Comparison of Six Major Microbial Strain Preservation Technologies: Scientific Selection to Stably Maintain Strain Fermentation Performance

In the fields of synthetic biology, microbial R&D, and industrial fermentation, engineered strains are core technological assets. The performance stability of high-yield strains, defective chassis strains, and high-expression engineered strains—obtained through multiple rounds of screening, mutagenesis, and gene editing—directly determines the yield and quality of fermentation batches. In actual R&D and production, issues such as strain performance degradation, reduced expression levels, contamination by foreign microorganisms, and loss of phenotypic traits are not typically caused by defects in the fermentation process, but rather by chronic deterioration resulting from inappropriate selection of strain preservation methods and non-standardized operation and maintenance.

The core logic of microbial strain preservation is to induce dormancy, reduce metabolic activity, and prevent genetic mutations through human intervention. Various preservation technologies in the industry have been developed and implemented around four core principles: low temperature, drying, oxygen exclusion, and metabolic inhibition. This article systematically reviews the six mainstream strain preservation methods in the industry, comprehensively analyzing the operating principles, scope of application, preservation periods, technical advantages and disadvantages, and practical considerations for each technology. It provides a standardized reference for establishing strain preservation systems in laboratory R&D, pilot-scale testing, and industrial production.

I. Slant Culture at Low Temperature: A Standard Short-Term Storage Method in Laboratories

Technical Principle

By maintaining a low-temperature environment of 4°C, the metabolic rate of microorganisms is suppressed, causing the strains to enter a semi-dormant state. Basic activity is sustained by trace nutrients in the slant culture medium, making this a simple and cost-effective short-term storage solution for microbial strains.

Storage Period

Bacterial strains: Pass culture once every 1–3 months; fungal, yeast, and Aspergillus strains: Pass culture once every 3–6 months.

Technical Advantages: No specialized equipment is required; the method is easy to perform and allows for flexible strain retrieval. It is well-suited for daily, high-frequency activation experiments and meets the needs for short-term transitional use of routine strains.

Limitations: This method has significant technical shortcomings and is not suitable for the preservation of high-value engineered strains. First, frequent passaging leads to the continuous accumulation of genetic mutations, which can easily result in the deterioration of traits and reduced expression levels in high-yield and gene-knockout strains. Second, the open operating environment makes it prone to the introduction of contaminating bacteria and mold, posing a high risk of contamination. Third, it only allows for short-term preservation and cannot lock in the original traits of the strain over the long term.

Applicable Scenarios

Limited to routine storage of standard strains, short-term experimental transitions, and frequent reactivation. Strictly prohibited for use with genetically modified strains, high-yield commercial strains, and the long-term storage of mass-production seed cultures.

II. Liquid Paraffin Coating Preservation: An Economical Mid-Term Strain Preservation Technique

Technical Principle

By covering the surface of the bacterial culture with sterile liquid paraffin, air and oxygen are isolated, thereby inhibiting the strain’s metabolic activity while preventing moisture evaporation from the culture medium, effectively extending the strain’s storage period.

Storage Period: Stable storage can last up to 6–12 months.

Technical Advantages: Requires no ultra-low temperature equipment; features low operational and maintenance costs and a simple procedure; offers superior strain stability compared to traditional low-temperature preservation on slant plates, making it a highly cost-effective medium-term preservation solution.

Technical Limitations: Strains still exhibit trace metabolic activity, which may lead to slight phenotypic degradation during long-term storage; the process of washing off the paraffin prior to strain reactivation is cumbersome; additionally, this method is not suitable for strictly anaerobic microbial strains.

Applications: Primarily suited for the medium-term storage of fungi such as yeasts, molds, and actinomycetes; ideal for low-cost transitional strain storage in small and medium-sized enterprises.

III. Glycerol Tube Cryopreservation: The Primary Long-Term Storage Solution for Research and Pilot-Scale Production

Glycerol cryopreservation is currently the most widely used and practical standardized long-term microbial strain preservation technique in laboratories and pilot-scale production facilities.

Technical Principle: A sterile glycerol solution with a final concentration of 15%–25% is prepared as a cell protectant. Under low-temperature conditions, this solution inhibits the formation of sharp ice crystals from water molecules, prevents cell membrane damage, induces a state of vitreous dormancy in the cells, significantly reduces metabolic activity, and maximizes the preservation of the strain’s original genetic traits.

Standard Operating Procedures

Mix fresh logarithmic-phase bacterial culture with 50% sterile glycerol in a 1:1 ratio and aliquot into cryopreservation tubes; first undergo a temperature gradient transition at –20°C, then store long-term at –80°C for ultra-low-temperature preservation.

Storage Duration: Strain characteristics can be stably preserved for 1–10 years.

Technical Advantages: Excellent genetic stability of strains with an extremely low probability of trait degradation; high strain revival rate and high degree of operational standardization; supports bulk aliquoting, suitable for large-scale preservation of engineered and recombinant strains; low overall cost, enabling rapid establishment of a standardized strain repository.

Technical Limitations: The primary contraindication is repeated freeze-thaw cycles; multiple freeze-thaw cycles can significantly reduce bacterial cell survival rates and disrupt microbial community structure and strain characteristics.

Applications: Widely applicable to various strains, including E. coli, Bacillus subtilis, yeast, Aspergillus niger, and lactic acid bacteria; it is the preferred method for routine long-term storage of genetically modified strains and high-yield fermentation strains.

