Bacterial Strain Cryopreservation Methods: Glycerol Preservation, Ultra-Low Temperature Storage, and Recovery Quality Control

Bacterial strain cryopreservation is used to preserve strain viability, genetic stability, and phenotypic characteristics over the long term. Common methods include glycerol cryopreservation, compound cryoprotectant-based cryopreservation, ultra-low temperature storage at -80°C, liquid nitrogen storage, and freeze-drying. The actual method should be selected according to strain tolerance, storage duration, recovery frequency, and downstream experimental requirements.

Keywords: bacterial cryopreservation; strain preservation; glycerol stock; bacterial cryopreservation solution; -80°C storage; compound cryoprotectant; strain recovery; cryopreservation quality control

1 Basic Principles of Bacterial Strain Cryopreservation

1.1 Purpose of Cryopreservation

(1) Maintaining strain viability

The core purpose of cryopreservation is to reduce bacterial metabolic activity so that strains remain recoverable over an extended period. Compared with continuous subculture, cryopreservation can reduce mutation accumulation, phenotypic drift, and contamination risk.

(2) Maintaining genetic stability

Experimental strains, engineered strains, reference strains, and quality control strains should be passaged as few times as possible. Establishing a master seed bank and a working seed bank can reduce genetic changes caused by repeated recovery and serial subculture.

(3) Maintaining experimental traceability

Bacterial cryopreservation records should include strain number, source, passage number, culture conditions, cryopreservation date, cryoprotectant system, and operator information. Unclear labeling or missing records can directly affect the reproducibility of subsequent experiments.

1.2 Factors to Confirm Before Cryopreservation

(1) Strain type

Different bacteria vary markedly in their tolerance to low temperature, osmotic stress, and freezing injury. Escherichia coli, Bacillus subtilis, lactic acid bacteria, anaerobes, actinomycetes, and clinical isolates may require different cryoprotectants and recovery conditions.

(2) Culture status

Cultures selected for cryopreservation should generally be healthy, uncontaminated, and phenotypically stable. Over-aged cultures may have reduced viability, whereas cultures at very early growth stages may have insufficient biomass; both can affect recovery success.

(3) Cryoprotectant system

Glycerol, DMSO, sucrose, trehalose, gelatin, BSA, skim milk, or medium components can be used as cryoprotectants or as part of compound cryoprotectant systems. Cryoprotectants reduce ice crystal damage, osmotic shock, and cell membrane disruption.

(4) Storage temperature

Short-term storage, long-term cryopreservation, and seed bank management have different temperature requirements. Ultra-low temperature storage at -80°C is a common strategy, while liquid nitrogen is suitable for longer-term storage or strains requiring higher stability.

Table 1 Comparison of commonly used bacterial strain preservation methods

Preservation Method	Applicable Duration	Main Use	Main Advantages	Main Limitations
Short-term slant preservation	Weeks to months	Temporary strain maintenance and short-term turnover	Simple operation and convenient recovery	Susceptible to passage drift and contamination
Short-term storage at 4°C	Short term	Temporary storage of cultured plates or slants	Suitable for short-term backup	Not suitable for long-term storage
Storage at -20°C	Short-term or transitional storage	Temporary storage of some tolerant strains	Low equipment requirement	More obvious ice crystal damage; not recommended for long-term strain preservation
-80°C glycerol cryopreservation	Long term	Routine bacterial strain bank	Broad applicability and relatively high recovery rate	Requires stable ultra-low temperature equipment
Liquid nitrogen storage	Long term	Valuable strains and low-temperature-sensitive strains	High storage stability	High equipment and management requirements
Freeze-drying preservation	Long term	Reference strains, transportation, and long-term preservation	Convenient transportation at room temperature or low temperature	High process requirements; some strains are not tolerant

2 Glycerol Cryopreservation

2.1 Method Positioning

Glycerol cryopreservation is one of the most commonly used bacterial cryopreservation methods and is applicable to most routine experimental strains. Glycerol reduces ice crystal formation and osmotic shock. When combined with -80°C storage, it enables long-term strain preservation.

(1) Applicable strains

Common laboratory strains, engineered strains, general environmental isolates, and most aerobic bacteria can be prioritized for glycerol cryopreservation. For special strains, the cryoprotectant system should be adjusted according to strain characteristics, recommendations from strain repositories, or pilot experimental results.

