Aseptic Technique: Streaking to Single Colonies

Introduction

Aseptic technique is essential to anyone working with microbes. It ensures that everything that touches your culture is sterile, so you avoid introducing contaminants into your culture. With pure cultures, contaminants introduce an incalculable number of variables, making it impossible to know if your results are attributable to the conditions you set for the experiment or the contaminant. It makes all your hard work, in a word, useless.

Streaking to single colonies can help you obtain a pure culture. Each individual colony - circle of growth - is the result of a single cell dividing into millions of clones. Thus, each cell within that colony is as genetically identical as you're ever going to get.

Materials

sterile agar plate
inoculation loop

• You can make your own by twisting a piece of metal wire (platinum is best, but aluminum is doable).

Bunsen burner
culture

• If you want to use the Penicillium culture featured in the video above, check here. 

Procedure

1. Turn on the Bunsen burner. The flame will heat the air above your work space, lifting the air and any contaminants that might have been disturbed.

2. Sterilize your inoculation loop. Hold the loop in the flame until it becomes red-hot. Then press it to the sterile surface of your agar. You will hear a hiss, indicating that that the loop is cool enough to touch your culture without singing it. Make sure the agar plate is uncovered no longer than necessary; this minimizes the risk of contamination.

3. Inoculate. Touch the inoculation loop to your culture, then touch the loop to the surface of the agar plate to make three parallel lines (again, make sure your agar plate is uncovered only as long as necessary). Sterilize your loop, touch it to an uninoculated part of the agar until you hear a hiss, then drag your loop through the end of the lines you just made. Make two more lines parallel to the last line, but not intersecting your previous three lines (this helps you "dilute" your inoculum). Sterilize your loop again, cool it with the sterile agar, then drag a line through the ending edge of your second set of three lines. If you do this until you have four sets of three lines, you will have single colonies.