Written and maintained by CASRAI Editorial Board
Last updated
An agar plate is a petri dish filled with a prepared, solid growth medium made from agar and nutrients, used to grow and isolate microorganisms. What separates one agar plate from another is the medium inside, and that medium is described by three labels that appear on almost every catalogue: enriched, selective and differential. This page explains what those labels mean, which common media fall into each, and how plates should be stored.
This page describes products and general use; it is not medical advice.
Who uses it: clinical and research microbiology laboratories, and the staff, students and buyers who read culture media labels when ordering or preparing plates.
What an agar plate is (and how it differs from a petri dish)
An agar plate is a petri dish plus the medium poured into it. The dish is the vessel; the plate is the dish once it holds solidified medium. If you only need to understand the empty vessel, see what a petri dish is. The two terms are often used loosely, but a catalogue entry for “agar plates” is describing the medium as much as the dish.
How it is built
Every agar plate has two parts: a shallow, lidded dish and a layer of solid medium. The medium supplies nutrients, and its composition decides which microorganisms grow and how they look. The CDC describes culture media as “useful for isolating and identifying microorganisms from various sites in the human body”.
Types of agar plate by media class
The CDC groups culture media by what they are designed to do. Its definitions, in short:
- Nutritive (enriched) media “contain added nutrients to support the growth of a wide range of microorganisms, including those with more demanding nutritional needs”.
- Selective media “contain substances (like dyes, acids, alcohols, or antibiotics) that encourage the growth of certain microbes while inhibiting others”.
- Differential media are “designed to show visible differences between microorganisms based on their biochemical or metabolic activity (e.g., color change due to acid production)”.
A single medium can belong to more than one class. The table below uses the classes the CDC assigns to each named medium.
| Plate (abbreviation) | Media class per CDC |
|---|---|
| Blood agar (BAP), also called sheep’s blood agar (SBA) | Nutritive (enriched) and differential |
| Chocolate agar (CHOC) | Nutritive (enriched) only |
| MacConkey agar (MAC) | Selective and differential |
| Hektoen enteric (HE), XLD, Salmonella Shigella (SS), TCBS | Selective and differential |
| Sorbitol-MacConkey (SMAC), used for E. coli O157 | Selective and differential |
| Columbia CNA; CHROMagar (chromogenic) | Selective and differential |
| Campylobacter agar; phenylethyl alcohol agar (PEA) | Selective only |
| Thayer-Martin / Martin-Lewis | Nutritive and selective |
Source: CDC REACH, “Basic Culture Media and Isolation Techniques Table”, checked 2026-10-29.
How to read a media label
The table explains why labels can look contradictory. MacConkey agar is both selective and differential: it encourages some microbes while inhibiting others, and it also makes differences between the ones that grow visible. Chocolate agar is enriched only, so it supports demanding organisms but does not suppress or distinguish any. Blood agar is enriched and differential, but not selective. When comparing plates, read the class first, then the named medium; the class tells you what the plate is designed to do.
The classes are a way to read the label, not a recommendation of which plate to use. Which medium suits a given specimen or organism is set by your laboratory’s procedures and the relevant method, not by this page.
Storage and handling basics
Thermo Fisher Scientific’s storage guide gives these general points:
- Store plates in a cold room or refrigerator at 2–8°C, not in a freezer, because freezing ruins plates.
- Store plates upside down, to prevent moisture condensing from the agar onto the lids.
- For self-poured plates, non-selective agars keep up to four weeks, non-selective blood agars up to seven days, and most selective agars up to seven days. Thermo Fisher calls these general guidelines that each lab should validate. Purchased prepared media keeps until the manufacturer’s expiration date.
- Check each plate for visible signs of deterioration and discard any plates that appear damaged.
- Let poured plates cool completely after pouring, so moisture does not condense on the lids during storage.
Source: Thermo Fisher Scientific, “Agar Plate Storage: A Complete Guide”, checked 2026-10-29. Storage requirements can differ between products, so follow the storage condition printed on the manufacturer’s label for the plates you actually hold. For wider context on keeping cultures clean, see cell culture contamination and the incubator guide; plates are often prepared in a laboratory that also uses an autoclave, and work with cultures falls under your site’s biosafety level rules.
Common mistakes
- Storing plates in a freezer. The storage guide says freezing ruins plates.
- Storing plates lid-side up, which lets condensation collect on the lids.
- Applying the four-week or seven-day figures to purchased plates. Those figures are for self-poured plates; purchased media follows its expiration date.
- Using a plate that looks damaged or deteriorated instead of discarding it.
- Treating “selective” and “differential” as the same thing. They are separate classes, and some plates are one without the other.
FAQ
What is the difference between an agar plate and a petri dish?
The petri dish is the empty vessel; an agar plate is a petri dish that holds solidified agar-based growth medium. See what a petri dish is for the vessel itself.
Are all agar plates selective?
No. Per the CDC table, chocolate agar is enriched only, and blood agar is enriched and differential. Others, such as MacConkey, are selective and differential.
How long do agar plates keep?
For self-poured plates, Thermo Fisher gives up to four weeks for non-selective agars, up to seven days for non-selective blood agars, and up to seven days for most selective agars, refrigerated at 2–8°C. These are general guidelines, so validate them for your own lab. Purchased prepared plates keep until the manufacturer’s expiration date.
Sources
- CDC REACH, Basic Culture Media and Isolation Techniques Table, checked 2026-10-29.
- Thermo Fisher Scientific, Agar Plate Storage: A Complete Guide, checked 2026-10-29.
