The Streaking Project
Historically, microbiologists used broths as the most common media for growing bacteria in the laboratory. If the desired a firm surface for cultivating microorganisms they might use an aseptically obtained piece of potato or carrot. Previous experiments in this lab have shown you that there are a vast number of microbes in most of the environments we've sampled. If you wished to identify just one organism out of these microbial cocktails you would have a very difficult time if you only had broths to work with. (Although it is possible with the serial dilution techniques we'll be covering a few weeks from now.)
It wasn't until the 1880's that Robert Koch's lab discovered how to turn broth media into solid media by adding agar. One advantage of agar was that no organisms of medical importance could digest it. The use of gelatin as a solidifying agent had been discarded precisely because so many bacteria could break it down, hence turning a solid medium into a liquid on. This is also a reason why potatoes and carrots weren't too useful either. Another characteristic of agar was that it melted at 100o C yet solidified at 45o C. How might this be advantageous? Gelatin, in contrast, melts and solidifies at the same temperature, about 35o C. Agar also is a gel which allows the free diffusion of water and other soluble nutrients.
Streaking for isolation takes advantage of the characteristics of agar and solid media and allows us to physically separate individual bacterial cells so that they each can grow up into discrete, isolated colonies. An isolated colony thus represents a "pure culture," a visible mass of genetically identical bacterial cells which originated from one "mother cell." Once you have obtained isolated colonies you can then identify and study the characteristics of that pure culture. Streaking for isolation, therefore, is a most fundamental technique important to the sucess of most microbiological investigations.
The purpose of the streaking project is to give you plenty of practice learning this vital technique. Every two weeks you will hand in a streaked plate. After an appropriate period of incubation the plate will be graded on the level of isolation achieved.
Procedure