Editing Periplasm (section)

== Terminology ==
Although [[bacteria]] are conventionally divided into two main groups—Gram-positive and Gram-negative, based upon their [[Gram stain|Gram-stain]] retention property—this classification system is ambiguous as it can refer to three distinct aspects (staining result, cell-envelope organization, taxonomic group), which do not necessarily coalesce for some bacterial species.<ref name="Gupta_1998">{{cite journal | vauthors = Gupta RS | title = Protein phylogenies and signature sequences: A reappraisal of evolutionary relationships among archaebacteria, eubacteria, and eukaryotes | journal = Microbiology and Molecular Biology Reviews | volume = 62 | issue = 4 | pages = 1435–1491 | date = December 1998 | pmid = 9841678 | pmc = 98952 | doi = 10.1128/MMBR.62.4.1435-1491.1998 }}</ref><ref name="Gupta_2009">{{cite journal | vauthors = Gupta RS | title = The natural evolutionary relationships among prokaryotes | journal = Critical Reviews in Microbiology | volume = 26 | issue = 2 | pages = 111–131 | date = 2000 | pmid = 10890353 | doi = 10.1080/10408410091154219 | s2cid = 30541897 }}</ref><ref name="Desvaux_2009">{{cite journal | vauthors = Desvaux M, Hébraud M, Talon R, Henderson IR | title = Secretion and subcellular localizations of bacterial proteins: a semantic awareness issue | journal = Trends in Microbiology | volume = 17 | issue = 4 | pages = 139–145 | date = April 2009 | pmid = 19299134 | doi = 10.1016/j.tim.2009.01.004 }}</ref><ref name="Sutcliffe_2010">{{cite journal | vauthors = Sutcliffe IC | title = A phylum level perspective on bacterial cell envelope architecture | journal = Trends in Microbiology | volume = 18 | issue = 10 | pages = 464–470 | date = October 2010 | pmid = 20637628 | doi = 10.1016/j.tim.2010.06.005 }}</ref> 
In most situations such as in this article, Gram-staining reflects the marked differences in the [[ultrastructure]] and chemical composition of the two main kinds of bacteria. The usual "Gram-positive" type does not have an outer lipid membrane, while the typical "Gram-negative" bacterium does.  The terms "diderm" and "monoderm", coined to refer to this distinction ''only'', is a more reliable and fundamental characteristic of the bacterial cells.<ref name="Gupta_1998" /><ref name="Gupta_1998b">{{cite journal | vauthors = Gupta RS | title = What are archaebacteria: life's third domain or monoderm prokaryotes related to gram-positive bacteria? A new proposal for the classification of prokaryotic organisms | journal = Molecular Microbiology | volume = 29 | issue = 3 | pages = 695–707 | date = August 1998 | pmid = 9723910 | doi = 10.1046/j.1365-2958.1998.00978.x | s2cid = 41206658 }}</ref>
[[File:Gram-Cell-wall.svg|thumb|Monoderm bacteria have a thin periplasm between the cell wall and the plasma membrane<ref name="Zuber_2006" />]]

All [[Gram-positive bacteria]] are bounded by a single unit lipid membrane (i.e. monoderm); they generally contain a thick layer (20-80&nbsp;nm) of peptidoglycan responsible for retaining the Gram-stain. A number of other bacteria which are bounded by a single membrane but stain gram-negative due to either lack of the peptidoglycan layer (viz., mycoplasmas) or their inability to retain the Gram-stain due to their cell wall composition, also show close relationship to the Gram-positive bacteria. For the bacterial (prokaryotic) cells that are bounded by a single cell membrane the term "monoderm bacteria" or "monoderm [[prokaryote]]s" has been proposed.<ref name="Gupta_1998" /><ref name="Gupta_1998b" /> In contrast to gram-positive bacteria, all archetypical Gram-negative bacteria are bounded by a cytoplasmic membrane as well as an outer cell membrane; they contain only a thin layer of peptidoglycan (2–3&nbsp;nm) between these membranes. The presence of both inner and outer cell membranes forms and define the periplasmic space or periplasmic compartment. These bacterial cells with two membranes have been designated as diderm bacteria.<ref name="Gupta_1998" /><ref name="Gupta_1998b" /> The distinction between the monoderm and diderm prokaryotes is supported by [[conserved signature indels]] in a number of important proteins (for example, [[DnaK]] and [[GroEL]]).<ref name="Gupta_1998" /><ref name="Gupta_2009" /><ref name="Gupta_1998b" /><ref name="Gupta_2011">{{cite journal | vauthors = Gupta RS | title = Origin of diderm (Gram-negative) bacteria: antibiotic selection pressure rather than endosymbiosis likely led to the evolution of bacterial cells with two membranes | journal = Antonie van Leeuwenhoek | volume = 100 | issue = 2 | pages = 171–182 | date = August 2011 | pmid = 21717204 | pmc = 3133647 | doi = 10.1007/s10482-011-9616-8 }}</ref>