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{{short description|Petri dish with agar used to culture microbes}}
{{Infobox laboratory equipment
|name = Agar plate
|alt = <!-- See Wikipedia:Alternative text for images -->
|image = File:Ecoli colonies.png
|caption =An agar culture of ''[[Escherichia coli|E. coli]]'' colonies
|acronym =
|other_names =
|uses = [[Microbiological culture]]<br>[[Microbial art|Art]]
|inventor =
|manufacturer =
|model =
|related = [[Petri dish]]<br />[[Growth medium]]
}}
[[File:Contamination on agar plate.jpg|thumb|Contamination on an agar plate]]
An '''agar plate''' is a [[Petri dish]] that contains a [[growth medium]] solidified with [[agar]], used to [[Microbiological culture|culture]] [[microorganisms]]. Sometimes selective compounds are added to influence growth, such as [[antibiotic]]s.<ref name="Brock">{{cite book|title=Brock Biology of Microorganisms|publisher=Prentice Hall|year=2005|isbn=0-13-144329-1|veditors=Madigan M, Martinko J|edition=11th}}</ref>[[File:96pinner.jpg|thumb|96 pinner used to perform spot assays with yeast, fungal or bacterial cells]]

Individual microorganisms placed on the plate will grow into individual [[colony (biology)|colonies]], each a [[cloning|clone]] genetically identical to the individual ancestor organism (except for the low, unavoidable rate of [[mutation]]). Thus, the plate can be used either to estimate the concentration of organisms in a [[Microbiological culture|liquid culture]] or a suitable dilution of that culture using a [[colony counter]], or to generate genetically pure cultures from a mixed culture of genetically different organisms.

Several methods are available to plate out cells. One technique is known as "[[Streaking (microbiology)|streaking]]". In this technique, a drop of the culture on the end of a thin, [[sterilization (microbiology)|sterile]] loop of wire, sometimes known as an inoculator, is streaked across the surface of the agar leaving organisms behind, a higher number at the beginning of the streak and a lower number at the end. At some point during a successful "streak", the number of organisms deposited will be such that distinct individual colonies will grow in that area which may be removed for further culturing, using another sterile loop.

Another way of plating organisms, next to streaking, on agar plates is the [[spot analysis]]. This type of analysis is often used to check the viability of cells and is performed with pinners (often also called froggers). A third technique is using sterile glass beads to plate out cells. In this technique, cells are grown in a liquid culture, in which a small volume is pipetted on the agar plate and then spread out with the beads. [[Replica plating]] is another technique used to plate out cells on agar plates. These four techniques are the most common, but others are also possible. It is crucial to work [[Asepsis|in a sterile manner]] to prevent contamination on the agar plates.<ref name="Brock" /> Plating is thus often done in a [[laminar flow cabinet]] or on the working bench next to a [[bunsen burner]].<ref>{{cite journal|url=https://www.jove.com/video/3064/aseptic-laboratory-techniques-plating-methods|title=Aseptic Laboratory Techniques: Plating Methods|first=Erin R.|last=Sanders|date=11 May 2012|journal=Journal of Visualized Experiments|issue=63|access-date=3 May 2018|doi=10.3791/3064|url-status=live|archive-url=https://web.archive.org/web/20171114202834/https://www.jove.com/video/3064/aseptic-laboratory-techniques-plating-methods|archive-date=14 November 2017|pmc=4846335|pmid=22617405|article-number=e3064}}</ref>

== History ==
In 1881, [[Fanny Hesse]], who was working as a technician for her husband [[Walther Hesse]] in the laboratory of [[Robert Koch]], suggested agar as an effective setting agent, since it had been commonplace in jam making for some time.<ref>{{cite magazine |url=http://www.labnews.co.uk/feature_archive.php/808/5/history-of-the-agar-plate |title=History of the agar plate |magazine=[[Laboratory News]] |access-date=2010-02-22 |archive-url=https://web.archive.org/web/20100211205151/http://www.labnews.co.uk/feature_archive.php/808/5/history-of-the-agar-plate |archive-date=11 February 2010 |url-status=dead }}</ref>

