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{{short description|Investigative procedure in microbiology}}
{{short description|Investigative procedure in microbiology}}
[[File:Gram positive coccus and gram negative rod.png|thumb|[[Micrograph]] of a [[gram-positive]] coccus and a [[gram-negative]] rod.]]
[[File:Gram stain 01.jpg|thumb|A Gram stain of mixed ''[[Staphylococcus aureus]]'' (''S. aureus'' ATCC 25923, [[gram-positive bacteria|gram-positive]] cocci, in purple) and ''[[Escherichia coli]]'' (''E. coli'' ATCC 11775, [[Gram-negative bacteria|gram-negative]] bacilli, in red), the most common Gram stain reference bacteria]]
[[File:Gram stain 01.jpg|thumb|A Gram stain of mixed ''[[Staphylococcus aureus]]'' (''S. aureus'' ATCC 25923, [[gram-positive bacteria|gram-positive]] cocci, in purple) and ''[[Escherichia coli]]'' (''E. coli'' ATCC 11775, [[Gram-negative bacteria|gram-negative]] bacilli, in red), the most common Gram stain reference bacteria]]


In [[microbiology]] and [[bacteriology]], '''Gram stain''' ('''Gram staining''' or '''Gram's method'''), is a method of [[staining]] used to classify [[bacteria]]l species into two large groups: [[gram-positive bacteria]] and [[gram-negative bacteria]]. The name comes from the Danish [[bacteriologist]] [[Hans Christian Gram]], who developed the technique in 1884.<ref>{{cite journal |last=Colco |first=R. |title=Gram Staining |journal=[[Current Protocols in Microbiology]] |date=2005 |volume=Appendix 3 |issue=1 |pages=Appendix 3C |doi=10.1002/9780471729259.mca03cs00 |pmid=18770544 |isbn=978-0471729259 |s2cid=32452815 }}</ref>
'''Gram stain''' ('''Gram staining''' or '''Gram's method'''), is a method of [[staining]] used to classify [[bacteria]]l species into two large groups: [[gram-positive bacteria]] and [[gram-negative bacteria]]. It may also be used to diagnose a [[fungal infection]].<ref name="medline2021">{{cite web |title=Gram Stain: MedlinePlus Medical Test |url=https://medlineplus.gov/lab-tests/gram-stain/#:~:text=What%20is%20it%20used%20for,used%20to%20diagnose%20fungal%20infections. |website=medlineplus.gov |language=en}}</ref> The name comes from the Danish [[bacteriologist]] [[Hans Christian Gram]], who developed the technique in 1884.<ref>{{cite journal |last=Colco |first=R. |title=Gram Staining |journal=[[Current Protocols in Microbiology]] |date=2005 |volume=Appendix 3 |issue=1 |pages=Appendix 3C |doi=10.1002/9780471729259.mca03cs00 |pmid=18770544 |isbn=978-0471729259 |s2cid=32452815 }}</ref>


Gram staining differentiates bacteria by the chemical and physical properties of their [[cell wall]]s. Gram-positive cells have a thick layer of [[peptidoglycan]] in the cell wall that retains the primary stain, [[crystal violet]]. Gram-negative cells have a thinner peptidoglycan layer that allows the crystal violet to wash out on addition of [[ethanol]]. They are stained pink or red by the [[counterstain]],<ref name="Beveridge_and_Davies_1983" /> commonly [[safranin]] or [[fuchsine]]. [[Lugol's iodine]] solution is always added after addition of crystal violet to strengthen the bonds of the stain with the [[cell membrane]].
Gram staining differentiates bacteria by the chemical and physical properties of their [[Cell wall#Bacterial cell walls|cell wall]]s. Gram-positive cells have a thick layer of [[peptidoglycan]] in the cell wall that retains the primary stain, [[crystal violet]]. Gram-negative cells have a thinner peptidoglycan layer that allows the crystal violet to wash out on addition of [[ethanol]]. They are stained pink or red by the [[counterstain]],<ref name="Beveridge_and_Davies_1983" /> commonly [[safranin]] or [[fuchsine]]. [[Lugol's iodine]] solution is always added after addition of crystal violet to form a stable complex with crystal violet that strengthen the bonds of the stain with the [[cell wall]].<ref>{{Cite journal |last=Libenson |first=L. |last2=McIlroy |first2=A. P. |date=1955-07-01 |title=On the Mechanism of the Gram Stain |url=https://academic.oup.com/jid/article-lookup/doi/10.1093/infdis/97.1.22 |journal=Journal of Infectious Diseases |language=en |volume=97 |issue=1 |pages=22–26 |doi=10.1093/infdis/97.1.22 |issn=0022-1899}}</ref>


Gram staining is almost always the first step in the identification of a bacterial group. While Gram staining is a valuable diagnostic tool in both clinical and research settings, not all bacteria can be definitively classified by this technique. This gives rise to ''gram-variable'' and ''gram-indeterminate'' groups.
Gram staining is almost always the first step in the identification of a bacterial group. While Gram staining is a valuable diagnostic tool in both clinical and research settings, not all bacteria can be definitively classified by this technique. This gives rise to ''gram-variable'' and ''gram-indeterminate'' groups.


