Agar Petri Dishes: Agar Types, Prepared Plates and Storage
A clinic laboratory orders dishes, then discovers that the plates it received hold an agar it has never worked with. The dish is fine. The medium decides whether the plate answers the question the clinician actually asked.
What is an agar petri dish?
An agar petri dish is a shallow, lidded dish holding a thin, solid layer of agar, the gelling agent that gives microbiological culture a surface to grow on. The dish is the container; the agar is the medium. That distinction matters because the two are ordered, stored and disposed of differently.
A 100 x 15 mm dish is the common clinic format. Agar is poured to a depth that keeps the plate from drying out during incubation while still allowing colonies to be counted and picked. Polystyrene dishes are optically clear for counting against a light box and are single use.
Which agar should be on the plate?
The agar is chosen for the organism or the question, not for convenience.
| Agar type | What grows on it | Typical clinic use | Notes |
|---|---|---|---|
| Nutrient agar | A wide range of non-fastidious bacteria | General environmental and surface checks | Broad but not selective; colonies are hard to tell apart |
| Tryptic soy agar | General purpose, supports most bacteria | Swab and surface monitoring | Often paired with a selective plate for the same sample |
| MacConkey agar | Gram-negative bacteria, lactose fermenters | Water, food and environmental screening | Selective and differential through bile salts and a pH indicator |
| Blood agar | Fastidious organisms including streptococci | Clinical reference work | Requires blood components and careful cold chain |
| Sabouraud dextrose agar | Yeasts and moulds | Air, surface and product spoilage checks | Lower pH suppresses most bacteria |
How are prepared plates stored before use?
Prepared plates are cold-chain items with a short working life. They arrive sealed, and they stay sealed and inverted until the moment of inoculation. Storage guidance is consistent across suppliers: refrigerate, keep away from the freezer compartment, keep away from the door where temperature cycles, and use within the shelf life printed on the pack.
Bringing a plate to room temperature before inoculation is not optional. A cold lid collects condensation, and that moisture drips onto the agar, turning a countable plate into a smear of colonies that cannot be attributed to a location or a time.
What should the label on a plate record?
A plate has to be traceable on its own, because the lid and the base can be separated and the incubator holds several plates at once. Laboratories that pass audits label the base, not the lid, and record sample identity, site, date and time of inoculation, medium and lot. Writing on the lid is a common finding in audits, because a stack of inverted plates shows only the bases.
How do agar plates fit into waste handling?
An inoculated plate is infectious waste. It leaves the bench in a sealed bag or container that is later autoclaved, and it never goes into general refuse. Because polystyrene warps well below sterilisation temperature, plates are autoclaved for decontamination and then discarded as treated waste rather than reused. The consumables that travel with the plate - specimen containers, bags and swabs - follow the same single-use logic.
The consumables that decide whether a plate is usable sit next to it on the bench. A clinic-scale laboratory needs sterile 100 x 15 mm polystyrene dishes in a case quantity, graduated leak-proof specimen cups for the sample that is being plated, and 95 kPa specimen bags for anything that has to travel before it reaches the plate. Ordering these as one group keeps a plating session from stopping halfway. A plate is only as clean as the bench and the cycle behind it, and a five-pack trial pack of biological indicators covers the weekly spore test for the steriliser that serves the bench.
References
- Public Health Ontario - infection prevention and control guidance for clinical settings (checked 17 September 2026)
- Public Health Ontario - environmental cleaning and laboratory surface practice (checked 17 September 2026)
Ordering for a clinic, lab or care home? Wholesale and multi-site ordering covers case pricing and account setup, and the B2B wholesale collection lists the lines stocked for institutional buyers.
Related Reading
- Laboratory consumables for Canadian clinics and food testing labs
- Learning Hub - compliance and product guides for clinics and labs
- Sterile petri dishes in a clinic lab: handling, labelling and incubation basics
- Diagnostic laboratory supply checklist
- Request bulk pricing for laboratory consumables
- diagnostic & research lab supplies
- free Ontario sterilization compliance log
Frequently Asked Questions
What is the difference between an agar petri dish and a broth culture?
Agar gives you a solid surface, so each surviving organism grows in place and becomes a countable colony. Broth gives you turbidity, which tells you that something grew but not how much or where. Plates are used when a count or an isolation is needed; broths are used when the question is simply present or absent.
Are prepared agar plates or dehydrated agar powder the better purchase for a clinic?
Prepared plates need refrigeration, have a short working life once poured and arrive ready to use. Dehydrated powder keeps far longer at room temperature but needs a balance, water, an autoclave and a pouring routine that a small clinic rarely has. Most clinic-scale laboratories buy prepared plates and keep the powder question inside a reference lab.
How should a 100 mm agar plate be stored before use?
Inverted, in a sealed sleeve or bag, refrigerated and away from the door of the fridge where temperature swings. Plates are taken out and allowed to reach room temperature before inoculation, because condensation on a cold lid drips onto the agar surface and spreads colonies across the plate.
Can a clinic reuse or re-pour agar plates?
No. Once a plate has been opened and inoculated it is a contaminated article, and it must be autoclaved as infectious waste before disposal. Re-pouring into the same dish is not a reprocessing route, and polystyrene dishes are not intended for repeat autoclaving in any case.
Last updated: September 2026. CliniEco Medical holds MDEL #35334 issued by Health Canada, and supplies clinics, long-term care homes and home-care programs across Canada.
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