== Types ==
[[Image:Quintote colony counter.jpg|thumb|upright|An agar plate being viewed in an electronic [[colony counter]]]]
[[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. Different agar plates are used for different specimen sources, as seen in the upper left quadrant.]]
Like other [[growth medium|growth media]], the formulations of agar used in plates may be classified as either "defined" or "undefined"; a defined medium is synthesized from individual chemicals required by the organism so the exact molecular composition is known, whereas an undefined medium is made from natural products such as [[yeast extract]], where the precise composition is unknown.<ref name=Baron>{{cite book | editor = Baron S | display-editors = etal | title = Baron's Medical Microbiology | edition = 4th | publisher = [[University of Texas]] Medical Branch | year = 1996 | id = [https://www.ncbi.nlm.nih.gov/books?cmd=Search&doptcmdl=GenBookHL&term=Culture%20AND%20mmed%5Bbook%5D%20AND%20147423%5Buid%5D&rid=mmed.section.5451 (via NCBI Bookshelf)] | isbn = 0-9631172-1-1 }}</ref>

Agar plates may be formulated as either permissive, with the intent of allowing the growth of whatever organisms are present, or restrictive or selective, with the intent of only allowing the growth of a particular subset of those organisms.<ref name=Sherris>{{cite book | editor = Ryan KJ | editor2 = Ray CG | title = Sherris Medical Microbiology | edition = 4th | publisher = McGraw Hill | year = 2004 | isbn = 0-8385-8529-9 }}</ref> This may take the form of a nutritional requirement, for instance providing a particular compound such as [[lactose]] as the only source of [[carbon]] and thereby selecting only organisms which can [[metabolism|metabolize]] that compound, or by including a particular antibiotic or other substance to select only organisms which are [[antibiotic resistance|resistant]] to that substance. This correlates to some degree with defined and undefined media; undefined media, made from natural products and containing an unknown combination of very many organic molecules, is typically more permissive in terms of supplying the needs of a wider variety of organisms. In contrast, defined media can be precisely tailored to select organisms with specific properties.

Agar plates may also be indicator plates, in which the organisms are not selected based on growth, but are instead distinguished by a color change in some colonies, typically caused by the action of an [[enzyme]] on some compound added to the medium.<ref>{{cite web|title=Indicator Plates|url=http://rothlab.ucdavis.edu/protocols/indicator-plates.shtml|access-date=12 July 2018}}</ref>

The plates are incubated for 12 hours up to several days, depending on the test that is performed.

Commonly used types of agar plates include:
[[Image:Agarplate redbloodcells edit.jpg|250px|thumb|right|[[Red blood cells]] on an agar plate are used to diagnose [[infection]]. On the left is a positive ''[[Staphylococcus]]'' infection, on the right a positive ''[[Streptococcus]]'' culture.]]

=== Blood agar ===
[[Image:Streptococcal hemolysis.jpg|thumb|right|350px|Hemolyses of [[Streptococcus|''Streptococcus'' spp.]] (left) α-hemolysis ([[Streptococcus mitis|''S. mitis'']]); (middle) β-hemolysis (''[[S. pyogenes]]''); (right) γ-hemolysis (= nonhemolytic, ''[[S. salivarius]]'')]]