== History ==
== History ==
The method is named after its inventor, the [[Denmark|Danish]] scientist [[Hans Christian Gram]] (1853–1938), who developed the technique while working with [[Carl Friedländer]] in the morgue of the city hospital in [[Berlin]] in 1884. Gram devised his technique not for the purpose of distinguishing one type of bacterium from another but to make bacteria more visible in stained sections of lung tissue.<ref>{{Cite journal |last=Austrian |first=R. |date=1960 |title=The Gram stain and the etiology of lobar pneumonia, an historical note |journal=Bacteriological Reviews |volume=24 |issue=3 |pages=261–265 |pmid=13685217 |pmc=441053 |doi=10.1128/MMBR.24.3.261-265.1960}}</ref> He published his method in 1884, and included in his short report the observation that the [[typhus]] [[Bacillus (shape)|bacillus]] did not retain the stain.<ref name=Gram_1884>{{Cite journal |author-link=Hans Christian Gram |last=Gram |first=Hans Christian |title=Über die isolierte Färbung der Schizomyceten in Schnitt- und Trockenpräparaten |journal=Fortschritte der Medizin |language=de |date=1884 |volume=2 |pages=185–189}}.<br />English translation in: {{Cite book |last=Brock |first=T. D. |date=1999 |title=Milestones in Microbiology 1546–1940 |edition=2nd |publisher=ASM Press |pages=215–218 |isbn=978-1-55581-142-6 |url= https://books.google.com/books?id=q5JHcs8w21gC&q=Milestones%20in%20Microbiology&pg=PA215}}<br />Translation is also at: {{Cite web |url= http://www.hoslink.com/pathology/lab_general/history2.htm#gram |last=Brock |first=T. D. |title=Pioneers in Medical Laboratory Science: Christian Gram 1884 |work=HOSLink.com |access-date=27 July 2010}}</ref>
The method is named after its inventor, the [[Denmark|Danish]] scientist [[Hans Christian Gram]] (1853–1938), who developed the technique while working with [[Carl Friedländer]] in the morgue of the city hospital in [[Berlin]] in 1884. Gram devised his technique not for the purpose of distinguishing one type of bacterium from another but to make bacteria more visible in stained sections of lung tissue.<ref>{{Cite journal |last=Austrian |first=R. |date=1960 |title=The Gram stain and the etiology of lobar pneumonia, an historical note |journal=Bacteriological Reviews |volume=24 |issue=3 |pages=261–265 |pmid=13685217 |pmc=441053 |doi=10.1128/MMBR.24.3.261-265.1960}}</ref> Gram noticed that some bacterial cells possessed noticeable resistance to decolorization. Based on these observations, Gram developed the initial gram staining procedure, initially making use of Ehrlich's aniline-gentian violet, Lugol's iodine, absolute alcohol for decolorization, and Bismarck brown for counterstain.<ref name=":2">{{Cite journal |last1=Bartholomew |first1=James W. |last2=Mittwer |first2=Tod |date=March 1952 |title=THE GRAM STAIN |journal=Bacteriological Reviews |language=en |volume=16 |issue=1 |pages=1–29 |doi=10.1128/br.16.1.1-29.1952 |issn=0005-3678 |pmc=180726 |pmid=14925025}}</ref> He published his method in 1884, and included in his short report the observation that the [[typhus]] [[Bacillus (shape)|bacillus]] did not retain the stain.<ref name=Gram_1884>{{Cite journal |author-link=Hans Christian Gram |last=Gram |first=Hans Christian |title=Über die isolierte Färbung der Schizomyceten in Schnitt- und Trockenpräparaten |journal=Fortschritte der Medizin |language=de |date=1884 |volume=2 |pages=185–189}}.<br />English translation in: {{Cite book |last=Brock |first=T. D. |date=1999 |title=Milestones in Microbiology 1546–1940 |edition=2nd |publisher=ASM Press |pages=215–218 |isbn=978-1-55581-142-6 |url=https://books.google.com/books?id=q5JHcs8w21gC&q=Milestones%20in%20Microbiology&pg=PA215 }}<br />Translation is also at: {{Cite web |url=http://www.hoslink.com/pathology/lab_general/history2.htm#gram |last=Brock |first=T. D. |title=Pioneers in Medical Laboratory Science: Christian Gram 1884 |work=HOSLink.com |access-date=27 July 2010 |archive-date=10 August 2016 |archive-url=https://web.archive.org/web/20160810225816/http://www.hoslink.com/pathology/lab_general/history2.htm#gram |url-status=dead }}</ref> Gram did not initially make the distinction between Gram-negative and Gram-positive bacteria using his procedure.<ref name=":2" />


