Agar plate

# Overview
An agar plate is a sterile Petri dish that contains a growth medium (typically agar plus nutrients) used to culture microorganisms. Selective growth compounds may also be added to the media, such as antibiotics.
Individual microorganisms placed on the plate will grow into individual colonies, each a 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 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, using a technique known as streaking. In this technique, a drop of the culture on the end of a thin, sterile loop of wire 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.
For information on agar plate preparation, please visit the Wikibooks Agar Plate page.

# Types of agar plates
Like other growth medium, the formulations of agar used in plates may be classified as either defined or undefined; defined medium being synthesized from the individual chemicals as required by the organism so that the exact molecular composition is known, while undefined medium is made up of 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 selecting for growth 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 for energy and material and thereby selecting only organisms which can metabolize that compound, or by including a particular antibiotic or other substance in order 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, while defined media can be precisely tailored to select organisms with very specific properties.
Agar plates may also be indicator plates, where the organisms are not selected on the basis of growth, but a compound in the agar is altered by an enzyme or similar in some colonies so as to change color and identify them from those lacking the enzyme.
Some commonly used agar plate types are:

## Blood agar types
- Blood agar plate (BAP)
- Chocolate agar (CHOC)
- Thayer-Martin agar (TM)

## General bacterial media
- Bile Esculin Agar (BEA)
- Cysteine Lactose Electrolyte Deficient agar (CLED)
- Hektoen Enteric (HE)
- MacConkey agar (MAC)
- Mannitol Salt Agar (MSA)
- Mueller Hinton agar
- Nutrient agar
- Onoz agar
- Phenylethyl Alcohol Agar (PEA)
- Tryptic (Trypticase) Soy Agar (TSA)
- Xylose-Lysine-Deoxycholate agar (XLD)

## Fungal media
- Sabouraud agar
- Hay Infusion agar
- Potato dextrose agar