Agar plate


An agar plate is a Petri dish that contains a growth medium solidified with agar, used to culture microorganisms. Sometimes selective compounds are added to influence growth, such as antibiotics.
Individual microorganisms placed on the plate will grow into individual colonies, each a clone genetically identical to the individual ancestor organism. Thus, the plate can be used either to estimate the concentration of organisms in a 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". 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 is performed with pinners. 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 in a sterile manner to prevent contamination on the agar plates. Plating is thus often done in a laminar flow cabinet or on the working bench next to a bunsen burner.

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.

Types

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.
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 as lactose as the only source of carbon and thereby selecting only organisms which can metabolize that compound, or by including a particular antibiotic or other substance to select only organisms which are 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.
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

Blood agar plate

Blood agar plates contain mammalian blood, typically at a 5–10% concentration. BAPs are enriched, and differential media is 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 and appear green or brown due to the conversion of hemoglobin to methemoglobin. An example of this would be Streptococcus viridans. γ-Hemolysis is the term referring to a lack of hemolytic activity. BAPs also contain meat extract or yeast extract, tryptone, sodium chloride, and 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 contained in the plate.

Thayer–Martin agar

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

Thiosulfate–citrate–bile salts–sucrose agar

enhances growth of Vibrio spp., including Vibrio cholerae.

General bacterial media

Fungal media

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 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.