I'm not a bio scientist and I cannot give you clear definition.
But that is my general idea and also what my master have said,
about bacterias.
Well, Sentient beings are those who have a Mind, and perhaps some
of the 5 senses that we have.
I think, tiny microscopic animals probably also have nervous
system, a body resembling an animal etc. They also have features
that resemble an animal - head, legs, etc.