(2) Cryoprotectant grade

Glycerol used for strain cryopreservation should preferably be of stable quality, such as molecular biology grade, anhydrous grade, ACS grade, AR grade, microbiology grade, or cell culture-compatible grade. Analytical standards or standard solutions are more suitable for quantitative analysis and are not preferred as routine cryoprotectants.

(3) Cryovial management

Cryovials should be sterile, resistant to low temperature, and well sealed. Each cryovial should be recovered only once to avoid reduced viability and contamination risk caused by repeated freeze-thaw cycles.

2.2 Key Control Points for Glycerol Cryopreservation

(1) Bacterial status

Before cryopreservation, the culture should be confirmed to be uncontaminated, morphologically consistent, and in good growth condition. For engineered strains requiring plasmid or expression system maintenance, selection pressure and target construct stability should also be confirmed.

(2) Mixing method

The bacterial suspension and glycerol cryoprotectant should be thoroughly mixed to avoid local inconsistency in cryoprotectant concentration. The mixing process should minimize aerosol generation and contamination risk, and room-temperature exposure should be shortened as much as possible.

(3) Cooling process

Routine bacterial glycerol stocks can usually be placed directly at -80°C for storage. For low-temperature-sensitive or difficult-to-recover strains, controlled-rate cooling or dedicated freezing containers may be considered to reduce injury caused by rapid cooling.

(4) Recovery method

During recovery, the cryovial should remain at room temperature for as short a time as possible. After sampling, the vial should either be immediately returned to low-temperature storage or discarded if thawed, to avoid repeated freeze-thaw cycles.

3 Compound Cryoprotectant-Based Cryopreservation

3.1 Significance of Compound Cryoprotectants

A single glycerol system is not suitable for all bacteria. Lactic acid bacteria, anaerobes, some environmental bacteria, and difficult-to-culture strains may require compound cryoprotectants to improve post-thaw survival and recovery stability.

(1) DMSO

DMSO can be used as a component of some cryoprotective systems, but it has certain toxicity to cells and microorganisms. Exposure time, working concentration, and removal after recovery all need optimization. For routine bacterial cryopreservation, glycerol is usually preferred, while DMSO is more suitable as an alternative cryoprotectant for special strains or optimized systems.

(2) Sugar-based cryoprotectants

Sucrose and trehalose can reduce cryopreservation injury by stabilizing membrane structures and protein conformations. They are often used in freeze-drying or compound cryopreservation systems. For lactic acid bacteria, probiotics, and some freeze-thaw-sensitive strains, sugar-based cryoprotectants can be combined with glycerol, protein protectants, or culture medium.

(3) Protein and colloidal protectants

Gelatin, BSA, skim milk, and related materials can provide colloidal protection and stabilize the microenvironment. They are suitable for freeze-drying systems or strains sensitive to freeze-thaw damage. Sterility, batch consistency, and compatibility with downstream detection methods should be considered when using these materials.

3.2 Applicable Scenarios

(1) Lactic acid bacteria and probiotics

Lactic acid bacteria are relatively sensitive to freezing and drying and often require sugars, skim milk, or compound cryoprotectants to improve post-storage viability. After recovery, colony morphology, acid production capacity, and growth curves should be evaluated.

(2) Anaerobes

For anaerobic bacteria, oxygen exposure must be controlled in addition to cryoprotection. The cryopreservation system, aliquoting, recovery, and culture process should maintain appropriate low-oxygen or anaerobic conditions as much as possible.

(3) Actinomycetes and filamentous bacteria

Some actinomycetes or filamentous bacteria can be cryopreserved according to their spore, mycelial, or dispersed cell state. If obvious bacterial aggregation occurs, uniformity of cell quantity within cryovials should be considered.

Table 2 Key points for selecting bacterial cryoprotectants

Cryoprotectant Type	Representative Materials	Main Function	Applicable Scenarios	Notes
Polyols	Glycerol, DMSO	Reduce ice crystal formation and osmotic injury	Routine bacteria, engineered strains, special cryoprotective systems	DMSO should not be used as the default option for all bacteria
Sugars	Sucrose, trehalose	Stabilize membrane structures and protein conformations	Lactic acid bacteria, freeze-drying protection, sensitive strains	Concentration needs optimization to avoid excessive osmotic pressure
Protein/colloid protectants	Gelatin, BSA, skim milk	Provide colloidal protection and reduce freeze-thaw injury	Sensitive strains and freeze-drying systems	Batch variation and sterility should be considered
Medium systems	TSB, SOC, R2A, etc.	Provide an adaptive recovery environment	Routine strains or strains with special nutritional requirements	Should match strain culture characteristics
Buffer systems	PBS, phosphate buffer	Control pH and ionic environment	Dilution, washing, or compound cryoprotective systems	Limited nutritional value; not suitable as a protective solution for all strains

