How should laboratory strains be preserved?

Microbial strains are one of the most fundamental and critical biological resources in a lab; preservation quality directly affects the reliability and reproducibility of downstream experiments. Different preservation strategies vary markedly in survival duration, genetic stability, operational difficulty, and cost. Simply “putting them in a fridge” often fails to promptly reveal contamination, trait drift, or loss of viability.

1. Low-temperature slant storage

【Principle】

Low-temperature slant storage uses ~4 °C to markedly slow microbial metabolism while solid medium provides basic nutrients, keeping strains active without overgrowth for a short period—thus suitable as a short-term working stock.

【Key steps】

Inoculate the target strain onto a slant; after uniform, healthy growth, seal the tube mouth with Parafilm or paraffin to reduce evaporation and external contamination. Store at 4 °C. Perform use and subculture under aseptic conditions to minimize contamination risk.When preparing slant media, high-purity agar and sodium chloride can be used together with buffering salts such as dipotassium hydrogen phosphate to adjust and stabilize the pH, helping maintain consistent medium properties and strain growth behavior during low-temperature storage.

【Storage time & applicable organisms】

Under proper conditions, most bacteria remain stable ~1–3 months; fungi ~3–6 months. Applicable to most common lab bacteria, yeast, and filamentous fungi, but overall better for short or medium-short term.

【Advantages】

Low equipment requirements, low consumable cost, simple workflow; convenient for quickly establishing working stocks; intuitive to retrieve—common in routine labs and teaching.

【Limitations & risks】

Shorter storage time requiring periodic subculture; frequent handling increases contamination risk; prolonged reliance on serial transfers can accelerate genetic variation/trait drift—undesirable when strict genetic stability is required.

【Typical use cases】

Short-term “working stock” for routine strains; teaching demos; temporary or mid-short-term storage in staged projects.

2. Liquid paraffin overlay

【Principle】

Building on cold slants, a sterile mineral oil layer physically isolates the culture from air, reducing O₂ exposure and evaporation, suppressing metabolism and medium cracking, thereby extending storage at 4 °C.

【Key steps】

Grow a pure, healthy lawn on a slant; under aseptic conditions slowly add pre-sterilized liquid paraffin to fully cover the lawn and medium surface; seal and store upright at 4 °C. Control addition speed to avoid bubbles or disturbing the lawn, reducing introduced contamination.The overlaying liquid paraffin is preferably a sterile, filtered mineral oil with appropriate viscosity and low impurity levels, forming a stable layer at 4 °C that reduces water evaporation and minimizes the entry of particulates and microorganisms.

【Storage time & applicable organisms】

Typically stable ~6–12 months if done correctly. Especially suitable for strains that do not readily sporulate (e.g., actinomycetes, yeast) requiring mid-term storage; useful where strains are moisture-sensitive and ultra-low freezers are unavailable.

【Advantages】

Significantly extends usable period versus cold slants alone, reducing subculture workload and drift risk while keeping equipment/skill barriers low.

【Limitations & risks】

Each retrieval must pierce the oil layer—poor asepsis can introduce contaminants; paraffin is flammable—follow safety rules in melting, sterilizing, and storage. Not all strains tolerate low-oxygen, oil-covered environments (strict aerobes or special nutrient demands may be unsuitable).

【Typical use cases】

Mid-term storage of actinomycetes, yeast, etc., in labs lacking ultra-low freezers; extends storage under routine refrigeration when lyophilization or liquid nitrogen are not options.

3. Glycerol cryotube storage

【Principle】

Glycerol acts as a cryoprotectant at −80 °C, reducing ice-crystal damage and markedly suppressing metabolism, placing strains in a stable frozen state for long-term viability and genetic integrity—one of the most common long-term lab methods.

【Key steps】

Grow dense, healthy culture; mix culture 1:1 with 40% glycerol, mix well, aliquot to cryotubes, minimize bubbles and freeze–thaw cycles; place immediately at −80 °C. For recovery, inoculate a small amount directly into fresh medium; minimize room-temperature exposure to avoid temperature cycling.