IV. Vacuum Freeze-Drying Preservation: Standardized Preservation Technology for National Standard Strain Banks

Technical Principle

Through a combination of low-temperature pre-freezing and a vacuum rapid dehydration process, the microbial cells are placed in a deep dormant state characterized by the absence of water and zero metabolism, thereby completely eliminating the environmental conditions that could lead to genetic mutations or phenotypic degradation of the strains.

Preservation Period: Strain characteristics can be stably preserved for 10–20 years.

Technical Advantages: Exceptionally strong ability to lock in traits, with an extremely low probability of strain mutation; the final product can be stored at room temperature away from light, without relying on low-temperature storage equipment; compact size and convenient transportation, compliant with national standards for microbial strain preservation.

Technical Limitations: High equipment investment costs and complex operational procedures; low survival rates after freeze-drying for some filamentous fungi and hyphal strains; relatively high daily operational and maintenance costs, making it unsuitable for high-frequency, routine preservation in industrial settings.

Applicable Scenarios: Suitable for the long-term preservation of resources in national standard strain repositories, patented strains, standard reference strains, and core corporate archive strains.

V. Cryopreservation in Liquid Nitrogen: The Ultimate Preservation Solution for Industrial-Grade Core Strains

Cryopreservation in liquid nitrogen is currently the industry’s most stable and advanced technology for the permanent preservation of microbial strains, making it ideal for high-value core strain resources.

Technical Principle

In an ultra-low temperature environment of -196°C, enzymatic reactions, metabolic activity, and genetic replication processes are completely halted, placing the strains into a state of absolute dormancy. Theoretically, this enables permanent preservation.

Preservation Duration: Strain activity and traits show virtually no degradation over several decades.

Technical Advantages: Among all preservation technologies, this method ranks first in terms of cell survival rate and trait stability. It completely eliminates issues such as strain mutation, aging, and degradation, making it ideally suited for the preservation of multi-gene knockout strains, rare mutant strains, and core industrial platform strains.

Technical Limitations: The procurement and operational costs of liquid nitrogen tanks are high, and liquid nitrogen must be replenished periodically to maintain the low-temperature environment; practical operations involve safety risks such as frostbite and hypoxia, and the strain transportation process is cumbersome.

Applicable Scenarios: Specifically designed to provide permanent backup storage for enterprises’ core patented strains, commercial mass-production master cultures, and irreplaceable, precious platform strains.

VI. Sand-Soil Tube Preservation: A Preservation Technique Exclusively for Spore-Forming Microorganisms

Technical Principle

This method utilizes sterile, dry sand to adsorb microbial spores. By creating a dry, oxygen-free environment, metabolic activity is suppressed, thereby specifically meeting the preservation needs of spore-forming microorganisms.

Storage Duration: Strain characteristics can be stably preserved for several years.

Applicability and Limitations: Suitable only for spore-forming microorganisms such as Bacillus subtilis and Bacillus licheniformis. This method is highly specialized and cannot be applied to non-spore-forming bacteria, fungi, or genetically engineered strains, resulting in limited applicability.

Guide to Selecting Microbial Strain Preservation Methods

· Based on practical R&D and mass production needs, preservation solutions can be precisely matched to specific scenarios: Frequent temporary reactivation and routine short-term experiments: Preferably preserve on slants at 4°C

· Low-cost, medium-term transitional preservation: Use preservation covered with liquid paraffin

· Routine laboratory engineering strains and long-term daily storage: Preferably freeze-preserve in glycerol tubes at -80°C

· Standard strains and long-term archival resources: Use vacuum freeze-drying preservation

· Core patented strains and permanent resource backups: Use liquid nitrogen cryopreservation

· Strains specific to spore-forming microorganisms: Use sand-filled tube storage

Common Misconceptions in Microbial Strain Preservation

The decline in strain performance and abnormal batch-to-batch variability are largely attributable to improper preservation practices. The following four common misconceptions must be avoided:

1. Relying exclusively on slant inoculation for preservation: Frequent passaging of recombinant strains, knockout strains, and high-yield engineered strains accelerates trait degradation and directly leads to a decline in fermentation yield; therefore, this method should not be used for long-term preservation.

2. Repeated freeze-thaw cycles of glycerol vials: Thawing and reusing a single vial multiple times causes massive cell death and disrupts the microbial balance, leading to stability variations across fermentation batches. Standard practice requires aliquoting into multiple vials, using only one vial per batch, and discarding it after use.

3. Lack of Standardized Traceability Controls: Without clear strain identification, passage records, or freezing and reactivation logs, it is impossible to trace the root cause of subsequent fluctuations in production capacity or abnormal traits.

4. Single Preservation Method Without Backup: A mature strain management system requires a two-tier backup mechanism. A glycerol bank at -80°C should meet daily needs, while liquid nitrogen storage should serve as permanent backup for core strains, mitigating risks associated with equipment failure and strain loss.

Summary

The core competitiveness of the fermentation industry lies not only in process optimization and purification technologies but also in standardized, systematic strain preservation and management capabilities. While processes can be iteratively optimized, the degradation or loss of core strain resources will result in irreversible losses in R&D and production.

Slope culture is suitable for short-term transition; glycerol freezing supports daily mass production; freeze-drying technology is used for standard archiving; and liquid nitrogen preservation serves as the cornerstone of core strain preservation. While conventional preservation methods may suffice for ordinary strains, high-value engineered strains and commercial high-yield host strains must be preserved using high-standard protocols. Only by establishing a scientific, two-tiered strain preservation system, standardizing passage management, and eliminating unnecessary genetic variation caused by passage can a company stably maintain strain fermentation performance, ensure batch-to-batch consistency, and safeguard its core biological assets and production capacity.