====Blood agar plate====
Blood agar plates (BAPs) contain mammalian blood (usually sheep or horse), typically at a 5–10% concentration. BAPs are enriched, and differential media is used to isolate [[Fastidious organism|fastidious]] organisms and detect [[Hemolysis (microbiology)|hemolytic]] activity. β-Hemolytic activity will show lysis and complete digestion of red blood cell contents surrounding a colony. Examples include ''Streptococcus haemolyticus''. α-Hemolysis will only cause partial lysis of the red blood cells (the cell membrane is left intact) and appear green or brown due to the conversion of hemoglobin to methemoglobin. An example of this would be ''Streptococcus viridans''. γ-Hemolysis (or nonhemolytic) is the term referring to a lack of hemolytic activity.<ref>{{cite web|url=http://www.microbelibrary.org/component/resource/laboratory-test/2885-blood-agar-plates-and-hemolysis-protocols |title= Blood Agar Plates and Hemolysis Protocols |access-date=2014-10-28 |url-status=dead |archive-url=https://web.archive.org/web/20120202204441/http://www.microbelibrary.org/component/resource/laboratory-test/2885-blood-agar-plates-and-hemolysis-protocols |archive-date=2012-02-02 }}</ref> BAPs also contain [[meat extract]] or [[yeast extract]], [[tryptone]], [[sodium chloride]], and agar.<ref>{{Citation |title=Blood Agar- Composition, Preparation, Uses and Pictures |url=https://microbiologyinfo.com/blood-agar-composition-preparation-uses-and-pictures/|work=Microbiology Info.com}}</ref>

====Chocolate agar====
[[Chocolate agar]] <!-- (CHOC) --> is a type of blood agar plate in which the blood cells have been [[Lysis|lysed]] by heating the cells to 80&nbsp;°C. It is used for growing fastidious respiratory bacteria, such as ''[[Haemophilus influenzae]]''. Chocolate agar is named for its color, and no [[chocolate]] is contained in the plate.

====Thayer–Martin agar====
[[Thayer–Martin agar]] <!-- (TM) --> is a chocolate agar designed to isolate ''[[Neisseria gonorrhoeae]]'' and ''[[Neisseria meningitidis]]''.

==== Thiosulfate–citrate–bile salts–sucrose agar ====
[[Thiosulfate–citrate–bile salts–sucrose agar]] <!-- (TCBS) --> enhances growth of [[Vibrio|''Vibrio'' spp.]], including ''[[Vibrio cholerae]]''.<ref name=Microbiology>{{cite book |author =Fisher, Bruce |author2 =Harvey, Richard P. |author3 =Champe, Pamela C. |title=Lippincott's Illustrated Reviews: Microbiology (Lippincott's Illustrated Reviews Series) |publisher=Lippincott Williams & Wilkins |location=Hagerstwon, MD |year= 2007|isbn=978-0-7817-8215-9 }}</ref>

