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What Is a Petri Dish? Glass vs Plastic Guide
A petri dish is one of the most recognisable pieces of lab equipment there is, even to people who've never set foot in a lab. Shallow, round, with a loose-fitting lid, it looks almost too simple to need explaining. But there's a fair amount going on behind that simplicity: glass versus plastic, sterile versus non-sterile, and a couple of rules about what you can and can't do to one without ruining it.
This guide covers what a petri dish actually is, when to use glass versus plastic, why sterility matters, and the everyday uses that go well beyond growing bacteria.
What Is a Petri Dish?
A petri dish (also called a Petri plate or cell-culture dish) is a shallow, cylindrical, transparent dish with a matching lid, used to hold a growth medium such as agar so that microorganisms, cells, or small samples can be observed or cultured. The lid sits loosely over the base rather than sealing it, which lets gas exchange happen while still keeping dust and contamination out. It's named after Julius Richard Petri, the German bacteriologist who developed it in 1887 while working in Robert Koch's laboratory.
Glass vs Plastic Petri Dishes
| Glass (Borosilicate) | Plastic (Polystyrene) | |
|---|---|---|
| Reusable | Yes, can be washed and re-sterilised many times | No, designed for single use |
| Sterilisation method | Autoclaving (steam, heat, and pressure) | Ethylene oxide gas or gamma irradiation |
| Cost per use | Higher upfront, cheaper over time | Lower upfront, adds up with repeated use |
| Environmental impact | Recyclable, less disposable waste | Generates plastic waste with repeated use |
| Typical use | Labs doing repeated culturing where reuse is practical | Routine microbiology, teaching labs, anywhere disposables are preferred |
Glass petri dishes are made from heat-resistant borosilicate glass, which is what makes them autoclavable in the first place. Plastic ones are almost always polystyrene, chosen because it's clear enough for microscope work and cheap enough to throw away after one use.
Why You Should Never Autoclave a Plastic Petri Dish
This is worth calling out on its own, because it trips people up. Polystyrene starts to soften at around 100°C and will visibly warp or melt well before it reaches the 121°C an autoclave runs at. Glass petri dishes are built to survive that heat and pressure cycle repeatedly, but plastic ones are not, and putting them through an autoclave to reuse them will just ruin a batch of dishes. If you need to sterilise plastic dishes before disposal, that's fine, autoclaving plastic waste for safe disposal is standard practice. The rule only applies to trying to reuse them afterwards.
Sterile vs Non-Sterile
Plastic petri dishes sold for microbiology work usually come pre-sterilised, most often by ethylene oxide (EO) gas, and are packaged to stay that way until opened. Glass petri dishes are typically sold non-sterile, since the expectation is that the lab will autoclave them before first use and again between uses. Neither is better across the board, it depends on whether you need a ready-to-use disposable or a dish you'll be sterilising yourself as part of routine lab work.
Design Features Worth Knowing
A few small design details make a real difference to how well a petri dish performs:
- Vented lids: many plastic dishes have small ridges or vents moulded into the lid to allow gas exchange while reducing condensation dripping onto the culture below.
- Stacking ridges: a raised ring on the lid lets dishes stack securely in an incubator without sliding around.
- Flatness: a well-made dish has a flat, even base, so the agar sets to a uniform thickness across the whole plate rather than pooling unevenly.
How Petri Dishes Are Used for Culturing
For microbiology work, a petri dish is filled with a nutrient growth medium, usually agar, while it's still liquid and warm, then left to cool and set into a firm gel. A sample is then introduced onto the surface, often using an inoculating loop or an L-shaped spreader to spread it evenly, and the dish is incubated at a controlled temperature. Over time, individual cells multiply into visible colonies that can be counted, examined, or isolated for further testing.
Beyond Microbiology: Other Uses
| Use | How the Petri Dish Helps |
|---|---|
| Seed germination | Seeds placed on damp cotton wool or filter paper let students observe root and shoot growth without soil in the way |
| Evaporation and crystallisation | A shallow, wide surface area speeds up evaporation for salt or crystal-growing experiments |
| Specimen observation | Small samples, insects, or plant material can be examined under a microscope or stereo microscope without handling them directly |
| Sample storage | Short-term holding of small solid samples during a practical, without needing full glassware |
Choosing the Right Size
Petri dishes are commonly available from around 35mm up to 150mm or larger in diameter, with 90mm and 100mm being the most common sizes for general microbiology work. Smaller dishes suit small sample volumes or situations where reagents or growth media are expensive, while larger dishes give more surface area for counting colonies or running side-by-side comparisons on one plate.
Shop the Petri Dish Range
SmartLabs stocks the full range covered in this guide, ready to order:
- Petri Dish Plastic, PS, EO Sterile, ready-to-use disposable dishes for routine microbiology
- Petri Dish, Borosilicate 3.3 Glass, reusable and autoclavable for repeated use
- Petri Dish Rack, for organised storage of up to 60 dishes
- L-Shape Spreader, for evenly plating liquid cultures
Frequently Asked Questions
Can you reuse a plastic petri dish?
No. Plastic petri dishes are designed for single use and can't be autoclaved for reuse, since polystyrene softens and warps well below autoclave temperature. Glass petri dishes are the reusable option.
Why do petri dishes have a lid that doesn't seal tightly?
The loose-fitting lid allows gas exchange, which most cultured organisms need, while still shielding the contents from dust and airborne contamination. A fully sealed container would trap gases and interfere with growth.
What is the difference between sterile and non-sterile petri dishes?
Sterile dishes, usually plastic, are pre-treated to be free of viable microorganisms and are ready to use straight from the pack. Non-sterile dishes, usually glass, need to be autoclaved before use since they're sold as reusable labware rather than ready-to-use disposables.
What size petri dish should I use?
90mm or 100mm covers most general microbiology work. Go smaller for limited sample volumes or expensive media, and larger when you need more surface area for colony counts or side-by-side comparisons.
Can petri dishes be used for anything other than growing bacteria?
Yes. They're commonly used for seed germination, evaporation and crystallisation experiments, and holding small specimens for observation under a microscope.
Who invented the petri dish?
Julius Richard Petri, a German bacteriologist working in Robert Koch's laboratory, developed the design in 1887.
The Final Word
A petri dish is one of the simplest tools in a lab, but it's worth knowing the few rules that actually matter: match glass or plastic to whether you need reusability, respect the sterile packaging on disposables, and never put a plastic dish through an autoclave expecting to use it again. Get that right and the rest is straightforward.