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STREAKING

Purpose

The streaking procedure is used in order to check

1) whether a culture consists of only one organism ( a "pure culture") or if there are more than one organisms in it (contaminated)

2) whether a culture is viable, i.e. able to grow (perhaps a "stock" culture kept for some time in the fridge)

Background

The basic tools used in this procedure include a (sterile) "loop", and a Petri dish containing an appropriate agar, well cooled and set, with a dry surface. The principle is that individual microbial cells can be separated by dragging them over the surface of the agar, and then given a chance to grow into individual colonies

Materials

Petri dish containing (set - fairly firm - dry) nutrient agar.

Wire loops, mounted in metal handle

Inoculum material in broth or other (liquid) medium

Procedure

Clear an area of bench, and wipe it with disinfectant.

Flame a wire loop, bringing it all to red heat, and leave it upright in a rack to cool.

(Optional - repeat to get several cooled loops)

streaking pattern

1) Using the loop, take a drop of the liquid culture medium (broth) provided and spread it carefully in a line across the surface of the agar as shown. With the same loop, a second, third and fourth line may be drawn parallel to the first. Close the lid of the Petri dish immediately.

2) Sterilise the loop in the flame once again, and allow it to cool.

3) Turn the Petri dish so that the end of the previous lines can be the start of the next ones.

4) Take a cooled loop and make 2 or 3 strokes as before. Close the lid of the Petri dish immediately.

Repeat 2,3,4 until there is no more space round the edge (4 or 5 times), then finish off with a single zigzag streak across the middle.

Seal and label the Petri dish with the culture reference and your name and the date. Place it in an inverted position in the incubator at an appropriate temperature.

Results

After an appropriate time:

Examine the Petri dish WITHOUT OPENING IT and look for individual colonies. Draw it.

space to draw your own plate

Has the technique caused the micro-organisms in the culture to become separated?

Was the original culture viable?

Was the original culture a pure culture?

How would you obtain a pure culture from your plate?

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