Standard Microbial Strain Lifecycle Management

Concepts and Applications of Reference Microbial Strains

Definition of Reference Microbial Strains
Reference microbial strains are microbial strains preserved and authenticated by national or international culture collections (e.g., ATCC, CMCC, CICC). They possess a definitive taxonomic status and genetic stability. They are typically preserved as lyophilized (freeze-dried) powders or ultra-low-temperature frozen stocks. Revival and cultivation must strictly follow the supplier's instructions, and their viability and purity must be confirmed through colony morphology, biochemical tests, and other methods.

Classification of reference microbial strains

Reference Microbial Strain: A strain defined at least to the genus or species level, classified and described based on its characteristics, with a clearly documented origin.

Reference Stock Culture: A strain of the same species obtained by subculturing a reference strain for one generation.

Working Culture: A strain of the same species obtained by subculturing a reference stock culture, typically used for routine laboratory operations.

Applications of reference microbial strains

Microbial Taxonomy and Identification: Serving as the "gold standard" in microbial taxonomy to determine the genus and species of newly isolated strains.

Method Validation: Used to verify the accuracy, sensitivity, and specificity when establishing new microbiological testing methods.

Laboratory Quality Control: Acting as the traceability starting point for laboratory strain management to ensure the traceability of experimental results.

Comprehensive Management Protocols for reference microbial strains

Acquisition and Procurement
Reference microbial strains must be obtained from national statutory institutions or their authorized channels; using strains of unknown origin is strictly prohibited. Upon acquisition, core information such as strain number, name, certificate of purity, and genetic stability report must be verified to ensure the strain perfectly matches experimental requirements. Upon receipt, a "Strain Receipt Registration Form" must be completed immediately, detailing the receipt date, strain number, preservation institution, transport conditions, and physical appearance (e.g., integrity of the lyophilized powder, contamination status of slant media) to prevent source contamination or misuse.

Revival and Activation
Frozen or lyophilized reference microbial strains must undergo "revival" to restore metabolic activity:

Lyophilized Microbial Strains: Dissolve in a sterile diluent (e.g., normal saline, nutrient broth) according to the manufacturer's instructions. Mix gently to avoid vigorous shaking that could damage cell structures.

Frozen Microbial Strains: Thaw slowly (typically overnight at 4°C or via a gradual temperature increase at room temperature). Repeated freeze-thaw cycles are strictly prohibited, as they can reduce strain viability or alter genetic characteristics.

After revival, inoculate the strain onto appropriate media (e.g., nutrient agar, MacConkey agar) and incubate under optimal conditions (temperature, oxygen, pH) until typical colonies form before use.

Subculturing and Utilization
(1) Revival of reference microbial strains:
① All strain manipulations must be performed under aseptic conditions to prevent contamination.
② For each use or revival, simply roll a small bead onto an agar plate or inoculate it into broth for cultivation.

(2) Confirmation of Reference Stock Cultures:
Each subculture of a reference stock culture must be confirmed. Confirmation methods generally rely on their unique morphology or their specific appearance on differential media.

(3) Subculturing Methods for Working Cultures:
① Prepare nutrient agar medium (add 3% sodium chloride for Vibrio parahaemolyticus). Autoclave at 121°C for 15 minutes, dispense into test tubes, and cool before use.
② Under aseptic conditions, use an inoculation loop to pick up the bacterial lawn and streak it in a zigzag (or "rice-character") pattern onto fresh test tubes. Incubate at 36°C for 24 hours.

(4) Utilization of Working Cultures:
① Internal Quality Control: Perform positive controls once a month; conduct acceptance testing for each batch of culture media.
② External Quality Control: Participate in proficiency testing and inter-laboratory comparisons.

(5) Periodic Verification of Working Cultures:
① Frequency: Conduct periodic verification of working strains every six months.
② Methods and Criteria: Same as the confirmation methods for working cultures.
③ Maintain detailed records of the periodic verification for reference microbial strains.

