system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it that we can flash-freeze bacteria indefinitely but not humans or similar organisms?

Working in a biochem research lab for a while and I got wondering why is it that I can flash-freeze a glycerol stock of bacteria, store it at -80°C, and thaw them out months later with no apparent problems in growth/function but not in say a human. I am not very familiar with the current state of cryogenics, but my impression is that a human would die instantly if flash frozen in liquid nitrogen.  Is it simply just an example of the simplicity/hardiness of bacterial systems compared to humans?
gpt
Your question includes your answer: it's the glycerol.
The glycerol prevents formation of ice crystals which are what destroy the cells and the reason we can't simply freeze and thaw real humans.

Because the bacteria are very small single cell organisms the glycerol is distributed evenly within the cells while simply administering glycerol to humans would not result in sufficient distribution of glycerol in every cell of the body (there are other issues as well, our bodies simply aren't designed to be frozen).

Remember also that it isn't necessarily every cell in the stock that survives the freezing, but because there are so many and they grow independently it's not really an issue when you thaw them again.

Some animals (particularly certain species of frogs) produce their own anti-icecrystal proteins and are able to survive being frozen because of this.