Bacteria do not have a nucleus because they are prokaryotic organisms. Prokaryotic organisms are characterized by their lack of a nucleus and other membrane-bound organelles. Instead, prokaryotic cells have a single, circular chromosome that is located in the cytoplasm. This chromosome is not enclosed by a nuclear membrane, but is instead attached to the cell membrane.

Prokaryotic cells also lack other membrane-bound organelles, such as mitochondria, chloroplasts, and endoplasmic reticulum. This means that prokaryotic cells must carry out all of their cellular functions in the cytoplasm.