== Uses ==
== Uses ==
[[File:Candida Gram stain.jpg|thumb|right|Gram stain of ''Candida albicans'' from a vaginal swab. The small oval [[chlamydospore]]s are 2–4 [[micrometre|µm]] in diameter.]]
[[File:Candida Gram stain.jpg|thumb|right|Gram stain of ''[[Candida albicans]]'' from a vaginal swab. The small oval [[chlamydospore]]s are 2–4 [[micrometre|μm]] in diameter.]]
Gram staining is a [[bacteriology|bacteriological]] [[laboratory]] technique<ref name=Sherris2>{{cite book |editor1-last=Ryan |editor1-first=K. J. |editor2-last=Ray |editor2-first=C. G. |title=Sherris Medical Microbiology |pages=232f |edition=4th |publisher=McGraw Hill |date=2004 |isbn=978-0838585290}}</ref> used to differentiate [[Bacterium|bacterial]] species into two large groups ([[gram-positive]] and [[gram-negative]]) based on the physical properties of their [[cell wall]]s.<ref name="Madigan_2004">{{cite book |last1=Madigan |first1=M. T. |last2=Martinko |first2=J. |last3=Parker |first3=J. |title=Brock Biology of Microorganisms |edition=10th |publisher=Lippincott Williams & Wilkins |date=2004 |isbn=978-0-13-066271-2 |url-access=registration |url= https://archive.org/details/brockbiologyofmi00madi}}</ref>{{page needed|date=March 2016}} Gram staining is not used to classify [[archaea]], since these microorganisms yield widely varying responses that do not follow their [[Phylogenetics|phylogenetic groups]].<ref>{{cite journal |last=Beveridge |first=T. J. |title=Use of the Gram stain in microbiology |journal=Biotechnic & Histochemistry |volume=76 |issue=3 |pages=111–118 |date=2001 |pmid=11475313 |doi=10.1080/714028139}}</ref>
Gram staining is a [[bacteriology|bacteriological]] [[laboratory]] technique<ref name=Sherris2>{{cite book |editor1-last=Ryan |editor1-first=K. J. |editor2-last=Ray |editor2-first=C. G. |title=Sherris Medical Microbiology |pages=232f |edition=4th |publisher=McGraw Hill |date=2004 |isbn=978-0838585290}}</ref> used to differentiate [[Bacterium|bacterial]] species into two large groups ([[gram-positive]] and [[gram-negative]]) based on the physical properties of their [[cell wall]]s.<ref name="Madigan_2004">{{cite book |last1=Madigan |first1=M. T. |last2=Martinko |first2=J. |last3=Parker |first3=J. |title=Brock Biology of Microorganisms |edition=10th |publisher=Lippincott Williams & Wilkins |date=2004 |isbn=978-0-13-066271-2 |url-access=registration |url= https://archive.org/details/brockbiologyofmi00madi}}</ref>{{page needed|date=March 2016}} Gram staining can also be used to diagnose a [[fungal infection]].<ref name="medline2021"/> Gram staining is not used to classify [[archaea]], since these microorganisms yield widely varying responses that do not follow their [[Phylogenetics|phylogenetic groups]].<ref>{{cite journal |last=Beveridge |first=T. J. |title=Use of the Gram stain in microbiology |journal=Biotechnic & Histochemistry |volume=76 |issue=3 |pages=111–118 |date=2001 |pmid=11475313 |doi=10.1080/714028139}}</ref>


Some organisms are gram-variable (meaning they may stain either negative or positive); some are not stained with either dye used in the Gram technique and are not seen. In a modern environmental or molecular microbiology lab, most identification is done using genetic sequences and other molecular techniques, which are far more specific and informative than differential staining.{{citation needed|date=July 2022}}
Some organisms are gram-variable (meaning they may stain either negative or positive); some are not stained with either dye used in the Gram technique and are not seen.{{cn|date=May 2024}}


=== Medical ===
=== Medical ===
[[File:Diagnostic algorithm of possible bacterial infection.png|thumb|Example of a workup algorithm of possible bacterial infection in cases with no specifically requested targets (non-bacteria, mycobacteria etc.), with most common situations and agents seen in a New England community hospital setting. In this setting, Gram stain is used even before agar plate cultures for blood and cerebrospinal fluid specimens. Furthermore, for cases having undergone culture, Gram stain is shown as a major determinant for further workup.]]
{{see also|Pathogenic bacteria}}
{{see also|Pathogenic bacteria}}
Gram stains are performed on [[body fluid]] or [[biopsy]] when [[infection]] is suspected. Gram stains yield results much more quickly than [[microbiological culture|culturing]], and are especially important when infection would make an important difference in the patient's treatment and prognosis; examples are [[cerebrospinal fluid]] for [[meningitis]] and [[synovial fluid]] for [[septic arthritis]].<ref name="Sherris" /><ref name="Søgaard_2007">{{cite journal |last1=Søgaard |first1=M. |last2=Nørgaard |first2=M. |last3=Schønheyder |first3=H. |title=First notification of positive blood cultures: High accuracy of the Gram stain report |journal=Journal of Clinical Microbiology |volume=45 |issue=4 |pages=1113–1117 |date=2007 |pmid=17301283 |doi=10.1128/JCM.02523-06 |pmc=1865800}}</ref>
Gram stains are performed on [[body fluid]] or [[biopsy]] when [[infection]] is suspected. Gram stains yield results much more quickly than [[microbiological culture|culturing]], and are especially important when infection would make an important difference in the patient's treatment and prognosis; examples are [[cerebrospinal fluid]] for [[meningitis]] and [[synovial fluid]] for [[septic arthritis]].<ref name="Sherris" /><ref name="Søgaard_2007">{{cite journal |last1=Søgaard |first1=M. |last2=Nørgaard |first2=M. |last3=Schønheyder |first3=H. |title=First notification of positive blood cultures: High accuracy of the Gram stain report |journal=Journal of Clinical Microbiology |volume=45 |issue=4 |pages=1113–1117 |date=2007 |pmid=17301283 |doi=10.1128/JCM.02523-06 |pmc=1865800}}</ref>
Line 38: Line 38:
| Primary dye || [[crystal violet]] || purple || purple
| Primary dye || [[crystal violet]] || purple || purple
|-
|-
| mordant || [[iodine]] || purple || purple
| Trapper || [[iodine]] || purple || purple
|-
|-
| Decolorizer || [[ethanol|alcohol]]/[[acetone]] || purple || colorless
| Decolorizer || [[ethanol|alcohol]]/[[acetone]] || purple || colorless
Line 49: Line 49:
Iodide ({{chem|I|-}} or {{chem|I|3|-}}) interacts with {{chem|CV|+}} and forms large complexes of crystal violet and iodine (CV–I) within the inner and outer layers of the cell. Iodine is often referred to as a [[mordant]], but is a trapping agent that prevents the removal of the CV–I complex and, therefore, colors the cell.<ref name="Stainsfile - Trapping Agents">{{cite web |url= http://stainsfile.info/StainsFile/theory/notmrdnt.htm |title=Stain theory – What a mordant is not |work=StainsFile.info |access-date=9 March 2016 |archive-url= https://web.archive.org/web/20160309162426/http://stainsfile.info/StainsFile/theory/notmrdnt.htm |archive-date=9 March 2016 |url-status=dead}}</ref>
Iodide ({{chem|I|-}} or {{chem|I|3|-}}) interacts with {{chem|CV|+}} and forms large complexes of crystal violet and iodine (CV–I) within the inner and outer layers of the cell. Iodine is often referred to as a [[mordant]], but is a trapping agent that prevents the removal of the CV–I complex and, therefore, colors the cell.<ref name="Stainsfile - Trapping Agents">{{cite web |url= http://stainsfile.info/StainsFile/theory/notmrdnt.htm |title=Stain theory – What a mordant is not |work=StainsFile.info |access-date=9 March 2016 |archive-url= https://web.archive.org/web/20160309162426/http://stainsfile.info/StainsFile/theory/notmrdnt.htm |archive-date=9 March 2016 |url-status=dead}}</ref>


