Agar plates are always labeled on the bottom (the half containing the agar), not on the lid. You write around the outer edge using a lab marker, and include at minimum your name, the date, the media type, and the organism being plated. This keeps your label permanently attached to the correct sample, visible during incubation, and out of the way when you need to read colonies.
Why You Label the Bottom, Not the Lid
Petri dish lids are interchangeable. If a lid gets swapped during stacking or handling, any information written on it goes with it, and your sample is now unidentified. The agar base is the half that actually holds your culture, so labeling it means the identity stays with the sample no matter what.
There’s a second practical reason: agar plates are incubated upside down. Flipping them prevents condensation from collecting on the lid and dripping onto your colonies, which would cause them to smear together and become unreadable. When plates sit inverted in an incubator, the bottom faces up, making your label the first thing you see when scanning a stack.
What to Write on Every Plate
At minimum, every plate needs four pieces of information written along the outer rim:
- Your name or initials: identifies who the plate belongs to, especially in shared incubators or teaching labs where dozens of plates sit side by side.
- The date: lets you track incubation time and know when colonies are ready to read or when a plate has been sitting too long.
- Media type: the kind of agar in the plate (LB, TSA, MacConkey, blood agar, etc.). This matters because different media select for or indicate different organisms.
- Organism or sample: what you plated, whether that’s a named bacterial species, a patient sample ID, an environmental swab location, or a plasmid/strain designation.
Depending on your lab or course, you may also need to add a class section number, the dilution factor, the antibiotic in the plate, or the incubation temperature. If your instructor or protocol specifies additional fields, fit them in around the edge or add a second line of text closer to the rim.
Where Exactly to Write
Write along the outermost edge of the bottom half, curving your text to follow the rim. This keeps the center of the plate clear so you can observe colonies, count them, and photograph them without marker lines running through your data. A common mistake is writing across the middle of the plate, which blocks your view of exactly the area where growth matters most.
Small, compact handwriting helps. You’re fitting several pieces of information into a narrow band around the circumference of a dish that’s typically only about 10 cm (4 inches) across. Abbreviations are fine as long as they’re consistent and your lab partners or graders can decode them.
Use the Right Marker
Standard felt-tip pens and Sharpies can smear or dissolve when they contact alcohol, which is common during disinfection steps in aseptic technique. Lab-grade markers are designed with quick-dry ink that resists both water and alcohol, and they write cleanly on dry, wet, or cold plastic surfaces like Petri dishes. A fine-tip version gives you more control in the small space along the plate’s edge.
If you don’t have access to a dedicated lab marker, a regular permanent marker will work for most classroom settings. Just be aware that wiping the plate with ethanol can blur the text. Let the ink dry completely before handling.
Color Coding for Multiple Plates
When you’re working with several media types or antibiotic concentrations at once, color-coded markers save time. Many labs assign a specific marker color to each antibiotic or medium. For example, you might use blue for ampicillin plates, red for kanamycin, and black for plates with no antibiotic. This lets you identify a plate’s contents at a glance without reading every label, which is especially helpful when you’re pulling 20 or 30 plates out of an incubator at once.
Whatever system you use, keep it consistent across your entire experiment so there’s no ambiguity when you return to read results a day or two later.
Labeling Plates With Infectious Material
If your plates contain blood, clinical specimens, or known pathogens, federal workplace safety rules require biohazard labeling on the container. The standard biohazard symbol must appear in a contrasting color against a fluorescent orange or orange-red background. In practice, many teaching and clinical labs use pre-printed biohazard tape or stickers placed on the plate or on the secondary container (a bag or rack) holding the plates. A red container can substitute for the label in some cases.
Even in labs where formal biohazard stickers aren’t required, it’s good practice to mark any plate containing a Biosafety Level 2 organism with a clear warning so others handling it know to take extra precautions.
Step-by-Step Summary
Before you pour or streak anything, grab your marker and label first. It’s easier to write on an empty or freshly poured plate than on one you’re trying to keep sterile mid-procedure. Flip the plate so the agar side faces you, write your name, date, media type, and organism around the rim, then set it aside to use. After inoculation, stack your plates and place them upside down (agar on top, lid on the bottom) in the incubator. Your label will be right on top of the stack, readable without opening or unstacking anything.