4 Ultra-Low Temperature and Liquid Nitrogen Storage

4.1 Ultra-Low Temperature Storage at -80°C

Storage at -80°C is suitable for most laboratory strain banks and is the most common management strategy for long-term bacterial cryopreservation. The key factors are not only temperature, but also aliquoting strategy, cryopreservation records, and equipment stability.

(1) Master bank and working bank

The master bank is used for long-term storage and should be opened and recovered as rarely as possible. The working bank is used for routine experimental recovery. Separate management of the master bank and working bank can reduce loss of valuable strains.

(2) Multiple aliquots

Multiple cryovials should be established for the same strain to avoid repeated sampling from a single vial. One vial should be used for each recovery to reduce contamination and freeze-thaw injury.

(3) Temperature fluctuation control

Cryoboxes should be placed in fixed positions to reduce frequent door opening and prolonged exposure. Temperature fluctuations accelerate loss of viability, especially for sensitive strains.

4.2 Liquid Nitrogen Storage

Liquid nitrogen storage is suitable for strains requiring long storage duration, high value, or difficult recovery. Compared with -80°C storage, liquid nitrogen further reduces metabolic activity and injury risk, but requires more stringent management.

(1) Applicable strains

Valuable reference strains, preserved engineered strains, slow-growing strains, and strains with unstable recovery rates can be considered for liquid nitrogen storage.

(2) Safety management

Liquid nitrogen storage requires compatible cryovials and storage racks to avoid tube rupture, label detachment, and loss of cryopreservation information.

(3) Recovery verification

Liquid nitrogen storage does not eliminate the need for quality control. Long-term preserved strains should still be periodically verified for recovery rate, purity, and key phenotypes.

5 Freeze-Drying Preservation

5.1 Method Positioning

Freeze-drying is suitable for some reference strains, industrial strains, and strains requiring transportation. After freeze-drying, strains can be stored long term at low moisture content, making transportation and distribution more convenient.

(1) Applicable strains

Spore-forming bacteria, some desiccation-tolerant bacteria, and process-optimized reference strains are more suitable for freeze-drying. For freeze-drying-sensitive strains, -80°C or liquid nitrogen storage should be prioritized.

(2) Cryoprotectant system

Freeze-drying protectants often contain sugars, proteins, or compound protective systems to reduce structural damage during freezing, drying, and rehydration. Trehalose, sucrose, gelatin, BSA, and skim milk-related materials are commonly used for optimization of such systems.

(3) Rehydration conditions

During recovery of freeze-dried strains, rehydration medium, temperature, and standing time can affect recovery performance. After rehydration, the strain should be transferred to a suitable culture system as soon as possible to reduce stress duration.

5.2 Limitations of Freeze-Drying Preservation

(1) Strong process dependency

Freeze-drying performance is affected by pre-freezing, sublimation, protectants, bacterial status, and residual moisture. An unoptimized freeze-drying process may lead to a marked decrease in recovery rate.

(2) Not suitable for all strains

Some Gram-negative bacteria, non-spore-forming bacteria, or desiccation-sensitive strains may show low survival after freeze-drying and should be validated through pilot experiments.

(3) High quality control requirements

After freeze-drying preservation, purity, viability, phenotype, and key functions still need to be verified, especially for industrial strains and reference strains.

