Jupiterimages/Photos.com/Getty Images

The human body hosts roughly nine times more bacterial cells than human cells. In microbiology, a pure culture refers to a laboratory preparation—such as a petri dish—that contains a single bacterial species. Identifying and isolating that species is essential for accurate characterization and downstream applications.

John Foxx/Stockbyte/Getty Images

1. Sterilize the Inoculating Loop

Ignite a Bunsen burner and pass the inoculating loop through the hottest portion of the flame until it glows red. This step eliminates any residual microbes.

2. Pick Up Bacteria

Allow the loop to cool for about 10 seconds, then gently dip it into the microbial sample.

3. First Streak

Open the agar plate and swipe the loop back and forth across approximately one‑third of the plate’s surface. Re‑sterilize the loop in the flame.

4. Second Streak

Touch the loop to an uninoculated region of the agar to cool it. Rotate the plate so the inoculated section lies on the left or right, then slide the loop through the inoculated area and across the top third of the remaining surface several times. Re‑sterilize the loop.

5. Third Streak

Cool the loop on a clean area of the agar, rotate the plate another 90°, and drag the loop through the previously inoculated zone before sweeping across the remaining uninoculated area.

6. Incubate the Plate

Place the plate in an incubator (or at room temperature) and wait until visible colonies appear—typically 24 to 48 hours or longer depending on the organism.

7. Transfer the Isolated Bacteria

Re‑sterilize a new inoculating loop. Touch it to a single, well‑formed colony on the plate, then transfer it to an agar slant tube or a tube of nutrient broth.

8. Incubate the Pure Culture

Incubate the slant or broth under the same conditions as before. After sufficient growth, the culture will be a pure bacterial preparation.

Things Needed

• Microbial sample
• Inoculating loop
• Bunsen burner
• Agar plate
• Agar slant tube or nutrient broth

TL;DR (Too Long; Didn’t Read)

Streaking a plate in successive stages dilutes the bacterial load, allowing individual colonies to form. Avoid excessive cross‑contamination between streak zones. Incubation can occur at room temperature, but using a controlled incubator optimizes growth conditions for the specific organism.

Warning

When working with unknown microorganisms, safety is paramount: wear gloves, thoroughly sterilize surfaces after exposure, and ensure the inoculating loop is sterilized before and after each step.

Br
Br
Br