What Are Petri Dishes?

Petri dishes are shallow, flat-bottomed cylindrical vessels with a loose overlapping lid, used to hold a solid or semi-solid growth medium on which microorganisms or cultured cells are grown and observed. They are named for Julius Richard Petri, an assistant in Robert Koch's laboratory in Berlin who described the design in an 1887 note titled "A Small Modification of the Plating Technique of Koch." The modification was procedural rather than chemical: earlier plating used flat glass slabs under bell jars, which had to be uncovered for handling, while a dish with its own close-fitting cover could be inoculated, stacked, incubated, and examined under a microscope without exposing the culture to airborne contamination. The biographical account maintained by the Science History Institute sets that change against the wider push in Koch's group to isolate pure bacterial strains.

As an item of biomedical equipment the dish is deliberately simple, and its persistence for well over a century reflects that simplicity. It remains the standard container for colony isolation, antimicrobial susceptibility testing, environmental and food monitoring, and adherent cell culture.

Materials and Physical Design

Two material families are in use. Borosilicate glass dishes tolerate repeated steam sterilization in an autoclave and are reused, which suits laboratories with heavy throughput and limited consumable budgets. Polystyrene dishes are molded, sterilized by gamma irradiation or ethylene oxide, sealed at the factory, and discarded after use, which removes the cleaning step and the risk of carryover. The common bacteriological format is roughly 100 millimeters in diameter and 15 millimeters deep, holding about 20 to 25 milliliters of agar, with 35, 60, and 150 millimeter variants and rectangular plates for specialized work. Lids sit above small ribs or vents rather than sealing, so that oxygen and carbon dioxide exchange while the gap remains tortuous enough to exclude settling particles. For mammalian cell work the polystyrene surface is treated with plasma or corona discharge to introduce oxygen-containing groups that make the hydrophobic polymer wettable and allow cells to attach and spread, and dishes may be further coated with collagen, fibronectin, or poly-lysine.

Culture Media and Plating Technique

The dish is a container for a medium, and the medium determines what grows. Nutrient agar supports general growth, selective media suppress unwanted organisms through antibiotics, salt, or dyes, and differential media contain indicators that make colonies of different species visibly distinct. Inoculation methods control the resulting colony density: streak plating dilutes a sample progressively across quadrants to yield isolated colonies, spread plating distributes a measured volume across the surface, and pour plating mixes the sample into molten agar so colonies develop within the depth of the medium. Counting isolated colonies gives an estimate of viable organisms in the original sample expressed as colony-forming units. Disk diffusion susceptibility testing uses the same platform, placing antibiotic-impregnated paper disks on a uniformly seeded lawn and measuring the diameter of the zone where growth is inhibited.

Instrumentation and Automation

Reading plates by eye is slow and subjective, so much of the recent work around Petri dishes concerns instrumentation. Automated colony counters image the plate under controlled illumination and segment colonies by size, shape, and color, which removes operator variance from enumeration. Total laboratory automation systems in clinical microbiology now handle specimen streaking, incubation in imaging incubators that photograph each plate at intervals, and machine learning classification of growth, so that a technologist reviews images rather than physical plates. Optical methods can go further than counting: elastic light scattering from a laser passing through a colony produces a diffraction pattern that encodes colony morphology, and work on laser optical scattering sensors applied to Petri plates has shown that these patterns can identify bacterial genus and species without labeling or destroying the sample. The Emerging Infectious Diseases etymology note on the Petri dish records how firmly the original design became embedded in laboratory practice.

Applications

Petri dishes have applications in a wide range of fields, including:

• Clinical microbiology and infectious disease diagnosis
• Antimicrobial susceptibility testing
• Food safety and water quality monitoring
• Pharmaceutical sterility and environmental monitoring
• Mammalian cell culture and tissue engineering
• Plant tissue culture and seed germination testing