Table 3 Selection of cryopreservation strategies for different bacterial strain types

Strain Type	Recommended Preservation Method	Protective System Selection	Quality Control Focus
Routine laboratory strains	-80°C glycerol cryopreservation	Glycerol + compatible medium	Recovery rate, colony morphology, contamination check
Engineered strains	-80°C master/working bank	Glycerol + selective culture background	Plasmid stability, target gene or expression function
Lactic acid bacteria	-80°C storage or freeze-drying	Glycerol/sugars/skim milk compound system	Viable count, acid production capacity, growth curve
Anaerobes	-80°C storage or liquid nitrogen	Compound cryoprotectant + low-oxygen handling	Anaerobic recovery, purity, and growth viability
Spore-forming bacteria	-80°C storage or freeze-drying	Glycerol system or freeze-drying protectants	Spore formation status and recovery consistency
Valuable reference strains	-80°C + liquid nitrogen backup	Validated protective system	Periodic recovery, identity confirmation, phenotypic stability
Difficult-to-culture strains	Liquid nitrogen or optimized cryopreservation	Compound cryoprotectants	Recovery rate and reproducibility of culture conditions

6 Recovery and Quality Control

6.1 Recovery Principles

The goal of recovering cryopreserved strains is to rapidly restore growth while avoiding contamination and selection bias. Recovery conditions should be consistent with, or as close as possible to, the pre-cryopreservation culture conditions.

(1) Minimize freeze-thaw cycles

Cryovials should be processed as soon as possible after removal from low temperature. Thawed cryovials are not recommended to be returned to the long-term storage bank.

(2) Select an appropriate recovery medium

The recovery medium should match the characteristics of the strain. Routine bacteria can be recovered using systems such as TSB, nutrient broth, or SOC. Nutrient-poor or environmental isolates may be recovered using media such as R2A, while anaerobes may require compatible systems such as thioglycollate medium.

(3) Observe colony morphology

After recovery, colony size, color, margin, transparency, and growth rate should be observed. Abnormal colonies should be assessed for contamination, mutation, or selection bias caused by recovery stress.

6.2 Cryopreservation Quality Control

(1) Purity check

After recovery, purity should be confirmed by plate streaking, microscopic observation, or molecular identification when necessary. Mixed cultures or contamination can affect downstream experimental results.

(2) Identity confirmation

Key strains should be confirmed by strain-specific PCR, sequencing, mass spectrometry identification, or phenotypic testing, especially after long-term storage or multiple transfers.

(3) Functional verification

Engineered strains, enzyme-producing strains, fermentation strains, and standard quality control strains should be verified for key functions such as antibiotic resistance, product formation, enzyme activity, plasmid retention, or standard reaction characteristics.

(4) Distinction from RNA sample preservation

If cryopreserved strains will later be used for transcript-level analysis, “viable strain preservation” and “RNA sample preservation” should be distinguished. Bacterial cryopreservation solution is used to maintain strain recoverability, whereas non-freezing bacterial RNA preservation solution is used to stabilize bacterial RNA and is suitable for subsequent RNA extraction, RT-qPCR, or transcriptome analysis. It cannot replace viable bacterial cryoprotectants.

(5) Record management

Cryopreservation records should include strain number, cryopreservation date, cryoprotectant system, medium, passage number, storage location, and recovery results. Strain bank management determines long-term traceability more than a single cryopreservation operation.

Table 4 Quality control points for bacterial cryopreservation and recovery

Step	Key Control Point	Possible Problem	Recommended Treatment
Pre-cryopreservation culture	Select a healthy culture	Aged cells and low viability	Use cultures in good growth condition
Cryoprotectant mixing	Consistent concentration and thorough mixing	Insufficient protection or osmotic injury	Standardize formula and mixing method
Aliquoting and labeling	Multiple aliquots and clear labels	Label detachment or sample confusion	Use cryogenic labels and electronic records
Storage temperature	Stable -80°C or liquid nitrogen	Temperature fluctuation and repeated freeze-thaw	Reduce door opening and handling time
Recovery culture	Match culture conditions	Slow recovery or selection bias	Use appropriate medium and recovery conditions
RNA sample preservation	Distinguish RNA preservation solution from viable cryopreservation solution	Misuse of RNA preservation solution as a cryoprotectant	Select preservation system according to downstream detection purpose
Purity check	Streaking and morphology observation	Contamination or mixed culture	Recover from a backup vial
Functional verification	Phenotypic or molecular validation	Plasmid loss or functional decline	Trace back to the master bank or rebuild the strain bank

7 Common Problems and Troubleshooting

7.1 No Growth After Recovery

Recovery failure may result from poor bacterial status before cryopreservation, unsuitable cryoprotectants, temperature fluctuation, excessive storage duration, or mismatched recovery conditions. Other cryovials from the same batch should be checked first, and cryopreservation records and culture conditions should be reviewed.

