Choosing culture dishes by what the protocol needs rather than by what is in the cupboard: why a 100 mm petri dish, catalogued equally often as a 100mm petri dish or a 10cm petri dish, is the default and a 150 mm petri dish is not simply a larger one, where a 60 mm petri dish written as a 60mm petri dish or a 60 x 15 mm petri dish sits beside a 35 mm petri dish and the same item listed as a 35mm petri dish, when a glass petri dish or glass petri dishes are still the right answer among the polymer ones, what a petri dish holder does for a stack, and what cloning rings add on top of any of them

Choosing a size and a surface

1. Work from growth area and yield. The useful figures are the surface area of the dish and the number of cells it holds at confluence for your line. Write those down once for each format you use, and scaling a protocol becomes arithmetic rather than guesswork.
2. Medium volume follows area, not dish depth. Medium depth affects gas exchange, so the volume is set by the area rather than by how much the dish will hold. Too little dries and exhausts; too much slows oxygen transfer and changes the result. The supplier's recommended volume is a good starting point and worth checking against your own feeding schedule.
3. Surface treatment is part of the product. Untreated polymer is hydrophobic and suits suspension work or bacteriology. Tissue-culture treated surfaces are modified to let adherent cells attach, and specialised coatings go further. An adherent line put into an untreated dish simply will not attach, which is easy to mistake for a cell problem.
4. Vented or not, and stacking. Lids with moulded ridges admit gas exchange and reduce condensation; tight lids suit anaerobic and long incubations. Dishes also have to stack stably in the incubator, and a stack that slides is a contamination route as well as a nuisance.
5. Sterility and single use. Polymer dishes arrive sterile and are used once. Glass is reusable, autoclavable and used where a solvent would attack polymer or where a long incubation or a high temperature is involved, at the cost of washing, wrapping and sterilising.

Why a bigger dish is not a simple scale-up

Growth area rises with the square of the diameter, so a step up the ladder changes cell number, medium volume, reagent consumption and cost far faster than the name suggests. The largest formats also warm and cool more slowly and are harder to handle inside a cabinet.

Edge and centre conditions differ more in large dishes, which shows as uneven confluence. Where uniformity matters more than yield, several smaller dishes are usually better than one large one, and they also let a replicate be lost without losing the experiment.

Bacteriological and tissue culture dishes are different products

Dishes sold for microbiology are untreated, because a colony sits on the agar rather than attaching to plastic. Dishes sold for cell culture are surface-modified so that adherent cells can attach and spread, and they cost more for exactly that reason.

The two look identical. Keeping them separately stored and clearly labelled prevents an afternoon spent wondering why a reliable line has stopped attaching, which is the single most common confusion in this part of a store cupboard.

Isolating a clone on the dish

A small ring sealed to the dish surface around a single colony lets that colony be trypsinised and collected without disturbing its neighbours, which is the classic way to pick a clone from an adherent population. Sterile grease or silicone makes the seal.

It is fiddly and it works. The alternatives are limiting dilution into a multiwell plate and sorting, each with its own trade between speed and the certainty that a well started from one cell. Which is right depends on how much that certainty matters to the claim.

Common questions

Why will my cells not attach to a new dish?

Check whether the dishes are tissue-culture treated. Untreated dishes sold for microbiology look identical and adherent cells will not attach to them. Store and label the two kinds separately.

How much medium should a dish hold?

Enough to cover the area without slowing gas exchange, set by growth area rather than dish depth. Start from the supplier's recommended volume and check it against your own feeding schedule.

Is one large dish equivalent to several small ones?

Not for uniformity. Edge and centre conditions differ more in large dishes, and several smaller dishes also let one replicate be lost without losing the experiment. One large dish wins on cell yield per handling step.

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