Editing Gram stain (section)

== Examples ==

=== Gram-positive bacteria ===
[[File:Gram-positive bacteria and pus cells.jpg|thumb|Gram-stain of [[gram-positive]] [[streptococci]] surrounded by [[neutrophil| pus cells]]]]
{{Main|Gram-positive bacteria}}
Gram-positive bacteria generally have a single membrane (''monoderm'') surrounded by a thick peptidoglycan.
This rule is followed by two phyla: [[Bacillota]] (except for the classes [[Mollicutes]] and [[Negativicutes]]) and the [[Actinomycetota]].<ref name="Madigan_2004" /><ref name="Begey_essay">{{cite book |series=Bergey's Manual of Systematic Bacteriology |volume=2A |title=Introductory Essays |editor-first=George M. |editor-last=Garrity |first1=Don J. |last1=Brenner |first2=Noel R. |last2=Krieg |first3=James T. |last3=Staley |publisher=Springer |location= New York |edition=2nd |isbn=978-0-387-24143-2 |page=304 |url= https://www.springer.com/life+sciences/book/978-0-387-24143-2 |date=26 July 2005 |orig-year=1984 |id=British Library no. GBA561951}}</ref> In contrast, members of the [[Chloroflexota]] (green non-sulfur bacteria) are monoderms but possess a thin or absent (class [[Dehalococcoidetes]]) peptidoglycan and can stain negative, positive or indeterminate; members of the [[Deinococcota]] stain positive but are diderms with a thick peptidoglycan.<ref name="Madigan_2004" />{{page needed|date=March 2016}}<ref name="Begey_essay" />

The cell wall's strength is enhanced by teichoic acids, glycopolymeric substances embedded within the peptidoglycan. Teichoic acids play multiple roles, such as generating the cell's net negative charge, contributing to cell wall rigidity and shape maintenance, and aiding in cell division and resistance to various stressors, including heat and salt. Despite the density of the peptidoglycan layer, it remains relatively porous, allowing most substances to permeate. For larger nutrients, Gram-positive bacteria utilize exoenzymes, secreted extracellularly to break down macromolecules outside the cell.<ref name=":3">{{Cite book |last=Bruslind |first=Linda |url=https://open.oregonstate.education/generalmicrobiology/ |title=General Microbiology |date=2019-08-01 |publisher=Oregon State University |language=en}}</ref>

[[Bacterial taxonomy|Historically]], the gram-positive forms made up the [[phylum (biology)|phylum]] [[Bacillota|Firmicutes]], a name now used for the largest group. It includes many well-known genera such as ''[[Bacillus|Lactobacillus, Bacillus]]'', ''[[Listeria]]'', ''[[Staphylococcus]]'', ''[[Streptococcus]]'', ''[[Enterococcus]]'', and ''[[Clostridium]]''.<ref>{{Cite journal |last=Galperin |first=Michael Y. |date=27 December 2013 |title=Genome Diversity of Spore-forming Firmicutes |journal=Microbiology Spectrum |volume=1 |issue=2 |pages=TBS-0015-2012- |doi=10.1128/microbiolspectrum.tbs-0015-2012 |issn=2165-0497 |pmc=4306282 |pmid=26184964}}</ref> It has also been expanded to include the Mollicutes, bacteria such as ''[[Mycoplasma]] and Thermoplasma'' that lack cell walls and so cannot be Gram-stained, but are derived from such forms.<ref>{{cite web |last=Hashem |first=Hams H. |title=Practical Medical Microbiology |work=University of Al-Qadisiya |url=http://qu.edu.iq/el/mod/resource/view.php?id=1391 }}{{Dead link|date=June 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

Some bacteria have cell walls which are particularly adept at retaining stains. These will appear positive by Gram stain even though they are not closely related to other gram-positive bacteria. These are called [[acid-fastness|acid-fast bacteria]], and can only be differentiated from other gram-positive bacteria by [[Ziehl–Neelsen stain|special staining procedures]].<ref>{{Cite web |url= http://www2.highlands.edu/academics/divisions/scipe/biology/labs/rome/acid_fast_stain.htm |title=The Acid Fast Stain |work=www2.Highlands.edu |publisher=Georgia Highlands College |access-date=10 June 2017 |archive-url= https://web.archive.org/web/20170610135044/http://www2.highlands.edu/academics/divisions/scipe/biology/labs/rome/acid_fast_stain.htm |archive-date=10 June 2017 |url-status=dead}}</ref>

