Sometimes there is a hidden logic in what we do
28 January 2026
Recently, our post about maintaining bacterial stocks in 40% glycerol sparked some pointed questions. The subtext was clear: "How could you have been so stupid?" Keeping stocks at –20°C, and in 40% glycerol so that they remain liquid, is not standard practice. But there is a logic here that is worth unpacking, one that highlights a deeper issue: the relationship between strain stability and recovery methodology.
Two approaches, two philosophies
When retrieving cells from frozen stocks, microbiologists generally fall into two camps. The traditional approach involves streaking frozen material to single colonies, selecting one colony and using it to inoculate an overnight culture. This method is microbiologically sound and absolutely critical for unstable strains prone to suppressor mutations. On a streak plate, suppressor colonies often reveal themselves, because they grow faster, form larger colonies and, using the methodology can be avoided. For strains with distinctive growth defects, this visual screening is invaluable.
The alternative approach, which in my experience is more common in yeast genetics, takes a different view. Rather than streaking to singles, researchers patch material onto plates or inoculate cultures directly from a larger aliquot of frozen stock. The logic? Maintain the population structure present in the original culture, including any low-frequency variants.
Both methods are correct. Both methods can also be catastrophically wrong. It depends entirely on your strain.
When numbers tell a different story
Consider mutation rates. In E. coli, spontaneous mutation rates sit around 10-9 to 10-10 per base pair per generation. With a 4.6 Mbp genome, this translates to roughly 0.005 mutations per genome per replication. In practical terms, starting from a single cell, you'll encounter your first mutant after approximately 200 cell divisions. The fidelity is remarkable – but when working with cultures containing 109 bacteria, the absolute number of mutants becomes substantial. It is simply a numbers game.
Now consider strains with elevated recombination rates carrying tandem repeat reporter cassettes, as we used in our recent study (see here for the study and here for the blog post about it). Reversion rates for these constructs run at 10-4 to 10-5, orders of magnitude higher than spontaneous mutations. Any overnight culture, and therefore any frozen stock derived from it, contains a significant fraction of reverted cells. This is where recovery methodology becomes critical.
The single colony problem
When you streak to singles and pick a colony for your overnight culture, you are gambling. That colony might already be reverted. We have seen this happen. An experiment begins with what appears to be a well-characterized strain, and results make no sense because the founding colony was not representative of the population you thought you were working with.
For strains with high reversion rates, streaking to singles amplifies risk rather than reducing it. You need the population structure of the original culture: reverted and unreverted cells in their natural proportions. This is why yeast geneticists, working with Saccharomyces cerevisiae and its highly active recombination pathways, often avoid single-colony isolation entirely. They patch. They inoculate directly. They preserve population ratios, at least in the labs I have worked.
In this context, liquid frozen stocks make perfect sense. You can inoculate overnight cultures directly from stock without intermediate plating steps. A small aliquot contains cells in the proportions established during the original purification – as close to the tested, validated population structure as you can get. Yes, frozen stocks deteriorate over time, but experimental activity typically peaks with freshly generated strains anyway.
This is the rationale behind 40% glycerol stocks. Not inexperience. Not carelessness. A deliberate choice based on strain characteristics and experimental requirements.
The converse problem
Of course, we have also seen the opposite failure. Using larger inoculation volumes can introduce suppressor mutations that compromise experiments. We have observed cultures behaving oddly when derived from single colonies, sometimes suggesting that mutation or recombination rates had somehow shifted in the chosen colony. Both approaches can fail. Both have succeeded. The difference lies in understanding what you are working with.
Know Your Strains
The critical point is not that one method is necessarily superior. It is that recovery methodology must match strain biology.
For unstable strains throwing suppressors frequently, streak to singles. Visual inspection on plates is your quality control. For strains with elevated recombination rates and reporter systems that revert readily, preserve population structure. Avoid the single-colony bottleneck.
The challenge is recognizing which situation you're in. This requires robust knowledge of your strains, their stability, their mutation landscape, their recombination activity – and a clear understanding of your experimental requirements. There is no universal protocol. The textbook method might be exactly wrong for your system.
The broader lesson
What sometimes appears foolish often has solid theoretical foundation. Methodological choices that seem unorthodox may reflect deep engagement with strain-specific biology rather than ignorance of standard practice. The key is recognizing when deviation from convention is justified and when it is not.
Getting this "just right" requires experience, careful observation, and willingness to think critically about what is actually happening in your cultures. Sometimes that means following the textbook. Sometimes it means writing your own protocols. Either way, understand your strains first. Everything else follows from there.
For related discussion on glycerol stock concentrations and their practical implications, see article here.