Pitfalls:
Skipping cryoprotectant or multiple freeze–thaw cycles, which reduce viability; always aliquot to avoid repeated thawing.

Final Answer:
Ultra-low temperature freezer (≈ −70°C)