7.2 Smaller Colonies or Slow Growth After Recovery

Smaller colonies may be associated with freeze-thaw stress, unsuitable culture medium, or reduced strain fitness. A recovery culture step can be performed first, followed by evaluation of whether the normal phenotype is restored.

7.3 Plasmid Loss After Recovery of Engineered Strains

Plasmid loss may be related to insufficient selection pressure during passaging, instability of the strain before cryopreservation, or improper selection pressure after recovery. Before establishing an engineered strain bank, the plasmid and target insert should be confirmed. After recovery, antibiotic resistance or molecular validation should be performed.

7.4 Precipitation or Phase Separation in Cryovials

Precipitation or phase separation may result from medium components, insufficient mixing of cryoprotectants, or differences during freezing. If recovery rate is abnormal, the strain bank should be rebuilt and the cryoprotectant system optimized.

7.5 Large Recovery Differences Among Cryovials of the Same Strain

Differences may result from insufficient mixing of the bacterial suspension before aliquoting, inconsistent cell quantity among cryovials, or temperature fluctuations at storage locations. During strain bank establishment, suspension mixing, aliquot volume, and cryovial location management should be standardized.

8 Related Reagent and Material Selection

Table 5 Reagent and material selection for bacterial strain cryopreservation and recovery

Cat. No.	Product Name	Grade/Purity	Application Module	Application Positioning
R777486	Ready-to-Use Bacterial Preservation Solution	BioReagent,sterile	Ready-to-use cryopreservation system	Used for standardized bacterial strain cryopreservation, reducing batch-to-batch variation caused by self-prepared protective solutions
B777341	Bacterial Preservation Solution	BioReagent,sterile	Ready-to-use cryopreservation system	Used for bacterial strain aliquoting, low-temperature storage, and working strain bank establishment
B1520266	Non-freezing Bacteria RNA Preservation Solution	BioReagent,Suitable for molecular biology,ready-to-use	RNA sample preservation/molecular detection support	Used for stabilizing bacterial RNA samples, suitable for subsequent RNA extraction, RT-qPCR, or transcriptome analysis; not used for long-term viable bacterial cryopreservation and recovery
G116208	Glycerol	Suitable for molecular biology, ≥99%	Glycerol cryoprotectant	Used for preparing routine bacterial glycerol stocks, suitable for preservation of molecular biology laboratory strains
G755728	Glycerol	Anhydrous Grade, UltraBio™, Suitable for molecular biology, ≥99.5%(GC)	High-quality glycerol cryoprotectant	Used for strain cryopreservation systems requiring stricter control of water content, impurities, and molecular biology background
G116206	Glycerol	ACS, ≥99.5%	Glycerol cryoprotectant	Used for preparing routine laboratory glycerol-based cryopreservation systems
G116205	Glycerol	≥99.5%(GC)	Glycerol cryoprotectant	Used for bacterial cryopreservation systems requiring higher-purity glycerol
G116203	Glycerol	AR, ≥99%	Glycerol cryoprotectant	Used for preparing general bacterial strain cryopreservation solutions
G116209	Glycerol	for cell culture, suitable for insect cell culture, ≥99%(GC)	Biological experiment-grade cryoprotectant	Can be used in cryoprotective systems requiring higher biocompatibility
D755829	Dimethyl sulfoxide（DMSO）	sterile-filtered, BioReagent, suitable for hybridoma, ≥99.7%	Special cryoprotectant	Can be used as an alternative low-temperature protective component for special strains or compound cryoprotectant systems
D103277	Dimethyl sulfoxide（DMSO）	Suitable for molecular biology, ≥99.9%	Special cryoprotectant	Used for optimization of DMSO-based protective systems requiring molecular biology grade materials
D755902	Dimethyl sulfoxide（DMSO）	sterile-filtered, USP, suitable for hybridoma, Ph.Eur., BioPerformance Certified	Sterile DMSO cryoprotectant	Used for special cryopreservation systems requiring sterile, pharmacopeial-grade, or highly consistent DMSO
D274279	Dimethyl sulfoxide（DMSO）	Ultra pure, ≥99.9%	Special cryoprotectant	Used for low-impurity DMSO protective systems or method optimization
D433293	Dimethyl sulfoxide（DMSO）	Anhydrous Grade, ≥99.9%	Special cryoprotectant	Used for DMSO systems requiring strict water content control
S112234	Sucrose	Moligand™, Suitable for molecular biology, ≥99.5%(HPLC)	Sugar-based cryoprotectant	Used for compound cryoprotectants, freeze-drying protection systems, and protection of sensitive strains
S112226	Sucrose	Moligand™, Ultra pure, ≥99.9%, RNase,DNase Free	High-purity sugar protectant	Used for strain protection systems sensitive to nuclease background or sample protection before molecular detection
S112236	Sucrose	Moligand™, for cell culture, suitable for insect cell culture, ≥99.5%	Biological experiment-grade sugar protectant	Used in compound cryoprotectant systems to help reduce freeze-thaw injury
S112231	Sucrose	Moligand™, ACS	Sugar protectant	Used for preparation of routine compound cryopreservation solutions or freeze-drying protective systems
T100012	D-(+)-Trehalose dihydrate	for cell culture, suitable for insect cell culture, ≥99%	Sugar protectant	Used in compound protective systems for lactic acid bacteria, probiotics, or freeze-drying-sensitive strains
T100010	D-(+)-Trehalose dihydrate	≥99%, from cassava starch	Sugar protectant	Used for freeze-drying protection, membrane stabilization, and optimization of cryopreservation conditions for sensitive strains
T777551	D-(+)-Trehalose dihydrate	≥99%	Sugar protectant	Used for compound cryoprotectant systems and screening of cryopreservation/freeze-drying protection conditions
