PPLO i.e. pleuro-pneumonia-like organisms.
Viruses could be the smallest living things, but they cannot be classified definitively as "living" since they do not carry out the processes associated with life. They only reproduce, and then only by altering the reproductive mechanisms of other organisms.
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atom
The smallest unit an image can be broken down into is a pixel.
The smallest unit in a bank is extension counter for bank branches. For money in bank that is one paisa.
cells
I think the smallest unit of culture is the meme. It travels fast, from person to person and helps define, shape or create a concept around an idea, thing, person or institution.