【Storage time & applicable organisms】

At −80 °C most bacteria remain stable ~1–2 years; many fungi ≥5 years. Well suited for E. coli, yeast, and most routine lab strains—high overall cost-effectiveness for long-term storage.

【Advantages】

Standardizable, reproducible, amenable to scale; simple recovery; high survival rates. Compared with liquid nitrogen or lyophilization, capital and maintenance costs are moderate—often first-choice for strain banks.

【Limitations & risks】

Relies on ultra-low freezers; power outages, failures, or temperature fluctuations risk viability loss. Some freeze-sensitive or fastidious strains may need optimized protectants and cooling profiles. For critical strains, glycerol alone poses residual risk—pair with lyophilization or LN₂ backups.

【Typical use cases】

General long-term storage for most lab strains, especially model organisms (E. coli, yeast); supports high-throughput screening and projects requiring repeated, long-term use with traceable sources.

4. Freeze-drying (lyophilization)

【Principle】

Mix cells with protectants, pre-freeze at low temperature, then under vacuum sublimate ice for low-temperature dehydration, driving microbes into a highly desiccated dormant state with near-halted metabolism—preserving genomes and structures for many years.

【Key steps】

Prepare cell suspension with protectant; pre-freeze; lyophilize under controlled vacuum/temperature; flame-seal or tightly cap ampoules/vials aseptically. Typical lyophilization protectant systems often include disaccharides such as trehalose and sucrose, combined with suitable buffers or basal media, which help alleviate osmotic and desiccation stress during freezing and sublimation and stabilize cell membranes and proteins.Optimize protectant type/concentration and pre-freeze/heating rates to improve survival through lyophilization and recovery.

【Storage time & applicable organisms】

With optimized process, storage easily reaches 5–10+ years, applicable to most bacteria and fungi tolerant of lyophilization. Extremely desiccation-sensitive strains require screening of protectants and parameters.

【Advantages】

Longest storage durations with good genetic stability; relaxed transport/storage—short-term ambient shipping without cold chain, facilitating inter-lab exchange and standardized repositories.

【Limitations & risks】

Requires specialized equipment with higher capital/maintenance; more complex process needing trained personnel; interspecies tolerance varies widely—suboptimal protectants/parameters can sharply reduce survival or hinder revival. Pilot optimization is recommended.

【Typical use cases】

Standard culture collections, key resources, patented strains for long-term or “quasi-permanent” storage; ideal where frequent distribution/exchange is needed while maintaining genetic stability.

5. Liquid-nitrogen ultralow storage

【Principle】

At −196 °C, cellular metabolism is essentially halted, placing strains/cells into deep-cold dormancy for extremely long-term preservation with maximal genetic/physiological stability—the most robust option in microbe/cell repositories.

【Key steps】

Mix culture with suitable cryoprotectant (e.g., DMSO or glycerol), apply controlled-rate or slow cooling to minimize ice damage, then store in gas or liquid phase of LN₂. For recovery, rapidly thaw and inoculate into fresh medium; avoid repeated excursions from LN₂ and unnecessary temperature cycling.

【Storage time & applicable organisms】

With proper management and reliable LN₂ supply, effective duration commonly exceeds 10 years; in theory, ultra-long-term. Broadly applicable to microbial strains and diverse animal/plant/fungal cell lines; excellent for engineered strains requiring stringent trait conservation.

【Advantages】

Maximizes long-term genetic and phenotypic stability, outperforming −80 °C and most other methods; ideal “ultimate backup” for precious, slow-growing, or hard-to-reconstruct strains/cell lines; integrates with other methods to form multilayer safety nets.

【Limitations & risks】

Higher equipment and operational demands; safety considerations (ampoule rupture, frostbite); requires strong safety management. LN₂ mismanagement or supply interruption risks uncontrolled warming; mandates thorough training, routine checks, and contingency plans.

【Typical use cases】

Long-term preservation of rare strains, critical engineered strains, and important cell lines—any core resource where storage failure is unacceptable. Serves as an ultimate backup alongside −80 °C glycerol stocks or lyophilization to provide redundant protection.

Aladdin: https://www.aladdinsci.com/