Petri Dish: Types, Uses, Why the Lid Doesn't Seal, and Why Plates Are Incubated Upside Down

A student pours a set of plates, inoculates them, and stacks them right-side up in the incubator like cups on a shelf. The next morning the colonies are streaked and run together, unreadable, with beads of water across the agar. Nothing contaminated them. Water that evaporated from the warm agar condensed on the lid and dripped straight back down, dragging the colonies across the surface.

Turned upside down, those same plates would have grown clean, separate colonies, because the condensation would have collected harmlessly on the lid above. The Petri dish is deceptively simple, but two things about how you use it, which way up it sits and how its lid rests, decide whether you can read the result at all.

The Petri dish, also called the Petri plate or the cell-culture dish was invented in 1887 by the German bacteriologist Julius Richard Petri. He was the assistant of the most renowned bacteriologist Robert Koch. Before the invention of the Petri dish, microorganisms were cultured in glass jars, liquor bottles, and test tubes. Petri dish provides a broader surface area for growing the organism than tubes and bottles.

A Petri dish is essential laboratory equipment for the growth of microorganisms in the artificial solid culture media. A Petri dish consists of two parts: the smaller one is a container, and the larger is a lid. The larger dish fits over, the smaller dish and forms a cap. The commonly used Petri dish in a laboratory has a diameter of 90 mm.

Why the Petri Dish Works the Way It Does

Two features of a Petri dish are counterintuitive, and understanding them is what separates a readable plate from a ruined one.

The lid does not seal, on purpose. The lid is slightly larger than the base and rests loosely over it, leaving a small gap all around. This is deliberate. Growing microorganisms need to exchange gases with the air, so a sealed dish would suffocate aerobic bacteria and let carbon dioxide build up. The loose lid lets air move in and out, while the overhanging rim and the still air trapped in the gap keep airborne contaminants from settling onto the media. It is a container designed to breathe but not to let anything fall in.

Plates are incubated upside down. Once inoculated, plates go into the incubator inverted, lid on the bottom, agar on top. The reason is condensation. As the agar warms, water evaporates and collects on the cooler lid. If the plate sat right-side up, those drops would fall onto the agar and smear the colonies together, making them impossible to count or isolate, and adding contamination risk. Inverted, the condensation gathers on the lid below the agar and never touches the growth. Inversion also slows the media from drying out during long incubations.

Together, the loose lid and the inverted position solve the two problems of growing organisms in an open dish: getting them air without contamination, and keeping water off the colonies.

Types

Different Petri dishes are available based on materials, size, and compartments.

Materials

Figure: plastic petri plate

Petri dishes are made from glass or plastic. Usually, glass Petri dishes are made from borosilicate glass, and plastic Petri dish from polystyrene or polycarbonate. The glass Petri dishes are reusable, while the plastic Petri dishes are discarded after use. Plastic Petri dishes are lighter than glass Petri dishes, which makes them easy to handle.

Size

Figure: Different sizes of Petri dish

The commonly used Petri dish in the laboratory has a 90 mm diameter. Petri dishes are available in different sizes like 35 mm, 50 mm, 60 mm, 90, 100 mm, 150 mm, etc., which vary according to the purpose of the laboratory work.

Compartments

Figure: Compartment Petri dish Image source:Thomas Scientific

Some Petri dishes have different compartments in which partition is made so that more than one culture media plate can be prepared in a single plate. These plates are of various sizes, having 2, 3, or 4 compartments.

Uses

• Petri dish prepares the culture media plates for growing the microorganisms. It provides the proper place for the growth of microorganisms and cells and prevents them from contamination. These culture media supports the growth of microorganisms and help to isolate and identify them from various clinical, food, and environmental samples. Various plating techniques are used for determining the viable microbial count and isolation of bacteria using the Petri dish.
• Petri plate is transparent because it is made from clear glass or plastic. So, microorganisms can be observed and studied from outside the plate too.
• It is used to store the preserved microorganism for a certain time. Cultured media plates can be easily wrapped with parafilm tape or aluminum foil and stored in the refrigerator.
• Beyond microbiology, Petri dishes are also used in tissue and cell culture (skin grafts, organoids), plant germination studies, entomology, and to evaporate solvents in chemistry.

How to use a Petri dish?

In the laboratory, Petri dishes are used for various purposes. Before using it, ensure the plates are clean and sterile.

Sterilizing plates

Plastic Petri dishes come in a sealed pack already sterilized during manufacture. You can open and use it directly in the aseptic condition.