When a decolorizer such as alcohol or acetone is added, it interacts with the lipids of the cell membrane.<ref>{{Cite web |url= http://www.austincc.edu/microbugz/gram_stain.php |title=Gram Stain |work=Microbugz |publisher=Austin Community College |access-date=2017-05-26}}</ref> A gram-negative cell loses its outer lipopolysaccharide membrane, and the inner peptidoglycan layer is left exposed. The CV–I complexes are washed from the gram-negative cell along with the outer membrane.<ref name=":0">{{Cite book |title=Pharmaceutical Microbiology: Essentials for Quality Assurance and Quality Control |last=Tim |first=Sandle |isbn=9780081000229 |oclc=923807961 |date=21 October 2015}}</ref> In contrast, a gram-positive cell becomes dehydrated from an ethanol treatment. The large CV–I complexes become trapped within the gram-positive cell due to the multilayered nature of its peptidoglycan.<ref name=":0" /> The decolorization step is critical and must be timed correctly; the crystal violet stain is removed from both gram-positive and negative cells if the decolorizing agent is left on too long (a matter of seconds).<ref name=":1">{{cite web |title=Gram's Serendipitous Stain |work=Hardy's Diagnostics |url=http://hardydiagnostics.com/wp-content/uploads/2016/05/Hans-Christian-Gram.pdf |archive-url=https://web.archive.org/web/20170324063915/http://hardydiagnostics.com/wp-content/uploads/2016/05/Hans-Christian-Gram.pdf |archive-date=2017-03-24 |url-status=live |last1=Hardy |first1=Jay |last2=Maria |first2=Santa}}</ref>
When a decolorizer such as alcohol or acetone is added, it interacts with the lipids of the cell membrane.<ref>{{Cite web |url=http://www.austincc.edu/microbugz/gram_stain.php |title=Gram Stain |work=Microbugz |publisher=Austin Community College |access-date=2017-05-26 |archive-date=2017-05-26 |archive-url=https://web.archive.org/web/20170526012838/http://www.austincc.edu/microbugz/gram_stain.php |url-status=dead }}</ref> A gram-negative cell loses its outer lipopolysaccharide membrane, and the inner peptidoglycan layer is left exposed. The CV–I complexes are washed from the gram-negative cell along with the outer membrane.<ref name=":0">{{Cite book |title=Pharmaceutical Microbiology: Essentials for Quality Assurance and Quality Control |last=Tim |first=Sandle |isbn=9780081000229 |oclc=923807961 |date=21 October 2015|publisher=Elsevier Science }}</ref> In contrast, a gram-positive cell becomes dehydrated from an ethanol treatment. The large CV–I complexes become trapped within the gram-positive cell due to the multilayered nature of its peptidoglycan.<ref name=":0" /> The decolorization step is critical and must be timed correctly; the crystal violet stain is removed from both gram-positive and negative cells if the decolorizing agent is left on too long (a matter of seconds).<ref name=":1">{{cite web |title=Gram's Serendipitous Stain |work=Hardy's Diagnostics |url=http://hardydiagnostics.com/wp-content/uploads/2016/05/Hans-Christian-Gram.pdf |archive-url=https://web.archive.org/web/20170324063915/http://hardydiagnostics.com/wp-content/uploads/2016/05/Hans-Christian-Gram.pdf |archive-date=2017-03-24 |url-status=live |last1=Hardy |first1=Jay |last2=Maria |first2=Santa}}</ref>


