What prevents a microbe from growing on a candy bar?

Answer 1

Nothing. Doesn't that make you feel safe?

Actually, many microbes do not grow well when the sugar concentration is too high, which is at least part of why "preserves" are called "preserves" and not "jars of rotten fruit." Most of them also require rather more moisture for optimum growth conditions than is found in a typical candy bar. Finally, many candy bars contain small amounts of preservatives, which also serve to inhibit bacterial growth.