Agar plate

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Agar plate
UsesMicrobiological culture
Art
Related itemsPetri dish
Growth medium
Contamination on an agar plate

An agar plate is a

microorganisms. Sometimes selective compounds are added to influence growth, such as antibiotics.[1]

96 pinner used to perform spot assays with yeast, fungal or bacterial cells

Individual microorganisms placed on the plate will grow into individual

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". In this technique, a drop of the culture on the end of a thin, 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 performed with pinners (often also called froggers). A third used technique is the use of sterile glass beads to plate out cells. In this technique cells are grown in a liquid culture of which a small volume is pipetted on the agar plate and then spread out with the beads. Replica plating is another technique in order to plate out cells on agar plates. These four techniques are the most common, but others are also possible. It is crucial to work in a sterile manner in order to prevent contamination on the agar plates.[1] Plating is thus often done in a laminar flow cabinet or on the working bench next to a bunsen burner.[2]

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.[3]

Types

colony counter
An agar culture of E. coli colonies
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 upper left quadrant.

Like other 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.[4]

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 growth a particular subset of those organisms.

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, while 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 on the basis of 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.[6]

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:

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

S. salivarius
)

Blood agar plate

Blood agar plates (BAPs) contain mammalian blood (usually sheep or horse), typically at a concentration of 5–10%. BAPs are enriched, differential media used to isolate fastidious organisms and detect 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 will 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.[7] BAPs also contain meat extract or yeast extract, tryptone, sodium chloride, and agar.[8]

Chocolate agar

Chocolate agar is a type of blood agar plate in which the blood cells have been lysed by heating the cells to 80 °C. It is used for growing fastidious respiratory bacteria, such as Haemophilus influenzae. Chocolate agar is named for its color, and no chocolate is actually contained in the plate.

Horse blood agar

Horse blood agar is a type of blood-enriched microbiological culture media. As it is enriched, it allows the growth of certain fastidious bacteria, and allows indication of haemolytic activity in these bacterial cultures.

Thayer–Martin agar

Thayer–Martin agar is a chocolate agar designed to isolate Neisseria gonorrhoeae and Neisseria meningitidis.

Thiosulfate–citrate–bile salts–sucrose agar

Thiosulfate–citrate–bile salts–sucrose agar enhances growth of Vibrio spp., including Vibrio cholerae.[9]

General bacterial media

Four types of agar plate demonstrating differential growth depending on bacterial metabolism

Fungal media

  • Bottom view of a Sabouraud agar plate with a colony of Trichophyton rubrum var. rodhaini
    Bottom view of a Sabouraud agar plate with a colony of Trichophyton rubrum var. rodhaini
  • CHROMAgar (a chromogenic agar) with its distinctive presentation of some major fungal pathogens.
    CHROMAgar (a chromogenic agar) with its distinctive presentation of some major fungal pathogens.
  • Fungi (ascomycetes) growing in axenic cultures, 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
    axenic cultures
    , 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
  • Aspergillus niger growing on potato dextrose agar
    Aspergillus niger growing on potato dextrose agar

Moss media

Yeast media

  • 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.[13]
the yeast Candida albicans growing both as yeast cells and filamentous cells on YPD agar

See also

Different specific types of agar:

References

  1. ^ .
  2. from the original on 14 November 2017. Retrieved 3 May 2018.
  3. ^ "History of the agar plate". Laboratory News. Archived from the original on 11 February 2010. Retrieved 2010-02-22.
  4. .
  5. .
  6. ^ "Indicator Plates". Retrieved 12 July 2018.
  7. ^ "Blood Agar Plates and Hemolysis Protocols". Archived from the original on 2012-02-02. Retrieved 2014-10-28.
  8. ^ "Blood Agar- Composition, Preparation, Uses and Pictures", Microbiology Info.com
  9. ^ .
  10. ^ Miller, J. H. (1972). Experiments in molecular genetics. Cold Spring Harbor Laboratory, Cold Spring Harbor, New York.
  11. PMID 32491326
    , retrieved 2022-12-12
  12. .
  13. ^ "A cinematic approach to drug resistance". Harvard Gazette. 2016-09-08. Retrieved 2021-04-08.

External links