Spreading refers to a method of distributing microorganisms evenly over the surface of an agar plate. A small volume of microbial culture is spread evenly over the agar surface using a sterile spreader.

There are many different techniques for spreading. Some techniques use sterile glass beads that can be rolled over the agar surface to distribute cells. Often specialized tools called spreaders are used. The image below shows a disposable plastic spreader and a reusable metal spreader.

The following is an overview of the spreading procedure using a reusable glass spreader:

  • A glass spreader should be sterilized using a Bunsen burner flame. First, dip the glass spreader in alcohol (70% ethanol), shake off the excess alcohol, and ignite the residue. Then, allow the spreader to cool.
  • Spread the microbial culture evenly on the surface of the media plate. Pressure should be applied evenly as the glass spreads along the media surface.
  • Allow the microbial culture to be absorbed by the media plate.
  • Incubate the plate up-side down to avoid water droplets (condensation) forming on the plate.

Spreading is used to isolate single cells of highly diluted cultures such as transformed cells after cloning. After spreading the cells on the agar plate, each cell will produce colonies of clones. If the cell solution contains too many cells, it is not possible to isolate single colonies. In this case, the streaking method should be used to isolate single colonies.

It is important to pick a single colony to isolate pure genetic clones. A colony is a group of cells that originated from the same source; therefore, all the cells in a single colony contain identical genetic information.