SEPARATING BACTERIA: THE STREAK PLATE TECHNIQUE

Pure cultures (cultures in which only one kind of microorganism is growing) are peculiar to the microbiology laboratory. In Nature, whether in the soil, in a pond, on the surface of your skin, or in your gastrointestinal tract, many different kinds of microbes grow together. However, to study or identify a particular microbe, one must usually separate it from the others and grow it in pure culture. The most useful technique for separating bacteria is called the streak plate technique.

This technique is based on the principle of dilution. The aim is to dilute the bacteria to the point where individual bacteria are separated on a Petri plate. Each bacterium then multiplies to form colonies which are also separated from each other and are big enough to see and to work with.

Suppose you have a liquid medium with millions of bacteria in each milliliter and you stick in your loop and pick up a drop with 100,000 bacteria in it. You spread these bacteria over about a third of the Petri plate; instead of 100,000 bacteria in a tiny drop, you have 100,000 spread over a square inch or so of space. They have been diluted. After flaming your loop to destroy any bacteria that might still be on it, you cool your loop and drag it over a section of the Petri plate covered with bacteria and pick up maybe 5,000. These you spread over another section of the Petri plate. As you drag your loop, there are fewer and fewer bacteria left to fall off the loop. After flaming your loop again, you drag the loop over these last few "streaks" and pick up maybe only 20 bacteria. These you spread over the last section of the plate and leave on the agar.

Looking at a freshly streaked plate, you may see some bacteria where you started streaking where the bacteria are in the tens of thousands, otherwise you may see only light marks where your loop went. After incubating your plate and allowing the bacteria to grow, you see their millions of descendants, all bunched together where you started, but separate, isolated colonies coming up where your last set of streaks was.

Your instructor will demonstrate this technique to you with more detailed explanations. You will be given a Petri plate and asked to do your own. Question: What are some characteristics of bacteria that might make them more difficult to isolate this way?