Most micro-organisms are single-celled , or unicellular organisms , but there are unicellular protists that are visible to the human eye , and some multicellular species are microscopic . Most microorganisms are unicellular ( single-celled ) , but this is not universal , since some multicellular organisms are microscopic , while some unicellular protists and bacteria , like Thiomargarita namibiensis , are macroscopic and visible to the naked eye .