Plastic Petri Dishes

The standard 90 mm diameter dish is used for almost all agar plate work and general biology applications. The smaller 55 mm dish is used when sample volume is limited, for mini-plates in antibiotic sensitivity testing, or when incubator space is at a premium. Match the dish diameter to the volume of agar or medium used: 90 mm dishes typically hold 15 ml to 20 ml of agar.

Polystyrene (PS) Petri dishes are for single use only and cannot be autoclaved — PS melts at autoclave temperatures. They are the standard choice for most school and university microbiology where disposable sterile dishes are required. Polypropylene (PP) dishes can be autoclaved at 121 degrees Celsius and are for reusable applications, but are less common because re-sterilising used dishes is labour intensive.

Standard Petri dish lids are non-vented, providing a near-sealed environment that retains moisture and reduces contamination from airborne organisms. Vented dishes have small raised feet on the lid or rim that allow a small air gap, promoting gas exchange for cell culture and tissue culture work where CO2 and oxygen exchange is important. Standard non-vented dishes are appropriate for most school and general microbiology applications.

Disposable Petri dishes are a consumable and are ordered in packs of 10, 20, or larger bulk bags. Calculate the number of plates needed per experiment, multiply by the number of repeats and the number of student groups, and add 20% for breakage and failed pours. Over-ordering slightly is better than running out mid-experiment as dishes cannot be reused once contaminated.

The most common Petri dish diameter used in microbiology is 90 mm, which holds 15 ml to 20 ml of poured agar. Smaller 55 mm dishes are used for mini-plates and when sample volume is limited. In school biology, the 90 mm dish is the standard because it provides enough surface area for student inoculation techniques and clear colony observation.

Plates are incubated lid-down so that condensation water collects on the lid rather than dripping back onto the agar surface. Water droplets falling onto agar plates spread colonies, making it impossible to count or identify individual colony morphologies. Inverting the plate also reduces the risk of contamination from airborne organisms settling on the agar during incubation.

Disposable polystyrene Petri dishes should not be reused. PS is not autoclavable, and used dishes are contaminated with live microorganisms. Disposing of used agar plates in a sealed bag into the municipal waste stream is acceptable for school-safe organisms such as soil bacteria grown from environmental samples. Dishes contaminated with known pathogens must be autoclaved before disposal. PP Petri dishes can be autoclaved and reused.

A standard 90 mm Petri dish requires 15 ml to 20 ml of molten agar at approximately 50 degrees Celsius to form a uniform layer of 3 mm to 4 mm depth. Pour the agar in a single motion with the lid slightly open, swirl gently to level, and replace the lid immediately. Allow to set on a level surface for 20 to 30 minutes before inverting and storing at 4 degrees Celsius.

Yes, when standard precautions are followed. Use only environmental samples such as soil, air, water, or hand swabs with non-pathogenic growth media such as nutrient agar or MacConkey agar in a school setting. Seal all inoculated plates with a strip of tape before incubation so they cannot be opened by students. Incubate at room temperature (25 to 30 degrees Celsius) rather than at body temperature (37 degrees Celsius) to avoid selecting for human pathogens. Do not open sealed plates after incubation.