Coming in late, but thinking out loud....

If you culled rats that were sick, not horribly sick yet, but sick none the less, or around those who were sick, and you go to your freezer to thaw them out and feed them off, can't you run the risk of re-introduce the bacteria?

Freezing slows the bacteria growth down, doesn't eliminate it. It will still be present on your rats when you take them out and thaw them.