Editing Gram stain (section)

=== 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>