Making a glycerol stock is one of the most routine and essential skills in any biology lab: you mix a cell culture with sterile glycerol to reach a final concentration of about 15%, transfer the mixture to a cryovial, and store it at −80°C. That basic recipe covers most bacterial work, but the details shift depending on what you’re preserving, and getting those details wrong can mean thawing a tube months later only to find everything inside is dead.
Why Glycerol Keeps Cells Alive in the Freezer
When water freezes slowly, it forms large ice crystals that puncture cell membranes. Glycerol prevents this by inserting itself into the hydrogen-bonding network of water. As glycerol concentration rises, the pool of freely available “bulk” water shrinks. Water molecules cluster tightly around glycerol’s polar groups, while glycerol molecules begin bonding to each other. At high enough concentrations, the mixture doesn’t crystallize at all when cooled. Instead it forms a glass, a solid that lacks the sharp, repeating lattice of ice. This glassy state spares cells from mechanical damage during freezing and thawing.1PubMed. Hydrogen bonding and the cryoprotective properties of glycerol/water mixtures
That said, glycerol is not harmless. It is inherently toxic to most microorganisms at high concentrations, and even at standard levels only a fraction of cells survive a freeze-thaw cycle.2PubMed Central. Ice Recrystallization Inhibiting Polymers Enable Glycerol-Free Cryopreservation of Microorganisms The goal when preparing a glycerol stock is to hit the sweet spot: enough glycerol to block lethal ice formation, but not so much that the glycerol itself kills cells or interferes with downstream work.
The Basic Bacterial Protocol, Step by Step
For the vast majority of bacterial strains, a final glycerol concentration of 15% works well. One widely used approach starts with an 80% glycerol stock solution, which you autoclave or filter-sterilize ahead of time. For each cryovial, you combine 812 µL of your overnight liquid culture with 188 µL of sterile 80% glycerol, giving you a final concentration of 15%.3Methods in Enzymology. Storage of Bacteria and Yeast Mix gently by pipetting up and down or inverting the tube a few times. Vortexing isn’t ideal because it can shear fragile cells, though hardy species like E. coli tolerate it fine.
If you’re starting from colonies on an agar plate instead of a liquid culture, scrape or pick colonies into the cryovial and resuspend them in 812 µL of sterile deionized water, then add 188 µL of the 80% glycerol stock to reach the same 15% final concentration.3Methods in Enzymology. Storage of Bacteria and Yeast Label your tubes clearly with the strain name, date, antibiotic resistance markers, and any plasmid information. Then transfer them to −80°C as quickly as possible. Some labs snap-freeze vials in liquid nitrogen before placing them in the −80°C freezer, but for most bacterial stocks a direct transfer works fine.
Choosing the Right Glycerol Concentration
Fifteen percent is the default, but it isn’t universally optimal. When researchers tested different glycerol concentrations on several E. coli strains, they found that 15% was best for most, but one K-12 strain performed better at 20–25%.4PubMed Central. How Archiving by Freezing Affects the Genome-Scale Diversity of Escherichia coli Populations Even under optimized conditions, fewer than 40% of cells survived a single freeze-thaw cycle in that study, which underscores an important point: glycerol stocks aren’t meant to preserve every cell. They preserve enough viable cells to restart a culture. That’s all you need.
For organisms you haven’t worked with before, it’s worth checking the literature or running a quick pilot experiment with 10%, 15%, 20%, and 25% glycerol. Freeze aliquots, thaw one from each concentration after a week, and plate them to compare colony counts. The cost of doing this once is trivial compared to the cost of losing a strain.
Why −80°C Matters More Than You Think
Standard kitchen-style freezers run at about −20°C. That’s cold enough to freeze water, but it’s actually a dangerous temperature zone for glycerol stocks. At −20°C, glycerol-water mixtures can exist in a partly liquid, partly frozen state where slow ice recrystallization still damages cells over time. A study comparing Helicobacter pylori survival at −80°C versus −20°C found consistently higher recovery rates at −80°C, with log colony-forming units around 8 to 8.7 at −80°C. At −20°C, one stock culture dropped to zero recovery entirely.5PubMed Central. Evaluation of different cryoprotective agents in maintenance of viability of Helicobacter pylori in stock culture media
The practical takeaway: if your lab only has a −20°C freezer, glycerol stocks will still work short-term for hardy organisms, but don’t count on them for long-term archiving. For anything you want to preserve reliably beyond a few weeks, −80°C is the standard. Liquid nitrogen (−196°C) offers even longer stability and is preferred for high-value or irreplaceable strains, but it requires specialized storage infrastructure.
Avoiding Freeze-Thaw Cycles
Every time you thaw a glycerol stock and refreeze it, you kill a substantial fraction of the cells inside. After two or three cycles, recovery can drop to nearly nothing. The single best habit you can develop is to make multiple aliquots up front. If you expect to pull from a stock frequently, prepare ten or twenty tubes rather than one. Each time you need bacteria, take out a fresh tube, streak a plate from it, and discard the tube. Never thaw a stock, scrape off a bit, and put it back.
