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Over the next few sessions in lab you will be learning how to isolate bacterial colonies into individual species. This allows for the creation of a pure culture, a culture of bacteria containing only one type of microorganism. These pure cultures are important to microbiologists as they allow for the study of one species without the worry of contamination from other organisms. Obtaining a pure culture of bacteria is usually accomplished by spreading bacteria on the surface of a solid medium so that a single cell occupies an isolated portion of the agar surface. This single cell will go through repeated multiplication to produce a visible colony of similar cells, or clones. There are three methods commonly used to derive a pure culture:
1. Spread plate – the original culture is diluted serially and a small volume of the final dilution is spread on the surface of an agar plate.
2. Pour plate –the original culture is diluted serially and a small volume of the final dilution is added to molten agar which is poured over an agar plate and allowed to harden. Colonies develop sub-surface.
3. Streak plate – the original culture is directly diluted across an agar surface using an inoculating loop. This is a simple & rapid method.
All of these methods dilute or “thin out” a heavy population of bacteria across an agar surface. In this lab we will learn to streak a plate with a mixed culture containing more than one bacterial species. Since most bacterial samples encountered by a microbiologist (in the clinic, environment, industry, etc.) are mixed cultures, this is a very important microbiological technique. If this procedure is performed correctly, a number of isolated colonies will grow that will be a source of pure bacterial cultures.
There are many variations of the streaking technique, but in this lab we will use the triplet streak as described here. Be sure to note the diagrams and description in the procedure video.
T-streak : Smaller, yellower colonies are Micrococcus luteus Larger, whiter colonies are Staphylococcus aureus