After decolorization, the gram-positive cell remains purple and the gram-negative cell loses its purple color.<ref name=":1" /> Counterstain, which is usually positively charged safranin or basic fuchsine, is applied last to give decolorized gram-negative bacteria a pink or red color.<ref name="Beveridge_and_Davies_1983">{{cite journal |last1=Beveridge |first1=T. J. |last2=Davies |first2=J. A. |title=Cellular responses of ''Bacillus subtilis'' and ''Escherichia coli'' to the Gram stain |journal=Journal of Bacteriology |volume=156 |issue=2 |pages=846–58 |date=November 1983 |pmid=6195148 |pmc=217903 |doi=10.1128/JB.156.2.846-858.1983}}</ref><ref name="Davies_et_al_1983">{{cite journal |last1=Davies |first1=J. A. |last2=Anderson |first2=G. K. |last3=Beveridge |first3=T. J. |last4=Clark |first4=H. C. |title=Chemical mechanism of the Gram stain and synthesis of a new electron-opaque marker for electron microscopy, which replaces the iodine mordant of the stain |journal=Journal of Bacteriology |volume=156 |issue=2 |pages=837–845 |date=November 1983 |pmid=6195147 |pmc=217902 |doi=10.1128/JB.156.2.837-845.1983}}</ref> Both gram-positive bacteria and gram-negative bacteria pick up the counterstain. The counterstain, however, is unseen on gram-positive bacteria because of the darker crystal violet stain.
After decolorization, the gram-positive cell remains purple and the gram-negative cell loses its purple color.<ref name=":1" /> Counterstain, which is usually positively charged safranin or basic fuchsine, is applied last to give decolorized gram-negative bacteria a pink or red color.<ref name="Beveridge_and_Davies_1983">{{cite journal |last1=Beveridge |first1=T. J. |last2=Davies |first2=J. A. |title=Cellular responses of ''Bacillus subtilis'' and ''Escherichia coli'' to the Gram stain |journal=Journal of Bacteriology |volume=156 |issue=2 |pages=846–58 |date=November 1983 |pmid=6195148 |pmc=217903 |doi=10.1128/JB.156.2.846-858.1983}}</ref><ref name="Davies_et_al_1983">{{cite journal |last1=Davies |first1=J. A. |last2=Anderson |first2=G. K. |last3=Beveridge |first3=T. J. |last4=Clark |first4=H. C. |title=Chemical mechanism of the Gram stain and synthesis of a new electron-opaque marker for electron microscopy, which replaces the iodine mordant of the stain |journal=Journal of Bacteriology |volume=156 |issue=2 |pages=837–845 |date=November 1983 |pmid=6195147 |pmc=217902 |doi=10.1128/JB.156.2.837-845.1983}}</ref> Both gram-positive bacteria and gram-negative bacteria pick up the counterstain. The counterstain, however, is unseen on gram-positive bacteria because of the darker crystal violet stain.{{cn|date=May 2024}}


== Examples ==
== Examples ==


=== Gram-positive bacteria ===
=== Gram-positive bacteria ===
[[File:Gram-positive bacteria and pus cells.jpg|thumb|Gram-stain of [[gram-positive]] [[streptococci]] surrounded by pus cells]]
[[File:Gram-positive bacteria and pus cells.jpg|thumb|Gram-stain of [[gram-positive]] [[streptococci]] surrounded by [[neutrophil| pus cells]]]]
{{Main|Gram-positive bacteria}}
{{Main|Gram-positive bacteria}}
Gram-positive bacteria generally have a single membrane (''monoderm'') surrounded by a thick peptidoglycan.
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" />
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>
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[[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}}</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>
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>
Line 67: Line 69:
=== Gram-negative bacteria ===
=== Gram-negative bacteria ===
{{Main|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" />


<!--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> 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" />
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 ===
=== Gram-variable and gram-indeterminate bacteria ===
Line 77: Line 83:


