Answer
No mice die.
Work Step by Step
The endonuclease would destroy the DNA of the dead, pathogenic bacteria. This could then not be used by live non-pathogenic bacteria to become pathogenic. The mice would be safe and would survive.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.