Question about theoretical entities in Science

I always concerned about the theoretical entities in science while reading philosophy papers. For example, we know that according to logical positivists, the theoretical entities such as particles, bacteria are unobservable. For this reason, they tried to reduce such theoretical prepositions into observable propositions. However, I don't get what this mean exactly.

For example, maybe the particles are unobservable in the sense that they are defined by mathematical prepositions. However, we know that bacteria is observable through telescope and such term is not defined by mathematics. So I wonder why philosophers say that physical micro entities in science are not unobservable, and for this reason, they are theoreitical entities. (e.g., Wilfrid Sellars, Hilary Putnam)