Petri Dish: Uses and How to Use It

Laboratory equipment guide

Petri Dish: Uses and How to Use It

A Petri dish is a shallow round glass or plastic dish with a base and lid, used to grow microbes on agar media and observe their colonies. It is also used to hold small samples, dry substances and germinate seeds.

How to use a petri dish Petri Dish at AMI Scientific

What is a petri dish used for?

Growing bacteria and fungi on solid agar.
Counting microbial colonies (pour-plate or spread-plate method).
Testing antibiotic sensitivity with discs (agar diffusion).
Holding small samples, drying precipitates or germinating seeds.

Parts of a petri dish

1Lid – slightly larger than the base; covers without sealing
2Base – holds the medium; write labels here
3Agar medium – about 15–20 mL for a 90 mm dish

How to use a petri dish

Step 1

Label the base

Write the sample code, date and medium on the base, not the lid (1). Lids get swapped; the base always stays with its medium.

Step 2

Open slightly, near a flame or in a hood

Lift the lid at an angle like a clamshell, just enough for the loop or pipette (1), near a Bunsen flame or inside a laminar flow hood. Never lay the open lid on the bench.

Step 3

Incubate upside down

After inoculation, turn the dish over so the lid is underneath. Condensation then drips onto the lid rather than the agar, so colonies do not spread.

Common mistakes

Labelling the lid: the data is lost if lids get swapped.
Opening the lid wide or for too long: airborne contamination.

Care and safety

Wash glass dishes, then sterilise in a hot-air oven (160–170 °C, 2 h) or autoclave.
Sterile plastic dishes are single-use; discard after autoclaving.

SOP for using a petri dish

A summary of the standard operating procedure (SOP) for a petri dish that your laboratory can adapt. Each step is explained in the how-to section above.

1Protective equipment

A lab coat and nitrile gloves.
Work near a Bunsen flame or inside a laminar flow hood / biosafety cabinet matching the organism's hazard level.

2Before use

Glass dishes are sterilised; plastic dishes are in intact sterile packs.
Disinfect the bench with 70% ethanol.
The agar is not cracked, dried out or contaminated before use.

4After use

Autoclave dishes with cultures (121 °C, at least 15–20 min) before disposal or washing.
Disinfect the bench and wash your hands.

5Verification & calibration

Sterility check: incubate a few uninoculated plates; no colonies may grow.

6Emergencies

Culture dish dropped or broken: cover with tissue, pour disinfectant, leave for the contact time, then collect as infectious waste.
Chemical on skin or in eyes: rinse with running water for at least 15 minutes (eyewash or safety shower), then report to the lab supervisor.

A print-ready version with purpose, scope, illustrated procedure, verification and sign-off boxes. Fill in your own document number and effective date.

Download the SOP (PDF, A4)

Types and sizes

The most common size is 90 × 15 mm, also 60 × 15 mm and 100 × 15 mm. Borosilicate glass dishes are reusable; sterile polystyrene dishes are single-use. Divided dishes and gridded dishes for easier colony counting are also available.