We want to talk about how to grow cells over the holiday period spanning Christmas and New Year. (OK, so we have covered this before but we think it is worth repetition for new users of the baculovirus-insect cell system). Most labs, academic and industry, will have some time off over the next few weeks. It is often a problem keeping your cell cultures in good condition so that when you return in 2019 you can pick up that important protein expression project without too much delay. Given that most insect cells (usually Spodoptera frugiperda or Trichoplusia ni cell lines) need to be sub cultured at least twice a week if they are grown as suspension cultures, how will they survive for this protracted period?
It is probably pointless trying to keep a healthy suspension cell culture going for more than 5 days. Even if they are seeded very thinly they will eventually become very dense or at the other extreme possibly not grow at all (cells in serum-free medium don’t like to be over diluted). You will almost certainly return in the New Year to a dead flask of cells.
Therefore, you might think that it is time to return to your frozen stocks of healthy cells and start afresh in January. This isn’t a bad idea, particularly if you have had your current stocks in culture for a while. The only problem is that even with a batch of cells frozen at a high density it takes a while to amplify them to quantities sufficient for use in experiments.
An alternative is to assign the task of keeping healthy cells over the break to the most junior member of the lab. This isn’t very fair and runs the risk of a resentful or hungover member of staff contaminating the lot! Or you can assign the task of keeping healthy cells going over the break to the most senior member of staff and thus be almost certain of contaminating the lot!
Probably the best option is to remove your cells from suspension cultures to monolayers in flasks and grow them at a lower temperature for the duration. We recommend a temperature of 20-21°C, which is often the ambient level in most air conditioned labs. You might also have an incubator you could drop to this temperature for the period. This is particularly useful if your air conditioner system is turned off over the break. The size of flask you use depends on the numbers of healthy cells you wish to have available for rapid use in the New Year. These can be seeded with various densities of cells. For example, 175cm2 flasks could be set up with 106, 2.5 x 106 or 5 x 106 cells in 15mls medium. This gives a reasonable spread of cell densities so that at least one flask survives. (We routinely use this method for keeping our back up cultures of cells going during the year).
When you return after the holiday break and want to transfer your healthy cells to suspension culture don’t forget that the best way to remove them from the attached state is to tap them firmly on the lab bench. Don’t use a cell scraper as this can result in a very high mortality rate. The exception is S. frugiperda Sf21 cells in serum-containing medium, which seem to tolerate being scraped from plastic reasonably well.
If you follow the above advice, you should be able to return to the lab in 2019 and have your insect cell-based research up and running in no time.