If you regularly grow up the same bacterial culture, whether it’s the strain that expresses your favorite protein, the culture you make your competent cells from, or just your regular control strain, it can be a bit of a pain growing it up from scratch each time.
Before you even get to grow your actual culture, you have to take a loop from your frozen stock, streak it and grow colonies then pick colonies and grow a starter culture. What a drag.
Sounds like time for a shortcut, so here it is…. make a cell bank. A cell bank is essentially a rack-full of small, uniform, frozen starter cultures for the culture you grow regularly.
Making a cell bank
To make a cell bank, just streak your culture, pick a single clone and grow up a flask of cells until just before they are saturated, then add glycerol to a final concentration of 10%. Next aliquot it into screw-top tubes before freezing at -80°C. Make the aliquot sizes 100th of the culture size you normally grow, and judge how many aliquots you’ll need by how often you normally find yourself needing to grow the culture.
Now when you want to start a culture, you just need to take a tube from your cell bank, thaw slightly, then add your frozen starter culture straight into the waiting flask full of growth medium.
Not only does this save time in setting up the culture, but since each starter culture in the cell bank has exactly the same cell density, you can expect your culture growth to be reproducible; just monitor the growth the first time you grow a culture from the cell bank and you should get very similar growth curves for each subsequent culture. Also, since all of the cultures you grow from your cell bank will come from the same clone, this can make your results more reproducible.
Don’t forget to label
Now you can have a rack in the -80 containing row upon row of starter cultures for each strain you culture routinely. Just make sure to label them properly. It can be tedious labeling aliquot after aliquot, so my approach is to use a color-coding/symbol system. Give each batch its own, easily recognizable marking (e.g. a red, blue or green stripe – stripes, make it very easy to mark many tubes quickly -, or if you need more options, use circles, squares etc), then maintain an Excel file/paper sheet that lists each cell bank with its symbol, strain, date of production, cell density etc. And you’re ready to go.
Do you use cell banks already, or do you have another system for streamlining your routine culture preps?