If you are using reusable glass Petri dishes, you must sterilize them after each use. The standard method of sterilizing glass Petri dishes is dry heat sterilization using a hot air oven at 160 ℃ for 120 min. After sterilizing glass Petri dishes, turn off the hot air oven and let the plates cool down.

Culture media plate preparation

Then pour the sterile molten culture media, which provides the nutrients that support microbial growth. Media is held molten and tempered to about 45°C in a water bath before pouring, so it is cool enough not to destroy heat-sensitive additives like blood but still liquid enough to pour evenly.

After pouring, allow the media to set. About 25 ml of media is poured into a 90 mm dish. Molten agar is poured at around 45 to 50°C, while still liquid but cool enough not to damage heat-sensitive additives, and it solidifies as it cools below about 40°C.

Storage

Store the prepared culture media plates at 4℃ in the refrigerator.  Generally, prepared culture media can be stored for up to three weeks.

Inoculation and incubation

Before using stored culture media, bring them to room temperature. If there is moisture in the plates, dry them for some minutes in the hot air oven or incubator by slightly opening the lid of the plates at a temperature of around 30℃-35 ℃. Be sure not to dry it for a long time too.

Label the plates on the base (not the lid), inoculate the specimens, and incubate the plates inverted in an incubator. Most human pathogenic bacteria are mesophilic and are incubated overnight at 37°C.

Fungal plates are usually incubated at 25 to 30°C, though some dimorphic fungi are also incubated at 37°C to show their yeast phase. The fungi plates are incubated at both 28℃ and 37℃. Then the colony morphology and cultural characteristics of the microorganisms are studied from which the microorganisms can be identified.

Discard

To discard used culture media plates, sterilize them in the autoclave at 121℃ for 15 min. A reusable glass Petri dish can be washed, dried, sterilized, and used again, whereas plastic Petri plates are discarded after sterilization.

Why is the Petri dish always labeled at the bottom?

If you label the Petri dish or the culture media plates on the lid, swapping the lid changes the source/origin of the isolate. This may lead to false reporting laboratory. So to avoid these conditions, the Petri dish is always labeled on the bottom of the plate, not on the lid.

Automated Petri dish filler

Figure: Image source: Labcompare

Preparing and dispensing the media into the Petri dish in the laboratory by a manual method is time-consuming, and there is a higher chance of contamination. This problem can be solved using automated Petri dish fillers manufactured and marketed by companies like Systec, INTEGRA Biosciences, Neutec Group BT900000, etc.  These Petri-dish fillers can fill about a hundred to thousand plates in an hour.

How to Remember

Lid on top breathes, lid on the bottom during incubation. The two lid facts in one image. When you pour and store, the loose lid sits on top and lets the plate breathe. When you incubate, you flip it so the lid is on the bottom, catching condensation away from the colonies. The lid never seals either way.

Upside down keeps the water off. If you remember why plates are inverted, that condensation drips down, you will never incubate one right-side up. Picture the water beading on the lid: you want that lid below the agar, not above it.

Key Exam Facts in One Table

Where Students Get Confused

Why the lid is not sealed. Students assume a culture should be sealed shut to keep contaminants out. The loose lid is intentional: it lets the organisms breathe while the overhanging rim and still-air gap keep contaminants from falling in. Sealing it would suffocate aerobic bacteria.

Which way up to incubate. Plates are incubated upside down (lid on the bottom). This keeps condensation off the colonies. Incubating right-side up is the most common beginner error and ruins the plate.

Label the base, not the lid. Lids are interchangeable and get swapped between plates. Labeling the base keeps each result tied to the right sample.

Glass vs. plastic. Glass dishes are reusable and must be autoclaved or dry-heat sterilized between uses; plastic dishes come pre-sterilized and are discarded after one use. The choice depends on the lab's setup, not on the culture.

Pour temperature. Pour too hot and you kill heat-sensitive additives (like blood) and create excess condensation; pour too cool and the agar sets before it spreads. About 45°C, tempered in a water bath, is the target.

References

1. Mahajan, M. (2021). Etymologia: Petri Dish. Emerging Infectious Diseases, 27(1), 261. https://doi.org/10.3201/eid2701.et2701
2. Tille, P. M. (2022). Bailey & Scott's Diagnostic Microbiology (15th ed.). Elsevier.
3. Cheesbrough, M. (2006). District Laboratory Practice in Tropical Countries, Part 2 (2nd ed.). Cambridge University Press.
4. Willey, J. M., Sherwood, L. M., & Woolverton, C. J. (2021). Prescott's Microbiology (11th ed.). McGraw-Hill.