Gas exchange, condensation & glass versus plastic
Why plates are stored upside down, and five other things the format dictates.
A Petri dish is two shallow cylinders, the cover slightly larger than the base so it drops over it without sealing. That gap admits air for aerobic growth while a labyrinth path keeps most airborne contamination out. Understanding that the dish is a controlled leak rather than a container explains almost everything about how plates are handled.
The gap admits air and excludes most contamination
Air reaches the culture through the overlap between cover and base rather than through a vent, and the long, narrow path that overlap creates is what keeps settling spores out. It is not sterile in any absolute sense, which is why plates are poured, handled and incubated with technique that assumes the dish contributes some protection rather than all of it.
Plates are incubated inverted for a specific reason
Agar loses water as it warms, and that water condenses on whichever surface is coldest. Right way up, the drops form on the cover and fall onto the culture, spreading colonies into each other and ruining a count. Inverted, condensation collects on the lid below the agar and stays there. The dish design assumes this, which is why plates are labeled on the base.
Glass earns back its cost by being reused
Disposable polystyrene dominates routine microbiology because it arrives sterile and needs no washing. Glass wins where dishes are used continuously: it autoclaves indefinitely, does not distort, and is optically better for microscopy through the base. A laboratory pouring plates every day usually finds the washing labor cheaper than a continuing consumable line.
Autoclaving is what glass buys, and it has rules
Dishes are sterilized dry, stacked and wrapped or in a canister, with a cycle chosen for dry goods rather than for liquids. Wet steam cycles leave dishes damp and dishes stacked wet stick together and grow contaminants on the shelf. The washing and drying process, rather than the autoclave itself, is where most reusable-dish contamination originates.
Flatness is what makes a plate pourable
A base that is not flat gives agar of uneven depth, which changes diffusion distances in a zone-of-inhibition test and makes colony counting inconsistent across the plate. It is a real quality difference between dishes and it is easy to check: pour water into a batch of bases on a level bench and look at the edges before committing to a large order.
Diameter is chosen by what is being counted
The 90 and 100 mm sizes are the routine formats for spread and pour plates because they give a countable colony density at typical dilutions. The 60 mm size suits small volumes of expensive medium and screening work where many conditions run at once. Larger square and deep formats exist for other duties and are a different purchase.
