Gram-positive vs bacterium

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

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

Gram-positive (noun)	bacterium (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

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