While not all bacteria form spores, some species do when faced with unfavorable conditions like lack of nutrients, extreme temperatures, or radiation. Spores are highly resistant to these conditions and can remain dormant for extended periods, sometimes even centuries, until conditions become favorable again.

It's important to note that while the term "spore" is most commonly used, some bacteria may also have other forms of dormancy. However, "spore" is the most general and widely recognized term for a dormant bacterium.