STREAK PLATING

Streak plating spreads bacteria across an agar plate in a pattern that thins them out step by step. By the last section, single cells are far enough apart to grow into separate colonies, each one a pure population that started from one cell.

WHAT YOU NEED?

• Agar plate (plain or selective)

• Source culture or colony

• Sterile inoculation loops (or a metal loop and a Bunsen burner)

• Marker

STEPS

1. Label the bottom of the plate with the sample name, medium and date.

2. Using a sterile loop, pick up a small amount of bacteria from a single colony or culture.

3. Lift the lid slightly and streak back and forth across one edge of the plate (section 1).

4. Take a fresh sterile loop (or flame and cool a metal loop). Drag it once or twice through section 1, then streak into section 2.

5. Repeat for section 3 and section 4 from a single loop, each time starting from the end of the previous section.

6. Close the lid, let the plate sit for a few minutes, then turn it upside down.

7. Incubate upside down overnight. Isolated colonies should appear in the last sections.

TIPS

• Glide the loop lightly over the surface; don’t dig into the agar.

• Always use a new loop between samples to avoid cross-contamination.

• Incubating plates upside down stops condensation dripping onto the agar.