== Orthographic note ==
== Orthographic note ==
The term ''Gram staining'' is derived from the surname of [[Hans Christian Gram]]; the [[eponym]] (Gram) is therefore capitalized but not the common noun (stain) as is usual for scientific terms.<ref>{{cite book |url= https://books.google.com/books?id=btb1AwAAQBAJ&pg=PA105 |title=New Hart's Rules: The Oxford Style Guide |first=Anne |last=Waddingham |page=105 |publisher=Oxford University Press |date=28 August 2014 |isbn=978-0199570027 }}</ref> The initial letters of ''gram-positive'' and ''gram-negative'', which are [[Eponym#Orthographic conventions|eponymous adjectives]], can be either capital ''G'' or lowercase ''g'', depending on what [[style guide]] (if any) governs the document being written. Lowercase style is used by the US [[Centers for Disease Control and Prevention]] and other style regimens such as the [[AMA style]].<ref>{{cite web |url= http://wwwnc.cdc.gov/eid/pages/preferred-usage.htm |title=Preferred Usage |work=Emerging Infectious Diseases Style Guide |publisher=[[Centers for Disease Control and Prevention]]}}</ref> Dictionaries may use lowercase,<ref name="Dorlands">{{cite web |title=Dorland's Illustrated Medical Dictionary |edition=32nd |publisher=Elsevier |url= https://www.dorlands.com/dorlands/wsearch.jsp |access-date=5 June 2020}} Use search terms such as {{kbd|gram-negative}}.</ref><ref name="MWMD">{{Citation |title=gram–positive |work=Merriam-Webster.com |publisher=Encyclopædia Britannica, Inc. |url= http://www.merriam-webster.com/dictionary/gram%E2%80%93positive}}</ref> uppercase,<ref>{{cite web |url= http://www.collinsdictionary.com/dictionary/english/gram-positive |title=Gram-positive |work=CollinsDictionary.com |publisher=HarperCollins}}</ref><ref>{{cite web |url= https://www.lexico.com/definition/gram_stain |archive-url= https://web.archive.org/web/20200605094227/https://www.lexico.com/definition/gram_stain |url-status= dead |archive-date= June 5, 2020 |title=Gram stain |work=Lexico.com |publisher=Oxford University Press}}</ref><ref>{{cite web |url= http://www.medicinenet.com/script/main/art.asp?articlekey=9585 |title=Gram-positive |work=MedicineNet}}</ref><ref>{{cite web |url= http://www.businessdictionary.com/definition/Gram-negative-positive.html |title= Gram negative/positive |work= BusinessDictionary.com |access-date= 2016-10-20 |archive-date= 2016-10-20 |archive-url= https://web.archive.org/web/20161020173301/http://www.businessdictionary.com/definition/Gram-negative-positive.html |url-status= dead }}</ref> or both.<ref>{{cite web |url= https://ahdictionary.com/word/search.html?q=gram+positive&submit.x=0&submit.y=0 |title=gram-pos·i·tive or Gram-pos·i·tive |work=The American Heritage Dictionary |publisher=Houghton Mifflin}}</ref><ref name="dictionary">{{cite web |url= http://www.dictionary.com/browse/gram-positive |title=Gram-positive |work=Dictionary.com}}</ref> Uppercase ''Gram-positive'' or ''Gram-negative'' usage is also common in many [[scientific journal]] articles and publications.<ref name="dictionary"/><ref>{{cite journal |title=Through the wall: Extracellular vesicles in Gram-positive bacteria, mycobacteria and fungi |first1=Lisa |last1=Brown |first2=Julie M. |last2=Wolf |first3=Rafael |last3=Prados-Rosales |first4=Arturo |last4=Casadevall |journal=Nature Reviews Microbiology |volume=13 |issue=10 |pages=620–630 |date=2015 |doi=10.1038/nrmicro3480 |pmid=26324094 |pmc=4860279}}</ref><ref>{{cite journal |journal=Science |title=Detecting Gram-negative bacteria |date=12 June 2015 |volume=348 |issue=6240 |page=1218 |doi=10.1126/science.348.6240.1218-o |first=Kristen L. |last=Mueller}}</ref> When articles are submitted to journals, each journal may or may not apply house style to the [[postprint]] version. [[Preprint]] versions contain whichever style the author happened to use. Even style regimens that use lowercase for the adjectives ''gram-positive'' and ''gram-negative'' still typically use capital for ''Gram stain''.
The term ''Gram staining'' is derived from the surname of [[Hans Christian Gram]]; the [[eponym]] (Gram) is therefore capitalized but not the common noun (stain) as is usual for scientific terms.<ref>{{cite book |url= https://books.google.com/books?id=btb1AwAAQBAJ&pg=PA105 |title=New Hart's Rules: The Oxford Style Guide |first=Anne |last=Waddingham |page=105 |publisher=Oxford University Press |date=28 August 2014 |isbn=978-0199570027 }}</ref> The initial letters of ''gram-positive'' and ''gram-negative'', which are [[Eponym#Orthographic conventions|eponymous adjectives]], can be either capital ''G'' or lowercase ''g'', depending on what [[style guide]] (if any) governs the document being written. Lowercase style is used by the US [[Centers for Disease Control and Prevention]] and other style regimens such as the [[AMA style]].<ref>{{cite web |url= http://wwwnc.cdc.gov/eid/pages/preferred-usage.htm |title=Preferred Usage |work=Emerging Infectious Diseases Style Guide |publisher=[[Centers for Disease Control and Prevention]]}}</ref> Dictionaries may use lowercase,<ref name="Dorlands">{{cite web |title=Dorland's Illustrated Medical Dictionary |edition=32nd |publisher=Elsevier |url= https://www.dorlands.com/dorlands/wsearch.jsp |access-date=5 June 2020}} Use search terms such as {{kbd|gram-negative}}.</ref><ref name="MWMD">{{Citation |title=gram–positive |work=Merriam-Webster |publisher=Encyclopædia Britannica, Inc. |url= http://www.merriam-webster.com/dictionary/gram%E2%80%93positive}}</ref> uppercase,<ref>{{cite web |url= http://www.collinsdictionary.com/dictionary/english/gram-positive |title=Gram-positive |work=CollinsDictionary.com |publisher=HarperCollins}}</ref><ref>{{cite web |url= https://www.lexico.com/definition/gram_stain |archive-url= https://web.archive.org/web/20200605094227/https://www.lexico.com/definition/gram_stain |url-status= dead |archive-date= June 5, 2020 |title=Gram stain |work=Lexico.com |publisher=Oxford University Press}}</ref><ref>{{cite web |url= http://www.medicinenet.com/script/main/art.asp?articlekey=9585 |title=Gram-positive |work=MedicineNet}}</ref><ref>{{cite web |url= http://www.businessdictionary.com/definition/Gram-negative-positive.html |title= Gram negative/positive |work= BusinessDictionary.com |access-date= 2016-10-20 |archive-date= 2016-10-20 |archive-url= https://web.archive.org/web/20161020173301/http://www.businessdictionary.com/definition/Gram-negative-positive.html |url-status= dead }}</ref> or both.<ref>{{cite web |url= https://ahdictionary.com/word/search.html?q=gram+positive&submit.x=0&submit.y=0 |title=gram-pos·i·tive or Gram-pos·i·tive |work=The American Heritage Dictionary |publisher=Houghton Mifflin}}</ref><ref name="dictionary">{{cite web |url= http://www.dictionary.com/browse/gram-positive |title=Gram-positive |work=Dictionary.com}}</ref> Uppercase ''Gram-positive'' or ''Gram-negative'' usage is also common in many [[scientific journal]] articles and publications.<ref name="dictionary"/><ref>{{cite journal |title=Through the wall: Extracellular vesicles in Gram-positive bacteria, mycobacteria and fungi |first1=Lisa |last1=Brown |first2=Julie M. |last2=Wolf |first3=Rafael |last3=Prados-Rosales |first4=Arturo |last4=Casadevall |journal=Nature Reviews Microbiology |volume=13 |issue=10 |pages=620–630 |date=2015 |doi=10.1038/nrmicro3480 |pmid=26324094 |pmc=4860279}}</ref><ref>{{cite journal |journal=Science |title=Detecting Gram-negative bacteria |date=12 June 2015 |volume=348 |issue=6240 |page=1218 |doi=10.1126/science.348.6240.1218-o |first=Kristen L. |last=Mueller}}</ref> When articles are submitted to journals, each journal may or may not apply house style to the [[postprint]] version. [[Preprint]] versions contain whichever style the author happened to use. Even style regimens that use lowercase for the adjectives ''gram-positive'' and ''gram-negative'' still typically use capital for ''Gram stain''.{{cn|date=May 2024}}


