A Petri dish is a fundamental piece of laboratory equipment used for studying microscopic life. This shallow, cylindrical, transparent dish features a slightly larger lid that rests on top, providing a protected environment. The German bacteriologist Julius Richard Petri developed this design in 1887 while working in Robert Koch’s laboratory. Made from glass or disposable plastic, the dish allows scientists to observe biological processes without disturbing the contents.
Essential Role in Microbiology
The primary application of the Petri dish is culturing microorganisms, such as bacteria, yeast, and molds. To facilitate growth, the dish is partially filled with a solid culture medium called agar, a gelatinous substance derived from seaweed. This agar is mixed with specific nutrients—like carbohydrates, salts, and proteins—necessary for the target organisms to thrive, creating an agar plate.
A main use of the agar plate is isolating a pure culture from a mixed sample through streaking. Scientists use a sterile inoculating loop to spread a sample across the agar surface in a diminishing pattern, such as the quadrant streak technique. This serial dilution ensures that individual cells are far enough apart to grow into distinct, genetically identical colonies. Dishes are typically incubated upside-down to prevent water condensation from compromising the isolation. This controlled environment allows researchers to study colony morphology, test antibiotic sensitivity, and identify specific pathogens.
Applications in Eukaryotic Cell Culture
The Petri dish design is also adapted for growing complex eukaryotic cells, including human and animal cells. This process, known as cell culture, requires specialized liquid growth media and a surface that encourages cell attachment, unlike microbial work. Standard Petri dishes are unsuitable for mammalian cells because they are anchorage-dependent and cannot adhere to untreated plastic.
To address this, modern cell culture dishes are made from treated polystyrene, chemically modified to be hydrophilic and favorable for cell adhesion. These cells are maintained in an incubator that provides precise environmental control, including constant temperature and specific carbon dioxide concentration. Researchers use these cultures extensively for modeling human diseases and for the preclinical testing of new drug candidates.
Observation and Non-Biological Functions
The transparent, flat design of the Petri dish makes it an ideal platform for various observational and non-biological laboratory tasks. It is frequently employed in botanical research to monitor the early developmental stages of plants, such as observing seed germination and early root growth. The clear lid provides protection from contaminants while allowing for continuous visual inspection. The dish also serves as an effective enclosure for observing small organisms, such as insects or nematodes, under a low-power microscope or magnifying glass.
In chemistry and material science, its shallow form is useful for evaporating solvents or drying small precipitates and powders. The dish is also commonly used for the temporary storage of small, granular, or powdered samples.