<P> Although bacteria are traditionally divided into two main groups, Gram - positive and Gram - negative, based on their Gram stain retention property, this classification system is ambiguous as it refers to three distinct aspects (staining result, envelope organization, taxonomic group), which do not necessarily coalesce for some bacterial species . The Gram - positive and Gram - negative staining response is also not a reliable characteristic as these two kinds of bacteria do not form phylogenetic coherent groups . However, although Gram staining response is an empirical criterion, its basis lies in the marked differences in the ultrastructure and chemical composition of the bacterial cell wall, marked by the absence or presence of an outer lipid membrane . </P> <P> All Gram - positive bacteria are bounded by a single - unit lipid membrane, and, in general, they contain a thick layer (20--80 nm) of peptidoglycan responsible for retaining the Gram stain . A number of other bacteria--that are bounded by a single membrane, but stain Gram - negative due to either lack of the peptidoglycan layer, as in the Mycoplasmas, or their inability to retain the Gram stain because of their cell wall composition--also show close relationship to the Gram - positive bacteria . For the bacterial cells bounded by a single cell membrane, the term "monoderm bacteria" or "monoderm prokaryotes" has been proposed . </P> <P> 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 . </P> <P> In general, Gram - positive bacteria are monoderms and have a single lipid bilayer whereas Gram - negative bacteria are diderms and have two bilayers . Some taxa lack peptidoglycan (such as the domain Archaea, the class Mollicutes, some members of the Rickettsiales, and the insect - endosymbionts of the Enterobacteriales) and are Gram - variable . This, however, does not always hold true . The Deinococcus - Thermus bacteria have Gram - positive stains, although they are structurally similar to Gram - negative bacteria with two layers . The Chloroflexi have a single layer, yet (with some exceptions) stain negative . Two related phyla to the Chloroflexi, the TM7 clade and the Ktedonobacteria, are also monoderms . </P>

Why gram positive bacteria are called gram positive