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In dilution streaking a drop of liquid culture is collected with an inoculating loop and then streaked across the agar plate surface. At the end of the streaking the number of organisms clinging to the loop until only single cells are deposited at a given location and you get colony-forming units.

An agar plate with streak purification

I am wondering now what exactly the purpose of this streak purification is. If you want to grow your bacteria why would you not just put a few drops of the liquid culture on the agar plate? Why is it particularly useful to use this technique? Or, why would you choose this method over the "spread plate" technique, where a liquid culture undergoes a set of dilutions first and each dilution is then placed directly on the surface of separate agar plates.

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The purpose of this technique is to dilute the bacterial cells so much that you get single cells, as can be seen on the image of the plate. You can then pick the colonies grown from these single cells, characterize them, test them for the mutations/plasmids, or whatever change you introduced, and can be sure that you have only the descendants of a single cell in your culture.

This way you ensure that you don't grow a mixture of different cells with different characteristics, of which one will eventually overgrow the other one. The streaking is a fast and reliable technique, which works especially well when you want to test a larger number of bacterial cultures. It also allows to check a stock culture very fast for contaminations with other bacteria by simply inspecting the single colonies by eye (different bacteria often look different).

Diluting the cultures would work as well, but needs to be done in more than one step and is more work when you want to do it for more than one culture. Also for dilutions you often plate more than one plate to be sure to have a plate with single colonies (when the cell number is unknown).

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  • $\begingroup$ Maybe you were also implying this, but it's a fast method to see if your stock contains one strain or multiple (assuming you can distinguish them by eye). $\endgroup$ – VonBeche Oct 19 '16 at 8:29
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    $\begingroup$ And if you don't know how many cells you have you'd also use more than one plate when you plate dilutions. $\endgroup$ – VonBeche Oct 19 '16 at 8:31
  • $\begingroup$ @VonBeche Good points, I added them. $\endgroup$ – Chris Oct 19 '16 at 8:59
  • $\begingroup$ Great answer, I loved this technique as it is so quick to quickly get single-cells/mono-clones. One can not practically do serial dilutions when you have 10s of cultures or cell types to go through. $\endgroup$ – ktyagi Oct 19 '16 at 9:31

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