Editing Gram stain (section)

=== Gram-negative bacteria ===
{{Main|Gram-negative bacteria}}
[[File:Neisseria gonorrhoeae and pus cells Gram stain.jpg|right|thumb|Gram negative [[Neisseria gonorrhoeae]] and pus cells]]
<!--Definition of Gr- purposefully repeated-->Gram-negative bacteria generally possess a thin layer of peptidoglycan between two membranes (''diderm'').<ref name="pmid31975449">{{cite journal | vauthors=Megrian D, Taib N, Witwinowski J, Gribaldo S| title=One or two membranes? Diderm Firmicutes challenge the Gram-positive/Gram-negative divide | journal= [[Molecular Microbiology (journal)|Molecular Microbiology]] | volume=113 | issue=3 | pages=659–671 | year=2020 | url=https://hal.archives-ouvertes.fr/pasteur-02505848 | doi= 10.1111/mmi.14469 | pmid=31975449 | s2cid=210882600 | doi-access=free }}</ref> [[Lipopolysaccharide]] (LPS) is the most abundant [[antigen]] on the cell surface of most gram-negative bacteria, contributing up to 80% of the outer membrane of ''E. coli'' and ''Salmonella''.<ref name="pmid33746909">{{cite journal | vauthors=Avila-Calderón ED, Ruiz-Palma MD, Contreras-Rodríguez A| title=Outer Membrane Vesicles of Gram-Negative Bacteria: An Outlook on Biogenesis | journal= [[Frontiers in Microbiology]] | volume=12 | pages=557902 | year=2021 |  doi= 10.3389/fmicb.2021.557902 | pmc=7969528 | pmid=33746909 | doi-access=free }}</ref> These LPS molecules, consisting of the O-antigen or O-polysaccharide, core polysaccharide, and lipid A, serve multiple functions including contributing to the cell's negative charge and protecting against certain chemicals. LPS's role is critical in host-pathogen interactions, with the O-antigen eliciting an immune response and lipid A acting as an endotoxin.<ref name=":3" />

Additionally, the outer membrane acts as a selective barrier, regulated by porins, transmembrane proteins forming pores that allow specific molecules to pass. The space between the cell membrane and the outer membrane, known as the periplasm, contains periplasmic enzymes for nutrient processing. A significant structural component linking the peptidoglycan layer and the outer membrane is Braun's lipoprotein, which provides additional stability and strength to the bacterial cell wall.<ref name=":3" />

Most [[bacterial phyla]] are gram-negative, including the [[cyanobacteria]], [[green sulfur bacteria]], and most [[Pseudomonadota]] (exceptions being some members of the [[Rickettsiales]] and the insect-endosymbionts of the [[Enterobacteriales]]).<ref name="Madigan_2004" />{{page needed|date=March 2016}}<ref name="Begey_essay" />