Why do pickles last so long?

Pickles last because salt and acid make life miserable for spoilage microbes. Vinegar lowers the pH, while salt pulls water out of cucumber cells and leaves less moisture for bacteria and mold. A sealed jar also keeps new contamination and fresh air away.

Some pickles skip vinegar and let helpful bacteria do the souring. They feed on the cucumber's sugars and create lactic acid, which protects the jar in a different way. Once opened, even a good pickle needs the fridge, because new microbes can arrive.