What bacteria cannot be killed by boiling water?

What Survives When Water Boils?

Boiling water is one of the oldest and most reliable ways to make water safe to drink. At sea level, water boils at 100°C (212°F), and that heat is enough to kill most pathogens—yeast, molds, and the vast majority of bacteria don’t stand a chance.

But there’s an exception: some bacteria can survive boiling, not in their normal form, but as spores. These spores are like nature’s survival pods. When conditions get tough—like when exposed to high heat—certain bacteria go dormant, encasing themselves in a tough, protective shell that resists boiling.

Clostridium botulinum, the bacterium responsible for botulism, is one of the most well-known examples. Its spores can endure boiling temperatures, only to reactivate when conditions improve. This is why home canning requires special care—simply boiling jars isn’t always enough to destroy these resilient spores. Pressure canners, which reach temperatures well above boiling, are necessary to ensure safety.

Other spore-forming bacteria, like Bacillus species, also share this resistance. While many of them aren’t harmful, their hardiness shows just how clever some microbes can be at surviving extreme environments.

So while boiling remains a powerful tool for disinfection, it’s not foolproof. It kills nearly all common pathogens, but the spore-forming outliers remind us that nature often has a backup plan. For complete sterilization—especially in food preservation or medical settings—higher temperatures and specialized methods are often needed.

In everyday life, though, boiling your water is still one of the best ways to stay safe. Just remember: nature’s exceptions are rare, but they’re worth knowing about.