They're fully functional organisms that can reproduce on their own, and they can start growing outside of any cell and that's what they do, they live extracellularly.

细菌可是麻雀虽小五脏俱全,它们能够凭借自身繁殖,可以在任意一个细胞外开始增殖,它们就是这样做的,在细胞外生长繁殖

耶鲁公开课 - 生物医学工程探索课程节选