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

It's a picture of cells actually growing and moving in cell culture.

这幅图是真实细胞,在细胞培养液中生长和运动的样子

耶鲁公开课 - 生物医学工程探索课程节选

This makes sense; the rate at which the cell population is growing, the derivative dX/dt, is proportional to the number of cells I have.

这个说的通,细胞数目的增长速度,就是导数dX/dt,和细胞数目呈比例关系

耶鲁公开课 - 生物医学工程探索课程节选