=== 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" />

=== Gram-variable and gram-indeterminate bacteria ===
{{anchor|Gram-variable and gram-indeterminate bacteria|Gram-indeterminate bacteria|reason=Former names of this section; may have incoming links.}}
Some bacteria, after staining with the Gram stain, yield a gram-variable pattern: a mix of pink and purple cells are seen.<ref name=":0" /><ref name="Beveridge_1990">{{cite journal |url= |first=Terry J. |last=Beveridge |title=Mechanism of Gram Variability in Select Bacteria |journal=Journal of Bacteriology |date=March 1990 |volume=172 |issue=3 |pages=1609–1620 |pmid=1689718 |pmc=208639 |doi=10.1128/jb.172.3.1609-1620.1990}}</ref> In cultures of ''Bacillus, Butyrivibrio'', and ''Clostridium'', a decrease in peptidoglycan thickness during growth coincides with an increase in the number of cells that stain gram-negative.<ref name="Beveridge_1990" /> In addition, in all bacteria stained using the Gram stain, the age of the culture may influence the results of the stain.<ref name="Beveridge_1990" />

Gram-indeterminate bacteria do not respond predictably to Gram staining and, therefore, cannot be determined as either gram-positive or gram-negative. Examples include many species of ''[[Mycobacterium]]'', including ''Mycobacterium bovis'', ''[[Mycobacterium leprae]]'' and ''[[Mycobacterium tuberculosis]]'', the latter two of which are the causative agents of leprosy and tuberculosis, respectively.<ref>{{cite book |last=Black |first=Jacquelyn |title=Microbiology: Principles and Exploration |edition=8th |date=2012 |publisher=John Wiley & Sons |isbn=978-0-470-54109-8 |page=68}}</ref><ref name="pmid19885935">{{cite book |last1=Reynolds |first1=J. |last2=Moyes |first2=R. B. |last3=Breakwell |first3=D. P. |title=Differential staining of bacteria: Acid fast stain |journal=[[Current Protocols#Titles|Current Protocols in Microbiology]] |date=2009 |chapter=Appendix 3 |volume=Appendix 3 |pages=H |doi=10.1002/9780471729259.mca03hs15 |pmid=19885935 |isbn=978-0471729259|s2cid=45685776 }}</ref> Bacteria of the genus ''[[Mycoplasma]]'' lack a [[cell wall]] around their [[cell membrane]]s,<ref name=Sherris>{{cite book | author = Ryan KJ, Ray CG (editors) | title = Sherris Medical Microbiology | pages=409–12 |edition = 4th | publisher = McGraw Hill | year = 2004 | isbn = 978-0-8385-8529-0}}</ref> which means they do not stain by Gram's method and are resistant to the antibiotics that target cell wall synthesis.<ref name="pmid23085510">{{cite journal |vauthors=Lee EH, Winter HL, van Dijl JM, Metzemaekers JD, Arends JP |title=Diagnosis and antimicrobial therapy of Mycoplasma hominis meningitis in adults |journal=International Journal of Medical Microbiology |volume=302 |issue=7–8 |pages=289–92 |date=December 2012 |pmid=23085510 |doi=10.1016/j.ijmm.2012.09.003 |url=}}</ref><ref name="pmid30003864">{{cite journal |vauthors=Gautier-Bouchardon AV |title=Antimicrobial Resistance in Mycoplasma spp |journal=Microbiology Spectrum |volume=6 |issue=4 |pages= 425–446|date=July 2018 |pmid=30003864 |doi=10.1128/microbiolspec.ARBA-0030-2018 |isbn=9781555819798 |s2cid=51616821 |url=|pmc=11633602 }}</ref>