G108395	Gelatin	Suitable for microbiology, gel strength ~250 g Bloom	Colloidal protectant	Used in microbial freeze-drying or compound protective systems to reduce freeze-thaw and drying injury
G274269	Gelatin	Reagent Grade	Colloidal protectant	Used for general compound protective systems or exploration of freeze-drying protection systems
L434330	Low endotoxin gelatin from porcine skin	gel strength240-360(Bloom)＜10 EU/gEndotoxin	Low-endotoxin colloidal protectant	Used for development of protective systems requiring low endotoxin background
A104912	Bovine Serum Albumin(BSA)	Suitable for molecular biology,Component V	Protein protectant	Used in compound cryopreservation or freeze-drying protective systems to stabilize the bacterial microenvironment
B754973	Bovine Serum Albumin(BSA)	Low Endotoxin, for cell culture, ≥98%, chromatographically purified, New Zealand origin, pH 7	Low-endotoxin protein protectant	Used in strain protective systems requiring controlled endotoxin and impurity levels
B754985	Bovine Serum Albumin(BSA)	sterile-filtered, for cell culture, Low Endotoxin, 10% in DPBS, fatty acid free	Sterile protein protectant	Used for sterile compound protective systems or optimization of sensitive strain cryopreservation systems
B741824	Bovine Serum Albumin(BSA)	Low Endotoxin, Protease Free, ≥98%, Lyophilized	Low-endotoxin protein protectant	Used in freeze-drying protection, compound cryopreservation, and protective systems requiring controlled protein background
P488126	Phosphate Buffered Saline with 3% Non-Fat Milk, pH 7.4		Skim milk/buffer protective system	Can be used in milk protein-containing compound protective systems, freeze-drying condition exploration, or sensitive strain protection
P1509552	PBS (pH 7.4, Sterile)	BioReagent,Low Endotoxin,sterile-filtered,for cell culture	Buffer/dilution system	Used for bacterial washing, cryoprotectant preparation, and sample dilution before cryopreservation
P743267	PBS, DNase&RNase Free	sterile-filtered, BioReagent, DNase, RNase free, ready-to-use, for cell culture, 1×	Molecular experiment-compatible buffer	Used for bacterial processing requiring reduced nuclease background or sample treatment before molecular detection
T494526	PhosphateBuffered Saline（PBS）1X concentrate	1X,sterile,pH7.2-7.4	Buffer/dilution system	Used for routine bacterial washing, dilution, and compound cryoprotectant preparation
P492453	PhosphateBuffered Saline（PBS）20X concentrate	sterile	Concentrated buffer	Used for on-demand PBS preparation to support cryoprotectant formulation or pre-recovery processing
T1096883	Trypticase Soy Broth	BioReagent, Suitable for microbiology	Recovery medium	Used for recovery, expansion, and viability restoration of various cryopreserved bacteria
S778417	SOC Broth	BioReagent, Suitable for microbiology	Engineered strain recovery medium	Used for recovery after E. coli transformation, recovery of engineered strains, and nutrient-rich recovery culture
R1097610	General Broth Medium	BioReagent, Suitable for microbiology	Routine recovery medium	Used for general bacterial recovery and short-term expansion
R1096884	R2A Agar	BioReagent, Suitable for microbiology	Low-nutrient recovery/environmental bacterial culture	Used for recovery observation of environmental isolates, low-nutrient-requiring strains, or slow-growing strains
B1097609	BBL Agar Medium Base	BioReagent, Suitable for microbiology	Plate culture/purity check	Used for post-recovery streaking, colony morphology observation, and purity testing
T1097062	Thioglycollate Medium	BioReagent, Suitable for microbiology	Anaerobic/microaerophilic recovery	Used for recovery and activity restoration of anaerobes or strains requiring low-oxygen conditions
K1097702	King's B Medium	BioReagent, Suitable for microbiology	Specific strain recovery/phenotype observation	Used for culture and phenotypic observation of some Pseudomonas and related strains
E1097793	Eosin Methylene Blue Agar (EMB)	BioReagent, Suitable for microbiology	Selective and differential culture	Used for post-recovery identification of Gram-negative enteric bacteria and contamination screening
M1097309	Mannitol Salt Agar	BioReagent, Suitable for microbiology	Selective and differential culture	Used for post-recovery identification and purity observation of salt-tolerant staphylococcal strains
X1097732	Xylose Lysine Deoxycholate Agar (XLD)	BioReagent, Suitable for microbiology	Selective and differential culture	Used for selective culture and contamination screening of some intestinal bacteria after recovery
V1417948	Violet Red Bile Glucose Agar		Selective and differential culture	Used for selective culture and quality control of Enterobacteriaceae-related strains after recovery
B1442095	Bismuth Sulfite Agar Medium		Selective and differential culture	Used for specific Salmonella-related detection or selective culture scenarios
L1442790	Levine Eosin-Methylene Blue Agar Medium		Selective and differential culture	Used for isolation and identification of intestinal Gram-negative bacteria after recovery
S1096741	Sabouraud Dextrose Agar	BioReagent, Suitable for microbiology	Fungal/yeast recovery support	Used for recovery of yeast or fungal microorganisms; not used as the primary medium for bacterial cryopreservation
Y778413	YPD Broth	BioReagent, Suitable for microbiology	Yeast recovery support	Used for recovery and expansion of cryopreserved yeast strains; not used as the primary medium for bacterial cryopreservation
Y743366	YPD Broth with Agar (premixed powder)	BioReagent, Suitable for microbiology	Yeast plate recovery support	Used for plate culture and purity observation after recovery of cryopreserved yeast strains