== See also ==
== See also ==

Latest revision as of 20:12, 29 November 2024

Micrograph of a gram-positive coccus and a gram-negative rod.
A Gram stain of mixed Staphylococcus aureus (S. aureus ATCC 25923, gram-positive cocci, in purple) and Escherichia coli (E. coli ATCC 11775, gram-negative bacilli, in red), the most common Gram stain reference bacteria

Gram stain (Gram staining or Gram's method), is a method of staining used to classify bacterial species into two large groups: gram-positive bacteria and gram-negative bacteria. It may also be used to diagnose a fungal infection.[1] The name comes from the Danish bacteriologist Hans Christian Gram, who developed the technique in 1884.[2]

Gram staining differentiates bacteria by the chemical and physical properties of their cell walls. Gram-positive cells have a thick layer of peptidoglycan in the cell wall that retains the primary stain, crystal violet. Gram-negative cells have a thinner peptidoglycan layer that allows the crystal violet to wash out on addition of ethanol. They are stained pink or red by the counterstain,[3] commonly safranin or fuchsine. Lugol's iodine solution is always added after addition of crystal violet to form a stable complex with crystal violet that strengthen the bonds of the stain with the cell wall.[4]

Gram staining is almost always the first step in the identification of a bacterial group. While Gram staining is a valuable diagnostic tool in both clinical and research settings, not all bacteria can be definitively classified by this technique. This gives rise to gram-variable and gram-indeterminate groups.

History

[edit]

The method is named after its inventor, the Danish scientist Hans Christian Gram (1853–1938), who developed the technique while working with Carl Friedländer in the morgue of the city hospital in Berlin in 1884. Gram devised his technique not for the purpose of distinguishing one type of bacterium from another but to make bacteria more visible in stained sections of lung tissue.[5] Gram noticed that some bacterial cells possessed noticeable resistance to decolorization. Based on these observations, Gram developed the initial gram staining procedure, initially making use of Ehrlich's aniline-gentian violet, Lugol's iodine, absolute alcohol for decolorization, and Bismarck brown for counterstain.[6] He published his method in 1884, and included in his short report the observation that the typhus bacillus did not retain the stain.[7] Gram did not initially make the distinction between Gram-negative and Gram-positive bacteria using his procedure.[6]

Uses

[edit]
Gram stain of Candida albicans from a vaginal swab. The small oval chlamydospores are 2–4 μm in diameter.

Gram staining is a bacteriological laboratory technique[8] used to differentiate bacterial species into two large groups (gram-positive and gram-negative) based on the physical properties of their cell walls.[9][page needed] Gram staining can also be used to diagnose a fungal infection.[1] Gram staining is not used to classify archaea, since these microorganisms yield widely varying responses that do not follow their phylogenetic groups.[10]

Some organisms are gram-variable (meaning they may stain either negative or positive); some are not stained with either dye used in the Gram technique and are not seen.[citation needed]

Medical

[edit]
See also: Pathogenic bacteria

Gram stains are performed on body fluid or biopsy when infection is suspected. Gram stains yield results much more quickly than culturing, and are especially important when infection would make an important difference in the patient's treatment and prognosis; examples are cerebrospinal fluid for meningitis and synovial fluid for septic arthritis.[11][12]

Staining mechanism

[edit]
Purple-stained gram-positive (left) and pink-stained gram-negative (right)

Gram-positive bacteria have a thick mesh-like cell wall made of peptidoglycan (50–90% of cell envelope), and as a result are stained purple by crystal violet, whereas gram-negative bacteria have a thinner layer (10% of cell envelope), so do not retain the purple stain and are counter-stained pink by safranin. There are four basic steps of the Gram stain:

  1. Applying a primary stain (crystal violet) to a heat-fixed smear of a bacterial culture. Heat fixation kills some bacteria but is mostly used to affix the bacteria to the slide so that they do not rinse out during the staining procedure.
  2. The addition of iodine, which binds to crystal violet and traps it in the cell
  3. Rapid decolorization with ethanol or acetone
  4. Counterstaining with safranin.[13] Carbol fuchsin is sometimes substituted for safranin since it more intensely stains anaerobic bacteria, but it is less commonly used as a counterstain.[14]
Summary of Gram stain
Application of Reagent Cell color
Gram-positive Gram-negative
Primary dye crystal violet purple purple
Trapper iodine purple purple
Decolorizer alcohol/acetone purple colorless
Counter stain safranin/carbol fuchsin purple pink or red

Crystal violet (CV) dissociates in aqueous solutions into CV+
 and chloride (Cl−
) ions. These ions penetrate the cell wall of both gram-positive and gram-negative cells. The CV+
 ion interacts with negatively charged components of bacterial cells and stains the cells purple.[15]