Some experienced lab workers keep a “working stock” tube that they briefly open at −80°C and scrape a small amount of frozen material from the surface without fully thawing the tube. This extends the life of a single tube but is less reliable than simply using fresh aliquots, and it introduces contamination risk every time the cap is opened.
Thawing and Reviving a Culture
When you’re ready to use a glycerol stock, remove the tube from the freezer and thaw it quickly. Holding it in your hand or placing it in a 37°C water bath for 1–2 minutes both work. Speed matters here: the faster you pass through the temperature range where ice crystals can grow, the more cells survive.
Once thawed, streak a small amount onto a selective agar plate (with the appropriate antibiotic if your strain carries a resistance marker). Incubate overnight. Resist the urge to inoculate liquid media directly from the glycerol stock unless you’re in a rush. Plating first lets you confirm that the culture is pure, that colonies look normal, and that the antibiotic resistance hasn’t been lost. You can pick a single colony the next day and grow it up with confidence.
For strains carrying plasmids, this plating step doubles as a check on plasmid retention. A study testing direct recovery from glycerol stocks found that for seven out of eight plasmid constructs, skipping the plating step and going straight to liquid culture gave equivalent plasmid yields.6PubMed Central. To Plate or to Simply Unfreeze, That Is the Question for Optimal Plasmid Extraction So if you’re in a time crunch and just need plasmid DNA, direct inoculation from the glycerol stock can work for most constructs. But one of the eight constructs in that study did show a difference, so for critical work, plating first remains safer.
Preserving Bacteriophages
Phages are not cells, and they don’t respond to cryopreservation quite the same way. Many phages survive storage at −80°C or in liquid nitrogen for months or years with minimal titer loss, regardless of the stabilizer used.7PubMed Central. Comparative analysis of different preservation techniques for the storage of Staphylococcus phages aimed for the industrial development of phage-based antimicrobial products Glycerol at 15–50% is commonly added to phage lysates before freezing, and it generally does the job. However, glycerol at the concentrations typically used (especially 50%) can remain liquid at −20°C rather than freezing solid, which some researchers find messy and harder to manage.
Recent work has explored alternatives. Poly(ethylene glycol) at just about 1% by weight kept phages frozen solid at −20°C and delivered close to 100% recovery of plaque-forming units after two weeks, outperforming glycerol in several cases.8PubMed Central. Polymer-Mediated Cryopreservation of Bacteriophages For labs doing phage therapy research, where glycerol residues can complicate animal or clinical studies, polymer-based cryoprotectants could eventually replace glycerol. For routine lab storage of phages, though, glycerol at −80°C still works well.
Stool and Microbiome Samples
If you’re preserving fecal samples for microbiome analysis, the protocol changes depending on whether you need living bacteria or just their DNA. For DNA-only work, many stabilizing buffers and even room-temperature preservation methods exist. But if you want to culture organisms later or maintain a living snapshot of the community, glycerol-based cryopreservation is the standard approach.
A systematic comparison of stool preservation methods found that the best protocol for maintaining sample quality over 12 months was storage at −80°C (or in liquid nitrogen) with 10% glycerol added before cooling.9PubMed Central. Effects of Stool Sample Preservation Methods on Gut Microbiota Biodiversity: New Original Data and Systematic Review with Meta-Analysis The glycerol-PBS mixture preserved intracellular DNA well and maintained cell viability. Field work with mammalian gut samples has confirmed the same general pattern: samples stored in solutions containing cryoprotectants yielded the highest and most consistent bacterial densities after thawing.10Journal of Mammalogy. Reviving diversity: cryoprotectants and culturing methods enhance recovery of mammalian gut microbes from field samples
For microbiome work, homogenize the stool sample in a buffer like PBS, mix in glycerol to a final concentration of 10%, aliquot into cryovials, and freeze. As with bacterial stocks, make multiple aliquots. Microbiome researchers routinely run dozens of analyses from a single collection event, and each thaw introduces compositional shifts that can bias results.
Mammalian Cells Are a Different Story
Glycerol is the go-to cryoprotectant for bacteria and fungi, but mammalian cells are typically frozen in DMSO (dimethyl sulfoxide) rather than glycerol. The standard mammalian cell freezing medium is 10% DMSO in growth medium, sometimes with added serum. Cells are cooled at about 1°C per minute, often using a controlled-rate freezer or an isopropanol-filled container (the common “Mr. Frosty” device), and then transferred to liquid nitrogen for long-term storage.11PubMed Central. Cryopreservation: An Overview of Principles and Cell-Specific Considerations
Why DMSO over glycerol for mammalian cells? Both are cryoprotectants, but DMSO crosses cell membranes faster, which gives more uniform intracellular protection during the slow-cooling protocols that mammalian cells require. Glycerol can work for certain mammalian cell types, especially red blood cells, but DMSO is the default for cultured cell lines. If you’re ever unsure, check ATCC or the cell bank’s recommended protocol for the specific line you’re working with.
Long-Term Viability and Nutrient Supplements
A glycerol stock made today and stored properly at −80°C will typically remain viable for years. But “viable” and “unchanged” aren’t the same thing. Over very long storage times, slow DNA damage accumulates, and some fraction of cells will always die. For critical strain collections, refreshing stocks every few years is good practice: thaw an old tube, grow a fresh culture, and make new stocks.