=== General bacterial media ===
[[Image:Agarplates.jpg|right|thumb|Four types of agar plate demonstrating differential growth depending on bacterial [[metabolism]]]]
* [[Bile esculin agar]]  is used for the isolation of ''[[Enterococcus]]'' and [[Streptococcus|group D ''Streptococcus'' species]].
* [[CLED agar]] – [[cysteine]], [[lactose]], electrolyte-deficient agar <!-- (CLED) --> is used to isolate and differentiate urinary tract bacteria, since it inhibits ''[[Proteus (bacterium)|Proteus]]'' species swarming and can distinguish between lactose fermenters and nonfermenters.
* [[Granada medium]] is used to isolate and differentiate group B ''Streptococcus'', ''Streptococcus agalactiae'' from clinical samples. It grows in Granada medium as red colonies, and most of the accompanying bacteria are inhibited.
* [[Hektoen enteric agar]] is designed to isolate and recover fecal bacteria of the family [[Enterobacteriaceae]]. It is particularly useful in isolating ''[[Salmonella]]'' and ''[[Shigella]]''.
* [[Lysogeny broth]] is used to culture ''[[Escherichia coli]]''.<ref>Miller, J. H. (1972). Experiments in molecular genetics. Cold Spring Harbor Laboratory, Cold Spring Harbor, New York.</ref>
* [[MacConkey agar]] is a selective and differential medium used to differentiate between [[gram-negative]] bacteria while inhibiting the growth of [[gram-positive]] bacteria. Adding bile salts and [[crystal violet]] to the agar inhibits the growth of most gram-positive bacteria, making MacConkey agar selective. Lactose and [[neutral red]] are added to differentiate the lactose fermenters, which form pink colonies, from lactose nonfermenters that form clear colonies. An alternative medium, [[eosin methylene blue]] serves a similar purpose.<ref>{{Citation |last1=Jung |first1=Benjamin |title=MacConkey Medium |date=2022 |url=https://www.ncbi.nlm.nih.gov/books/NBK557394/ |work=StatPearls |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=32491326 |access-date=2022-12-12 |last2=Hoilat |first2=Gilles J.}}</ref>
* [[Mannitol salt agar]] is also a selective and differential medium. The [[mannitol]] indicates organisms that ferment mannitol: mannitol fermentation produces [[lactic acid]], lowering the pH and turning the plate yellow. The salt is to select for [[halophile]]s; organisms that cannot withstand a high salt content are unable to grow well.
* [[Mueller–Hinton agar]] contains beef infusion, peptone, and [[starch]], and is used primarily for antibiotic susceptibility testing. It can be in a form of [[Agar plate#Blood agar|blood agar]].
* [[Nutrient agar]] is usually used for growth of nonfastidious organisms and observation of pigment production. It is safe to use in school science laboratories because it does not selectively grow [[pathogen]]ic bacteria.
* [[Önöz agar]] allows more rapid bacteriological diagnosis, as ''Salmonella'' and ''Shigella'' colonies can be clearly and reliably differentiated from other Enterobacteriaceae. The yields of ''Salmonella'' from stool samples obtained, when using this medium, are higher than those obtained with LEIFSON agar or ''Salmonella–Shigella'' agar.
* [[Phenylethyl alcohol agar]] selects for ''[[Staphylococcus]]'' species while inhibiting Gram-negative bacilli (e.g., ''Escherichia coli'', ''Shigella'', ''Proteus'', etc.).
* [[R2A agar]], a nonspecific medium,  imitates water, so is used for water analysis.
* [[Trypticase soy agar|Tryptic (trypticase) soy agar]] (TSA) is a general-purpose medium produced by enzymatic digestion of [[soybean]] meal and [[casein]]. It is frequently the base medium of other agar types; for example, blood agar plates are made by enriching TSA plates with blood. TSA plates support growth of many semifastidious bacteria, including some species of ''[[Brucella]]'', ''[[Corynebacterium]]'', ''[[Listeria]]'', ''[[Neisseria]]'', and ''[[Vibrio]]''.
* [[XLD agar|Xylose]]-[[lysine]]-[[deoxycholate]] agar is used for the culture of [[human feces|stool]] samples and contains two indicators. It is formulated to inhibit Gram-positive bacteria, while the growth of Gram-negative [[bacilli]] is encouraged. The colonies of lactose fermenters appear yellow. It is also used to culture possible ''Salmonella'' that may be present in a food sample. Most ''Salmonella'' colonies produce a black centre on it.
* [[Cetrimide agar]] is used for the selective isolation of the Gram-negative bacterium ''[[Pseudomonas aeruginosa]]''.
* Tinsdale agar contains [[potassium tellurite]], which can isolate ''[[Corynebacterium diphteriae]]''.<ref name=Microbiology/>

