As nouns the difference between bacterium and protozoan
is that bacterium
is (microbiology) a single celled organism with no nucleus while protozoan
is (cytology) any of the diverse group of eukaryotes, of the phylum protozoa, that are primarily unicellular, existing singly or aggregating into colonies, are usually nonphotosynthetic, and are often classified further into phyla according to their capacity for and means of motility, as by pseudopods, flagella, or cilia.
As an adjective protozoan is
of, pertaining to, or characteristic of a protozoan.