Adding nutrient supplements to the cryoprotectant solution can improve long-term survival. A comparison of four different cryoprotectant formulations for Enterobacterales strains found that after 12 months at −20°C, a solution of 70% glycerin with added peptone and yeast extract preserved about 89% of strains, compared to only about 45% survival with glycerin alone (no supplements).12PubMed Central. Efficacy assessment of different cryoprotectants for preserving the viability of Enterobacterales strains at – 20 °C Adding DMSO to glycerin didn’t improve things much compared to glycerin plus nutrients. This finding is most relevant to labs that must store strains at −20°C. At −80°C, the advantage of nutrient supplementation is likely smaller, but for biobanks archiving hundreds of strains, even marginal improvements in survival rates matter.
Glycerol in Nature
Laboratory glycerol stocks are a human invention, but the underlying chemistry is borrowed from nature. Many cold-tolerant insects accumulate glycerol in their tissues before winter arrives. Larvae of the Arctic moth Gynaephora groenlandica, for example, build up glycerol as they cold-acclimate, and the compound is detectable in their tissues after freezing.13PubMed. Glycerol metabolism in a freeze-tolerant arctic insect: an in vivo 13C NMR study Wood frogs, certain beetles, and other overwintering organisms use similar strategies. The lab protocol of adding glycerol to protect cells from ice damage is, in a sense, just a standardized version of what evolution figured out long before anyone had a −80°C freezer.
Emerging Alternatives to Glycerol
Glycerol remains the gold standard for microbial cryopreservation, but researchers are increasingly exploring alternatives, especially for applications where glycerol’s toxicity or its interference with downstream assays becomes a problem. Some extremophile organisms survive repeated freezing not with glycerol but by producing antifreeze proteins that block the regrowth of ice crystals during thawing. Synthetic polymers that mimic this ice recrystallization inhibition, such as poly(vinyl alcohol) paired with poly(ethylene glycol), have been shown to produce a four-fold increase in E. coli yield after thawing compared to glycerol, and they work at concentrations as low as about 1% by weight.2PubMed Central. Ice Recrystallization Inhibiting Polymers Enable Glycerol-Free Cryopreservation of Microorganisms
These polymer-based approaches are still in the research phase and haven’t replaced glycerol in everyday lab use. But they point toward a future where cryopreservation could be gentler on cells and more compatible with sensitive assays. For now, if you’re setting up a standard microbiology lab, an autoclaved bottle of 80% glycerol and a box of cryovials will cover the vast majority of your preservation needs.
Common Mistakes and How to Avoid Them
A few errors come up again and again, especially in labs training new students:
- Non-sterile glycerol: Using glycerol straight from the bottle without autoclaving or filter-sterilizing it. Glycerol is viscous and doesn’t autoclave quite like water. Use a liquid cycle and don’t overfill the bottle, or filter-sterilize through a 0.22 µm filter (slowly, because of the viscosity).
- Wrong growth phase: Making stocks from a culture that’s been sitting on the bench for days. Cells in stationary phase or death phase freeze worse than those in healthy mid-log or late-log growth. Grow your culture to mid-to-late log phase before adding glycerol.
- Mislabeled tubes: This sounds trivial, but it’s the most destructive mistake in practice. A year from now, a tube labeled only “E. coli 3/14” is nearly useless. Include the strain designation, plasmid name, antibiotic resistance, your initials, and the date.
- Slow transfer to the freezer: Leaving freshly made glycerol stocks on the bench for an hour while you finish other tasks. The glycerol is already at its working concentration; letting the cells sit in it at room temperature gives the glycerol time to exert its toxic effects without any cryoprotective benefit. Move vials to −80°C within minutes.
- Storing in a frost-free freezer: Frost-free freezers cycle their temperature to prevent ice buildup. Those temperature fluctuations act like mini freeze-thaw events and can destroy stocks over months. A dedicated −80°C research freezer doesn’t have this problem, but if you’re ever forced to use a −20°C frost-free unit for short-term storage, be aware that your stocks will degrade faster than expected.
Scaling Up for Strain Collections
Individual researchers often maintain a few dozen glycerol stocks in a single freezer box. But culture collections, biobanks, and high-throughput screening labs may manage tens of thousands of strains. At that scale, organization becomes the real bottleneck. Labs typically use 96-well cryostorage plates instead of individual tubes, filling each well with culture and glycerol using multichannel pipettes or robotic liquid handlers. The plates are sealed with adhesive foil, labeled with barcodes, and cataloged in a database.
The biology doesn’t change at scale: the same 15% glycerol, the same −80°C storage, the same avoidance of freeze-thaw cycles. What changes is the logistics. Tracking which well contains which strain, ensuring that no two labs’ stocks are cross-referenced incorrectly, and having backup copies stored in a physically separate freezer (ideally in a different building) become the primary challenges. For any strain that would be costly or impossible to reconstruct, keeping at least two independent stocks in two separate locations is a basic precaution that every lab should adopt but many don’t until they experience their first freezer failure.