Microbiology is the study of very small living organisms, or micro-organisms. Micro-organisms are organisms too small to be visible to the naked eye. They include eukaryotes such as protists and fungi as well as prokaryotes such as bacteria. An eukaryote is an organism whose cells have a membrane-enclosed nucleus and membrane-enclosed organelles.In contrast, prokaryotic cells lack the membrane-enclosed nucleus and membrane-enclosed organelles.