Which type of microorganism has a single-celled structure?

Bacteria are categorized as single-celled microorganisms. These tiny organisms are prokaryotes, meaning they lack a defined nucleus and other membrane-bound organelles. This simplicity in structure allows them to reproduce quickly and thrive in various environments, from soil to the human body.

In contrast, viruses are not considered true cells; they require a host cell to replicate and do not possess the cellular machinery needed for metabolism or reproduction on their own. Fungi, while they can exist as single-celled organisms like yeast, generally include multicellular forms such as molds, making them a less clear fit for the category of single-celled microorganisms. Parasites can be unicellular or multicellular; for example, protozoa are single-celled, but larger parasites, such as worms, are multicellular.

So, bacteria stand out unequivocally as the representative of single-celled organisms within the context of this question.