High temperatures disrupt their enzymes, rendering them incapable of carrying out their cellular functions. Even higher temperatures cause their plasma membrane to burst open, causing the death of the bacterial cells.
Edit: At lower temperatures, their enzymes also get disrupted and crystallisation of their cytoplasm also causes cell death, however, they may have enough time to form spores, toxins, etc. which also get disrupted at high temperatures. So, boiling or UV radiation is generally preferred.
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