Petri dishes are usually cylindrical, mostly with diameters ranging from 30 to 200 millimetres (1.2 to 7.9 in),and a height to diameter ratio ranging from 1:10 to 1:4. Squarish versions are also available.
Petri dishes were traditionally reusable and made of glass; often of heat-resistant borosilicate glass for proper sterilization at 120–160 °C.
Since the 1960s, plastic dishes, usually disposable, are also common
The dishes are often covered with a shallow transparent lid, resembling a slightly wider version of the dish itself. The lids of glass dishes are usually loose-fitting. Plastic dishes may have close-fitting covers that delay the drying of the contents. Alternatively, some glass or plastic versions may have small holes around the rim, or ribs on the underside of the cover, to allow for air flow over the culture and prevent water condensation.
Some Petri dishes, especially plastic ones, usually feature rings and/or slots on their lids and bases so that they are less prone to sliding off one another when stacked or sticking to a smooth surface by suction.
Small dishes may have a protruding base that can be secured on a microscope stage for direct examination
Some versions may have grids printed on the bottom to help in measuring the density of cultures.
A microplate is a single container with an array of flat-bottomed cavities, each being essentially a small Petri dish. It makes it possible to inoculate and grow dozens or hundreds of independent cultures of dozens of samples at the same time. Besides being much cheaper and convenient than separate dishes, the microplate is also more amenable to automated handling and inspection.
Petri dishes are widely used in biology to cultivate microorganisms such as bacteria, yeasts, and molds. It is most suited for organisms that thrive on a solid or semisolid surface.
The culture medium is often an agar plate, a layer a few mm thick of agar or agarose gel containing whatever nutrients the organism requires (such as blood, salts, carbohydrates, amino acids) and other desired ingredients (such as dyes, indicators, and medicinal drugs). The agar and other ingredients are dissolved in warm water and poured into the dish and left to cool down. Once the medium solidifies, a sample of the organism is inoculated (“plated”).
The dishes are then left undisturbed for hours or days while the organism grows, possibly in an incubator. They are usually covered, or placed upside-down, to lessen the risk of contamination from airborne spores.
Virus or phage cultures require that a population of bacteria be grown in the dish first, which then becomes the culture medium for the viral inoculum.
While Petri dishes are widespread in microbiological research, smaller dishes tend to be used for large-scale studies in which growing cells in Petri dishes can be relatively expensive and labor-intensive.
Contamination detection and mapping
Petri dishes can be used to visualize the location of contamination on surfaces, such as kitchen counters and utensils, clothing, food preparation equipment, or animal and human skin.
For this application, the Petri dishes may be filled so that the culture medium protrudes slightly above the edges of the dish to make it easier to take samples on hard objects. Shallow Petri dishes prepared in this way are called Replicate Organism Detection And Counting (RODAC) plates and are available commercially.
Petri dishes are also used for cell cultivation of isolated cells from eukaryotic organisms, such as in immunodiffusion studies, on solid agar or in a liquid medium.
Botany and agriculture
Petri dishes may be used to observe the early stages of plant germination, and to grow plants asexually from isolated cells.
Petri dishes may be convenient enclosures to study the behavior of insects and other small animals.
Due to their large open surface, Petri dishes are effective containers to evaporate solvents and dry out precipitates, either at room temperature or in ovens and desiccators.
Sample storage and display
Petri dishes also make convenient temporary storage for samples, especially liquid, granular, or powdered ones, and small objects such as insects or seeds. Their transparency and flat profile allows the contents to be inspected with the naked eye, magnifying glass, or low-power microscope without removing the lid.