Quadrant Streaking: Isolating Bacteria

Quadrant streaking is a technique that is used in microbiology. This technique is frequently used to isolate pure bacterial colonies on a petri dish in the hopes that it will grow after the incubation period. The goal of this technique is to isolate pure bacteria from mixed bacterial cultures.

Gather materials: In order to start the quadrant streak you need to gather the materials in order to successfully perform the quadrant streak technique. First thing you are going to need is a petri dish that has the agar that will allow the isolate to grow. You are also going to need an inoculation loop, flame, and bacteria that you are working with. You also need to make sure to have disinfectant spray to clean the area you are working in to eliminate your changes of contaminating your samples.

Important reminders: It is very important to remember to flame your loop before you inoculate any agar plates. You do not want to introduce any contaminates to your bacterial sample because your isolate will not be pure. After you flame your loop you want to give it some time, usually 20-30 seconds to cool off. It is important to let your loop cool before inoculating because you do not want to kill any of the bacteria you have. If your flame is hot and you try to inoculate you will not be able to isolate any colonies due to the hot loop killing all of the bacteria you tried to pick up. You must always make sure that you are flaming your loop in between each quadrant.

Quadrant 1: The first quadrant is where the first part of the inoculation begins. The initial streak is the first streak that you put on the plate and it should be the heaviest because its where most of your growth is going to occur. You want to make sure that you are dragging your loop back and forth to make two make four rows that do not touch each other.

***flame your loop, and let it cool down. At this point you are no longer grabbing any more of your bacterial sample and adding it to your plate.

Quadrant 2: You are going to take your loop and go to the second row in from the first quadrant streak and repeat the first step. Take you loop and drag it back and fourth creating four rows that do not touch each other. By taking the loop and dragging it through the streak you are separating out the isolate.

***flame your loop and let it cool down

Quadrant 3: You are going to take your loop and go to the second row in from the first quadrant streak and repeat the first step. Take you loop and drag it back and fourth creating four rows that do not touch each other. At this point the streak should be looking thinner and you are probably starting to think that you do not have anything there.

***flame your loop and let it cool down

Quadrant 4: You are going to take your loop and go to the second row in from the first quadrant streak and repeat the first step. Take you loop and drag it back and fourth creating four rows that do not touch each other. You also want to make sure that this last streak you give the most room to and the streaks are longer and not as close together. This allows your isolate to really separate itself out for you to collect and sample after incubation.

***flame you loop and let it cool down, and start the incubation process.

The final result after incubation: