THE VIRTUAL EDGE: Lab 6 Cultivation of Bacteria II

Streak Plate

As mentioned in the previous lab, pure cultures are essential for microbiological studies.  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 and 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.  The spread plate technique was used in lab #5 to obtain isolated colonies.  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.

Streak plate - Triplet Streak.  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 of the technique on the following page before starting with your streak plate.

Rachel Watson, M.S.
AG 5010
Cell: 307-760-2942

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