Iodide (I−
 or I−
3) interacts with CV+
 and forms large complexes of crystal violet and iodine (CV–I) within the inner and outer layers of the cell. Iodine is often referred to as a mordant, but is a trapping agent that prevents the removal of the CV–I complex and, therefore, colors the cell.[16]

When a decolorizer such as alcohol or acetone is added, it interacts with the lipids of the cell membrane.[17] A gram-negative cell loses its outer lipopolysaccharide membrane, and the inner peptidoglycan layer is left exposed. The CV–I complexes are washed from the gram-negative cell along with the outer membrane.[18] In contrast, a gram-positive cell becomes dehydrated from an ethanol treatment. The large CV–I complexes become trapped within the gram-positive cell due to the multilayered nature of its peptidoglycan.[18] The decolorization step is critical and must be timed correctly; the crystal violet stain is removed from both gram-positive and negative cells if the decolorizing agent is left on too long (a matter of seconds).[19]

After decolorization, the gram-positive cell remains purple and the gram-negative cell loses its purple color.[19] Counterstain, which is usually positively charged safranin or basic fuchsine, is applied last to give decolorized gram-negative bacteria a pink or red color.[3][20] Both gram-positive bacteria and gram-negative bacteria pick up the counterstain. The counterstain, however, is unseen on gram-positive bacteria because of the darker crystal violet stain.[citation needed]

Examples

[edit]

Gram-positive bacteria

[edit]
Gram-stain of gram-positive streptococci surrounded by pus cells
Main article: 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.[9][21] 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.[9][page needed][21]

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.[22]

Historically, the gram-positive forms made up the phylum Firmicutes, a name now used for the largest group. It includes many well-known genera such as Lactobacillus, Bacillus, Listeria, Staphylococcus, Streptococcus, Enterococcus, and Clostridium.[23] 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.[24]

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-fast bacteria, and can only be differentiated from other gram-positive bacteria by special staining procedures.[25]

Gram-negative bacteria

[edit]
Main article: Gram-negative bacteria
Gram negative Neisseria gonorrhoeae and pus cells

Gram-negative bacteria generally possess a thin layer of peptidoglycan between two membranes (diderm).[26] 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.[27] 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.[22]

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.[22]

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).[9][page needed][21]

Gram-variable and gram-indeterminate bacteria

[edit]

Some bacteria, after staining with the Gram stain, yield a gram-variable pattern: a mix of pink and purple cells are seen.[18][28] 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.[28] In addition, in all bacteria stained using the Gram stain, the age of the culture may influence the results of the stain.[28]

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.[29][30] Bacteria of the genus Mycoplasma lack a cell wall around their cell membranes,[11] which means they do not stain by Gram's method and are resistant to the antibiotics that target cell wall synthesis.[31][32]

Orthographic note

[edit]

The term Gram staining is derived from the surname of Hans Christian Gram; the eponym (Gram) is therefore capitalized but not the common noun (stain) as is usual for scientific terms.[33] The initial letters of gram-positive and gram-negative, which are eponymous adjectives, can be either capital G or lowercase g, depending on what style guide (if any) governs the document being written. Lowercase style is used by the US Centers for Disease Control and Prevention and other style regimens such as the AMA style.[34] Dictionaries may use lowercase,[35][36] uppercase,[37][38][39][40] or both.[41][42] Uppercase Gram-positive or Gram-negative usage is also common in many scientific journal articles and publications.[42][43][44] When articles are submitted to journals, each journal may or may not apply house style to the postprint version. Preprint versions contain whichever style the author happened to use. Even style regimens that use lowercase for the adjectives gram-positive and gram-negative still typically use capital for Gram stain.[citation needed]

See also

[edit]

References

[edit]
  1. ^ a b "Gram Stain: MedlinePlus Medical Test". medlineplus.gov.
  2. ^ Colco, R. (2005). "Gram Staining". Current Protocols in Microbiology. Appendix 3 (1): Appendix 3C. doi:10.1002/9780471729259.mca03cs00. ISBN 978-0471729259. PMID 18770544. S2CID 32452815.
  3. ^ a b Beveridge, T. J.; Davies, J. A. (November 1983). "Cellular responses of Bacillus subtilis and Escherichia coli to the Gram stain". Journal of Bacteriology. 156 (2): 846–58. doi:10.1128/JB.156.2.846-858.1983. PMC 217903. PMID 6195148.
  4. ^ Libenson, L.; McIlroy, A. P. (1955-07-01). "On the Mechanism of the Gram Stain". Journal of Infectious Diseases. 97 (1): 22–26. doi:10.1093/infdis/97.1.22. ISSN 0022-1899.
  5. ^ Austrian, R. (1960). "The Gram stain and the etiology of lobar pneumonia, an historical note". Bacteriological Reviews. 24 (3): 261–265. doi:10.1128/MMBR.24.3.261-265.1960. PMC 441053. PMID 13685217.
  6. ^ a b Bartholomew, James W.; Mittwer, Tod (March 1952). "THE GRAM STAIN". Bacteriological Reviews. 16 (1): 1–29. doi:10.1128/br.16.1.1-29.1952. ISSN 0005-3678. PMC 180726. PMID 14925025.
  7. ^ Gram, Hans Christian (1884). "Über die isolierte Färbung der Schizomyceten in Schnitt- und Trockenpräparaten". Fortschritte der Medizin (in German). 2: 185–189..
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