Nobody thinks of a plate as a reagent, which is why it is the quietest source of variation in a molecular biology laboratory. The recipe is trivial and the outcome is not: the same medium made from two formulations gives different colony sizes, a plate poured hot kills the antibiotic that was added to it, a plate dried too little grows satellites and one dried too much stops growing anything, and a stack stored in the fridge for a month is not the plate it was on the day it was poured. All of that is controllable and none of it is on the bottle.
Which formulation you are buying, and what changes with it
The classical medium exists in more than one salt concentration, and the difference matters: the higher salt version supports denser growth of some strains and stresses others, and a plasmid maintained comfortably on one can be lost on the other. Powdered complete medium, powder plus separate agar, and ready poured plates are three different purchases with three different failure modes. Pick one formulation and stay with it for a project, and write the formulation rather than the medium's name in the method, because a reader who buys the other one is running a different experiment.
Antibiotics: when to add them and what survives
Heat is the first enemy. Add the selection when the molten medium is hand warm rather than steaming, because the commonly used beta lactam loses activity quickly at higher temperatures and the plate that results selects nothing. The second enemy is time: that same antibiotic is degraded by the enzyme the resistant colonies secrete, which is why satellite colonies appear around a large one on an old plate, and why the answer is a fresh plate rather than a longer incubation. Keep concentrated stocks frozen in single use aliquots and record the lot, since a stock thawed repeatedly is the usual reason selection weakens over a month.
Where ypd broth and the other standard media differ
The yeast medium is the same idea with a different nitrogen and carbon source, and the practical differences are pH, the growth rate it supports and the fact that it will grow moulds happily if a plate is left out. A laboratory working with both organisms should keep them physically separated at the pouring stage, because a shared water bath and a shared pouring bench is how a yeast plate ends up with a bacterial lawn. As with the bacterial medium, the recipe is not the specification: the supplier, the formulation and the lot belong in the record.
Drying, storage and the plate that stopped working
A freshly poured plate carries surface water that smears a streak and encourages satellites; a plate dried too long or stored uncovered has lost enough water to change the salt concentration and the agar's surface. Dry them at room temperature in a clean flow until the surface is matt rather than wet, then bag them inverted and refrigerate. Use them within a few weeks, sooner with an antibiotic, and write the pour date on the plate rather than on the bag, because the bag is not what ends up on the bench.
A 60 x 15 mm petri dish, and when glass petri dishes are worth washing
Plate size is chosen by what will be counted or picked rather than by habit: a small dish takes less medium and less incubator space and is right for a few colonies or a single clone, and a standard dish is right for anything that has to be spread and counted. The taller walls of a deeper dish matter when the plate will be incubated for days, because there is more room for condensation to go. Glass is worth the washing where the dish is reused for a soft agar overlay or where a solvent would craze polystyrene; for routine plating, single use plastic is cheaper than the labour of washing and sterilising.
Making plates reproducible between people
Write down the formulation and supplier, the agar concentration, the volume per plate, the temperature at which selection is added, the drying regime and the storage life, and keep that as the laboratory's own method rather than in somebody's memory. Then pour in batches large enough that a project runs on one or two lots. Most of the trouble people attribute to a construct or a strain traces back to plates poured differently by two people in the same week.
Questions people ask about lb agar
How long do plates with antibiotic last?
Days to a few weeks refrigerated, and much less than people assume for the beta lactam, which degrades both with heat and with time. If selection looks weak or satellites appear, pour fresh plates before changing anything else about the experiment.
Can I remelt agar that has set?
Once, gently, and not with antibiotic in it. Each melt drives off water and shears the polysaccharide a little, so a bottle remelted repeatedly sets softer and pours unevenly. Melt what you will use and add selection after it cools.
Ready poured plates or make our own?
Ready poured where the volume is small, where consistency between people is the problem, or where nobody has time to pour; your own where the medium is unusual, the selection is uncommon or the volumes are large. The comparison is labour against a per plate price, and the shelf life of a bought plate is the third variable.