Microbial Strain Preservation
The core objective of preservation is to "inhibit strain metabolism while maintaining unchanged genetic characteristics." Appropriate methods must be selected based on usage frequency and required storage duration. Storage requirements include:

Categorized Storage: Store different species and generations of reference microbial strains separately with clear labeling (strain name, number, generation, storage date, and person responsible).

Controllable Environment: For frozen storage (-80°C freezers or liquid nitrogen), ensure stable temperatures and avoid frequent power outages. For refrigerated storage (4°C), regularly check the refrigerator temperature to prevent media desiccation or contamination.

Periodic Re-evaluation: Conduct purity and characteristic verification (e.g., colony morphology, biochemical reactions, serotyping) on preserved reference microbial strains at least once a year. Any abnormal strains must be discarded immediately and replaced with new reference microbial strains.

Disposal
(1) Strains that have been used or have exceeded their storage period must be destroyed.
(2) Strains requiring destruction must be sterilized via autoclaving (121°C for 30 minutes).
(3) Proceed with cleaning and disposal only after sterilization.
(4) Maintain thorough records of destroyed strains. Destruction must be supervised by the laboratory manager and executed by the custodian.

5 Common Strain Preservation Methods

1)Low-Temperature Slant Storage

Principle: Utilizes low temperatures (4°C) to slow metabolism, with solid media maintaining short-term viability.

Procedure: Inoculate onto a slant medium → Incubate until a dense bacterial lawn forms → Seal with Parafilm → Store at 4°C.

Storage Duration: 1–3 months (bacteria), 3–6 months (fungi).

Pros: Low cost, simple operation, no special equipment required.

Cons: Prone to contamination, short storage life, and frequent subculturing increases the risk of mutation.

Application: Short-term experiments or routine daily use.

2) Mineral Oil Overlay Method

Principle: Mineral oil isolates oxygen, inhibits metabolism, and prevents media desiccation.

Procedure: Culture strain on a slant → Overlay with sterile mineral oil → Store upright at 4°C.

Storage Duration: 6–12 months.

Pros: Extends slant storage life; suitable for non-sporulating microorganisms.

Cons: Risk of contamination during retrieval; mineral oil is flammable and requires careful handling.

Application: Medium-term storage for Actinobacteria, yeasts, and other non-strict anaerobes.

3) Glycerol Cryopreservation

Principle: Glycerol acts as a cryoprotectant, while ultra-low temperatures (-80°C) prevent cellular damage.

Procedure: Mix cell suspension with 40% glycerol in equal volumes → Aliquot into cryovials → Store at -80°C.

Storage Duration: 1–2 years (bacteria), over 5 years (some fungi).

Pros: High revival rate (>90%), standardized operation, suitable for high-throughput storage.

Cons: Relies on -80°C freezers; requires backup plans for power outages.

Application: Long-term storage for common model organisms like E. coli and yeasts.

4) Lyophilization (Freeze-Drying)

Principle: Sublimation dehydration under vacuum forces microorganisms into a dormant state.

Procedure: Mix cell suspension with a protectant (e.g., skim milk) → Pre-freeze → Vacuum dry → Seal.

Storage Duration: 5–10 years or longer.

Pros: Extremely long shelf life, high stability, easy to transport.

Cons: Expensive equipment (lyophilizer), complex operation, low survival rate for certain strains.

Application: Permanent preservation of patent strains and standard culture collections.

5) Liquid Nitrogen Ultra-Low-Temperature Storage

Principle: Liquid nitrogen at -196°C completely halts metabolism, putting cells into a "cryptobiotic" state.

Procedure: Mix cell suspension with a cryoprotectant (e.g., DMSO) → Programmed cooling → Submerge in a liquid nitrogen tank.

Storage Duration: Over 10 years.

Pros: Longest storage duration, optimal genetic stability.

Cons: High cost of liquid nitrogen replenishment; hazardous operation (risks of frostbite and explosion).

Application: Ultimate preservation for rare strains, cell lines, or genetically engineered microorganisms.