9 Frequently Asked Questions

9.1 What is the most commonly used method for bacterial cryopreservation?

Most routine bacteria can be preserved in a glycerol-based protective system at -80°C. This method has broad applicability and is suitable for laboratory strain banks, engineered strains, and routine experimental strains.

9.2 Can glycerol stocks be repeatedly frozen and thawed?

Repeated freeze-thaw cycles are not recommended. They reduce bacterial viability and increase contamination risk. Multiple aliquots should be prepared, and one cryovial should be used for each recovery to avoid repeated sampling from the same vial.

9.3 Can bacteria be stored long term at -20°C?

In general, -20°C is not recommended for long-term strain preservation. Ice crystal injury and temperature fluctuations are more pronounced at -20°C. It is suitable for short-term or transitional storage, but not for long-term preservation of important strains.

9.4 What should be considered when cryopreserving engineered strains?

Before cryopreservation, plasmid, resistance marker, and target construct stability should be confirmed. After recovery, antibiotic resistance, target fragment, or expression function should be verified according to experimental purpose, and passage number should be controlled.

9.5 Why are compound cryoprotectants commonly used for lactic acid bacteria?

Lactic acid bacteria are sensitive to freezing, thawing, and drying damage, and a single glycerol system may sometimes provide insufficient protection. Sugars, skim milk, or protein-based protectants can improve survival rate and recovery stability.

Bacterial cryopreservation methods should be selected according to strain characteristics, storage duration, and recovery requirements. Routine strains can preferentially be preserved using -80°C glycerol cryopreservation, sensitive strains can be optimized with compound cryoprotectants, and valuable or long-term preserved strains can be backed up with liquid nitrogen or freeze-drying. Stable strain bank management, clear records, and recovery quality control are essential for ensuring long-term strain usability and reproducible experimental results.