Question: why gram positive bacteria are called gram positive

title: Gram-positive bacteria
text: In contrast to Gram - positive bacteria , all archetypical Gram - negative bacteria are bounded by a cytoplasmic membrane and an outer cell membrane ; they contain only a thin layer of peptidoglycan ( 2 -- 3 nm ) between these membranes . The presence of inner and outer cell membranes defines a new compartment in these cells : the periplasmic space or the periplasmic compartment . These bacteria have been designated as `` diderm bacteria . '' The distinction between the monoderm and diderm bacteria is supported by conserved signature indels in a number of important proteins ( viz . DnaK , GroEL ) . Of these two structurally distinct groups of bacteria , monoderms are indicated to be ancestral . Based upon a number of observations including that the Gram - positive bacteria are the major producers of antibiotics and that , in general , Gram - negative bacteria are resistant to them , it has been proposed that the outer cell membrane in Gram - negative bacteria ( diderms ) has evolved as a protective mechanism against antibiotic selection pressure . Some bacteria , such as Deinococcus , which stain Gram - positive due to the presence of a thick peptidoglycan layer and also possess an outer cell membrane are suggested as intermediates in the transition between monoderm ( Gram - positive ) and diderm ( Gram - negative ) bacteria . The diderm bacteria can also be further differentiated between simple diderms lacking lipopolysaccharide , the archetypical diderm bacteria where the outer cell membrane contains lipopolysaccharide , and the diderm bacteria where outer cell membrane is made up of mycolic acid .