=== Fungal media ===
* [[Sabouraud agar]] is used to culture [[fungi]] and has a low [[pH]] that inhibits the growth of most bacteria; it also contains the antibiotic [[gentamicin]] to specifically inhibit the growth of Gram-negative bacteria.
* [[Hay infusion agar]] is specific for the culturing of [[slime mould]]s (which are not fungi).
* [[Potato dextrose agar]]  is used to culture certain types of fungi.
* Malt extract agar has a high content of peptone and is acidic. It is essentially used in the isolation of fungal microorganisms.
* [[MMN medium|MMN (Modified Melin-Norkrans) medium]] and [[BAF medium]] are used for [[ectomycorrhiza]]l fungi.
* ''Chromogenic agars'' can distinguish some major types of fungal infection.
<gallery mode=packed heights=160>
Image:Trichophyton rubrum var. rodhaini PHIL 4248 lores.jpg|Bottom view of a Sabouraud agar plate with a colony of ''Trichophyton rubrum'' var. ''rodhaini''
File:CHROMAgar with N glabratus, P kudriavzevii, Candida albicans and Candida tropicalis, annotated.jpg|CHROMAgar (a ''chromogenic agar'') with its distinctive presentation of some major fungal pathogens.
Image:Ascomycetes.jpg|[[Fungi]] ([[ascomycetes]]) growing in [[axenic culture]]s, each of which is a culture of one selected organism and is free of all other organisms, enabling study of the cultured organism in isolation
File:Aspergillus niger 2.jpg|''[[Aspergillus niger]]'' growing on potato dextrose agar
</gallery>

=== Moss media ===
<!--
growing [[axenic]]ally on an agar plate ([[Petri dish]], 9&nbsp;cm diameter)]] -->
* Knop agar is used to [[axenic]]ally culture [[protonema]] and whole [[moss]] plants, such as those of ''[[Physcomitrella patens]]'', a [[model organism]].<ref>{{cite journal | last1 = Reski | first1 = Ralf | author-link = Ralf Reski | last2 = Abel | first2 = Wolfgang O. | year = 1985 | title = Induction of budding on chloronemata and caulonemata of the moss, Physcomitrella patens, using isopentenyladenine | journal = Planta | volume = 165 | issue = 3| pages = 354–358 | doi=10.1007/bf00392232| pmid = 24241140 | title-link = cytokinin | bibcode = 1985Plant.165..354R | s2cid = 11363119 }}</ref>

=== Yeast media ===
[[File:Candida albicans growing as yeast cells and filamentous (hypha) cells.jpg|thumb|the yeast ''[[Candida albicans]]'' growing both as yeast cells and [[Hypha|filamentous cells]] on YPD agar]]
* [[YEPD]] media is often used as a general growth media for yeasts like ''[[Saccharomyces cerevisiae]]'' and ''[[Candida albicans]]''
* Sporulation medium is medium used when spores have to be formed. It can also be used when working with fungi or bacteria depending on whether or not the strain is capable of forming spores.

=== Mega Plate ===
* A 2' x 4' petri plate filled with 14L (liters) of seaweed derived agar medium created by Harvard scientists that was used to see how ''[[E. coli]]'' evolved to be resistant to antibiotics. The mega plate also helped study more unique concepts of microbiology such as parallel evolution, mutation selection, colonial interference etc.<ref>{{Cite web|date=2016-09-08|title=A cinematic approach to drug resistance|url=https://news.harvard.edu/gazette/story/2016/09/a-cinematic-approach-to-drug-resistance/|access-date=2021-04-08|website=Harvard Gazette|language=en-US}}</ref>

== See also ==
* [[Microbial art]]
* [[Viral plaque]]

Different specific types of agar:
* [[Casein nutrient agar]]
* [[MRS agar]]
* [[New York City Agar]]

== References ==
{{reflist}}

== External links ==
{{Wikibooks|School Science|Agar plate}}
{{Commons category}}
* [http://www.tgw1916.net/movies.html Video of sheep blood agar preparation]
* [http://www.sciencebuddies.org/mentoring/project_ideas/MicroBio_Agar.shtml Science Buddies – All About Agar]
* [https://www.jove.com/video/3064/aseptic-laboratory-techniques-plating-methods Video on aseptic plating methods]
* [https://www.youtube.com/watch?v=ykSuO3gU4Ck Video on how to use glass beads to plate out samples]

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