Agar plate

Anagar plateis aPetri dishthat contains agrowth mediumsolidified withagar, used toculturemicroorganisms. Sometimes selective compounds are added to influence growth, such asantibiotics.
Individual microorganisms placed on the plate will grow into individualcolonies, each aclonegenetically identical to the individual ancestor organism (except for the low, unavoidable rate ofmutation). Thus, the plate can be used either to estimate the concentration of organisms in aliquid cultureor a suitable dilution of that culture using acolony 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,sterileloop 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 thespot 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 platingis 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 workin a sterile mannerto prevent contamination on the agar plates.Plating is thus often done in alaminar flow cabinetor on the working bench next to abunsen burner.
In 1881,Fanny Hesse, who was working as a technician for her husbandWalther Hessein the laboratory ofRobert Koch, suggested agar as an effective setting agent, since it had been commonplace in jam making for some time.
Like othergrowth 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 asyeast extract, where the precise composition is unknown.
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.This may take the form of a nutritional requirement, for instance providing a particular compound such aslactoseas the only source ofcarbonand thereby selecting only organisms which canmetabolizethat compound, or by including a particular antibiotic or other substance to select only organisms which areresistantto 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 anenzymeon some compound added to the medium.
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:
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 isolatefastidiousorganisms and detecthemolyticactivity. β-Hemolytic activity will show lysis and complete digestion of red blood cell contents surrounding a colony. Examples includeStreptococcus 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 beStreptococcus viridans. γ-Hemolysis (or nonhemolytic) is the term referring to a lack of hemolytic activity.BAPs also containmeat extractoryeast extract,tryptone,sodium chloride, and agar.
Chocolate agaris a type of blood agar plate in which the blood cells have beenlysedby heating the cells to 80 °C. It is used for growing fastidious respiratory bacteria, such asHaemophilus influenzae. Chocolate agar is named for its color, and nochocolateis contained in the plate.
Thayer–Martin agaris a chocolate agar designed to isolateNeisseria gonorrhoeaeandNeisseria meningitidis.
Thiosulfate–citrate–bile salts–sucrose agarenhances growth ofVibriospp., includingVibrio cholerae.
Different specific types of agar: