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Who this is for: bench microbiologists, lab staff and students who need to know why a streak plate is made and what it gives them before the next test.
A streak plate is made to turn a specimen or mixed culture into separate, isolated colonies on an agar plate, so that one colony can be picked for further testing. The CDC’s laboratory training job aid puts it this way: “Quadrant streaking of an agar plate is performed to obtain isolated colonies from a specimen.”
What a streak plate is
A streak plate is a sterile agar plate onto which a small amount of specimen or culture is spread with a loop in a set pattern. The common pattern is four-quadrant streaking. It is used in the microbiology laboratory to obtain isolated colonies from a specimen or from a mixed culture. For how it differs from other plating methods, see the comparison streak plate vs spread plate.
What is the purpose of a streak plate?
The purpose is isolation: separate colonies that can be picked and tested on their own. Each quadrant carries less inoculum than the one before. In the CDC’s words, four-quadrant streaking “results in increased dilution of the inoculum from one quadrant to the next. Typically, by the third or fourth quadrant, isolated colonies are present and are used for further testing.”
What an isolation streak is used for
An isolation streak is used at the point where a mixed or crowded sample has to become something testable. The chain described in the CDC sources is:
- A specimen or mixed culture is streaked onto a sterile agar plate.
- Dilution across the quadrants leaves isolated colonies, usually by quadrant three or four.
- The plate is incubated according to the laboratory’s testing protocol.
- After incubation, the plate is observed for isolated colonies and the lab proceeds with the appropriate testing.
Selective and differential media such as XLD agar and HE agar are read from isolated colonies in this way. Background on plates and media is in what is a Petri dish and culture media preparation.
How the four quadrants create dilution
The CDC job aid describes the pattern as follows. The first quadrant gets the inoculum, a downward streak of about 1 inch and then back and forth across it. After a quarter turn, the second quadrant is streaked by touching the edge of the first about four times and then continuing without going back into it. The third quadrant goes into the second only two or three times. The fourth is streaked like the third, with the streaks getting smaller and trailing off. Each pass drags less material forward, which is the dilution.
What you need
The CDC lists these supplies for quadrant streaking:
- Personal protective equipment (PPE)
- The specimen to be streaked
- 1.0 to 10 microliter metal or sterile plastic loops
- An incinerator, if using metal loops
- A sharps biohazard waste container
- A labeling pen
- A sterile agar plate
Metal loops and plastic loops
| Loop type | Between quadrants | Disposal |
|---|---|---|
| Metal | Sterilize in the incinerator for 5 to 10 seconds before streaking each quadrant, then let it cool. Check it is cool by touching an uninoculated area of the agar. | Reused after resterilizing |
| Sterile plastic | Use a new loop before each quadrant. | Discard into a sharps biohazard container after each quadrant |
Source: CDC REACH, Quadrant Streaking for Isolation job aid and 4 Quadrant Streak video transcript. The job aid adds that good laboratory practice suggests a new sterile loop or a resterilized metal loop before each quadrant, especially for beginners.
Before you streak
- Use PPE (gloves, laboratory coat, and face and eye protection) according to your laboratory’s standard operating procedures and safety manual.
- Check that the agar plate has not expired and has no defects.
- Label the bottom of the plate with an identifier such as the culture number, isolate number and date.
- After streaking, replace the lid and invert the plate, which the CDC says decreases condensation, then incubate according to your laboratory’s testing protocol.
Common mistakes
- Streaking a quadrant without a new sterile loop or a resterilized metal loop. The CDC suggests this before each quadrant, especially for beginners.
- Using a metal loop before it has cooled. The CDC advises letting it cool and checking on an uninoculated area of the agar.
- Touching other areas of a mixed plate when picking a colony. The CDC video notes this avoids contamination with the other colony types present.
- Labeling the lid instead of the bottom of the plate.
Who uses it
Microbiology laboratory staff and trainees use streak plates. The CDC publishes the technique in its REACH laboratory training resources, described as an important skill in the microbiology laboratory.
FAQ
What is the purpose of a streak plate?
To obtain isolated colonies from a specimen or mixed culture so one colony can be picked and tested separately.
What is an isolation streak plate used to do?
It separates organisms by serial dilution across the quadrants of an agar plate. Isolated colonies are typically present by the third or fourth quadrant and are used for further testing.
Why are there four quadrants?
Each quadrant is streaked from the one before with less carried over, so the inoculum is increasingly diluted from one quadrant to the next.
Why is the plate inverted for incubation?
The CDC states that inverting the plate decreases condensation.
Sources
- CDC REACH, Quadrant Streaking for Isolation (job aid V26015), checked 2026-11-03.
- CDC REACH, Basic Culture Media and Isolation Techniques: 4 Quadrant Streak (video transcript), checked 2026-11-03.
Related: aseptic technique.
