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Plate Spreading

Shown on the right is a device that is used by bacteriologists to spread a small amount of liquid uniformly over the agar surface in a petri plate. If, for example, the 0.1 ml of liquid were to contain 231 bacteria in it, those bacteria would be distributed across the length and breadth of the agar surface and grow up to form 231 colonies. Thus each bacterium grew up to form a colony. Thus living bacteria are called "colony-forming units" or CFU.

Labs usually make their own spreaders. Most commonly they are made of bent glass - either of glass rods or disposable pipets. Suggested is a less breakable type made of bent stainless steel rod. It is important that the portion that rests on the agar surface be straight. If you make yours of bent glass, DO NOT fire-polish that end of the glass because doing so will cause a round bead of glass that will not allow the straight portions of the rod near the bead to touch the agar surface. Similarly, if you cut yours from metal rod, be sure to file that end to a good bevel. Of course, if you make yours of metal rod, stainless steel is the best because it will neither rust nor corrode as will steel or copper.

To use the spreader:

1. Your plates should have been previously dried so that the spread liquid will soak in within minutes leaving the bacteria high and dry of the surface to grow into isolated colonies.
2. You will start with setting up a beaker of some sort of flammable alcohol, and into that alcohol you set your spreader for sterilization.
3. Pipet your 0.1 ml of bacterial suspension onto the middle of the agar surface.
4. Lift the spreader out of the alcohol, and pass the spreader rapidly through a flame. DO NOT hold the spreader in the fire. All you want to do is have the flame burn off any alcohol clinging to the spreader. You do NOT want to heat the spreader. If you do, you will sterilize the droplet on the plate and you will get a result of zero!
5. Place the spreader on the agar surface and use is to push and spread the liquid all over the surface. Some labs are wealthy enough to have small turntables upon which the plate spins and the spreader needs only to be pushed back and forth.
6. Once you have finished spreading for 5 to 10 seconds, lift the spreader away and replace the cover on the plate. Put the spreader back in the alcohol.

Here is a trick that will save you a little time, and ensure that your spreader is not too hot. When you are spreading a set of plates from a serial dilution / enumeration experiment, place the measured amounts of liquids onto upto five plates and then spread them starting with the plate that has received the most dilute sample. You may spread them all without resterilizing and reflaming the spreader IF if you go from most dilute towards the more concentrated. Thus an overheated spreader would be cooled on the first plate. (Likewise, when you are transferring the dilutions onto the agar surfaces, you may use the same pipetter if you always go from the most dilute to the most concentrated samples in your serial dilution.

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