Streaking for Single Colonies: The Streak Plate Challenge

By Jennifer Tsang

Streaking for single colonies is an integral part of any bacteriologist’s skill set. So when Dave Westenberg taught this concept in his microbiology lab course, he decided to add a bit of fun. He mixed together 10 E. coli strains producing different pigments, and tasked the students to separate out colonies from each individual strain using the streak plate technique.

Westenberg, a Professor of Biological Sciences at Missouri University of Science and Technology, has been teaching the microbiology lab every year for over 20 years. Previously, the lab used three different species for the challenge, but one species always outcompeted the other species. 

When he found the chromoprotein plasmids available from Addgene, he knew it would be a perfect fit for the streak plate challenge. Chromoproteins are a subset of the fluorescent protein family that give off color in ambient light. Expressing these chromoproteins in E. coli would make it easy to distinguish colonies from one another in the challenge.

To create a successful streak plate, first the bacteria are inoculated over a quadrant of an agar plate. Then, a stick is dragged through the first streak and spread over the second quadrant. The process continues until all quadrants are used, hopefully revealing single, isolated colonies in the last quadrant after incubation.

The challenge is usually successful for the students. “They always get a few isolated colonies. I should probably make it a bit more challenging, potentially by using a thicker bacterial suspension,” says Westenberg.

In addition to streaking for single colonies, these chromoprotein-expressing strains are easily amenable to other experiments to teach other aspects of molecular biology. “What I'd like to do with the plasmids from Addgene is have the students transform them into different strains of E. coli,” says Westenberg. During the transformation experiment, he plans to show how different genetic backgrounds of the host strains affect transformation efficiency. 

Aside from the Streak Plate Challenge, Westenberg’s course includes Winogradsky columns and characterizing the microbes from the termite hindgut.

Want to level up your or your class’s streak plate skills? Find the chromoprotein expressing plasmids here, and check out our streak plate protocol video on Youtube:



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Topics: Fluorescent Proteins, Education

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