bacterium vs Gram-positive

bacterium is a noun but Gram-positive is not a noun.

Word	Noun	Adjective	Verb	Adverb
bacterium	Yes	No	No	No
Gram-positive	No	Yes	No	No

bacterium (noun)	Gram-positive (noun)
(microbiology) single-celled or noncellular spherical or spiral or rod-shaped organisms lacking chlorophyll that reproduce by fission; important as pathogens and for biochemical properties; taxonomy is difficult; often considered to be plants

bacterium (adjective)	Gram-positive (adjective)
	(of bacteria) being or relating to a bacterium that retains the violet stain used in Gram's method