Gram-negative vs bacteria

Gram-negative is not a noun while bacteria is a noun.

Word	Noun	Adjective	Verb	Adverb
Gram-negative	No	Yes	No	No
bacteria	Yes	No	No	No

Gram-negative (noun)	bacteria (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-negative (adjective)	bacteria (adjective)
(of bacteria) being of or relating to a bacterium that does not retain the violet stain used in Gram's method