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I have a follow up question to this question: Does freezing microorganisms such as probiotics kill them?

If freezing bacteria kills some of them, then you are effectively putting selective pressure on bacteria when you freeze them (the bacteria more resistant to low temperatures survive, bacteria less resistant die). Why then is it a good idea to freeze bacteria?

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    $\begingroup$ When you freeze bacteria, you slow their reproduction. Example: Bacteria, as well as enzymes etc. need the correct temperature and environment to function properly. Is it something like this that you want to know, or are you searching something more in-depth? $\endgroup$ – user27740 Nov 21 '16 at 21:47
  • $\begingroup$ Mutations usually occur after a bacteria has divided into two, and usually not within an already existing bacteria. If the bacteria cannot multiply, then how can it mutate or build up resistance? $\endgroup$ – user27740 Nov 21 '16 at 21:54
  • $\begingroup$ Hmm ok you have a point. So bacteria are killed in a non-discriminant way by freezing? i.e. some just happen to die during freezing while others survive. $\endgroup$ – liyuan Nov 21 '16 at 22:15
  • $\begingroup$ If the cold shatters or ruptures the membrane, then that ought to weaken the bacteria or kill it. $\endgroup$ – user27740 Nov 21 '16 at 22:17
  • $\begingroup$ I was thinking of heterogeneous bacteria, but that's not correct it seems. $\endgroup$ – liyuan Nov 21 '16 at 22:17
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The ice crystals that form tear and shear the bacterial cells, unless you add a cryoprotectant (for example, glycerol) to the cells beforehand.

Remember, when water freezes its volume increases. This is why a closed soft drink can that you put into your freezer to chill faster, will burst if you leave it in until the contents are frozen.

Exactly the same thing happens to cells (animal cells, plant cells, yeast cells, etc.) if they are frozen without a cryoprotectant.

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    $\begingroup$ This does not answer the question. $\endgroup$ – iayork Nov 22 '16 at 13:56
  • $\begingroup$ @mdperry Your answer contain valuable relevant information but you should write it as a comment otherwise it may be deleted in future. $\endgroup$ – Always Confused Nov 22 '16 at 17:49
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    $\begingroup$ @AlwaysConfused I'm not so sure about it being deleted, since mdperry's answer does contain intelligible, accurate & [somewhat] relevant information. A downvote, however, would be a reasonable action to take, should you think the response deserves it. $\endgroup$ – Charles Oct 12 '17 at 20:13
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The long-term effectiveness of freezing or incinerating bacteria is rooted in how brutal and difficult to withstand such actions are.

Freezing generates ice crystals which, as mdperry points out, "tear and shear the bacterial cells." In theory this could create a selective pressure to survive these temperatures, and indeed akinetes are known to survive the freezing process. However, that is a non-trivial evolutionary step. To fit such a state into the lifespan of a bacteria would call for a substantial mutation. Freezing bacteria for a few hundred million years will likely be sufficient to "teach" them how to survive! Without such a step, it's just not reasonable to survive the abuse.

The same goes for incinerating. Once you get hot enough, the chemistry of organic molecules starts to become a permanent terminal issue. Evolving to beat this is simply really hard. In fact, an autoclave is enough to kill virtually everything, though one strain of Pyrolobus fumarii did survive at 121C in an autoclave.

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