Phage Discovery Guide: Spot Tests

A good time to test for the presence of bacteriophage in your sample using this method is after you have collected your enriched isolation samples.  Rather than performing a plaque assay using serial dilutions, a spot test can be performed with just a single plate.

The spot test can be used as a yes or no assay.  Is there bacteriophage in my sample?  A variation of this assay can also be used to estimate the bacteriophage titer in the sample.  A sample that is known to have bacteriophage can be serially diluted and then spots of each dilution can be made “spotted” onto the plate.  (Remember to note the volume of sample spotted onto the plate and factor it into your titer calculation.)

Major benefit:

  • Using the spot test, you can screen several of your samples (or dilutions of a sample) for bacteriophage.

Potential problems:

  • Spots should be well-spaced so that the samples don’t flow into each other.
  • Inverting the plate for incubation can be problematic if the samples haven’t fully been absorbed into the top agar.  (Gravity will make spots move on your top agar surface.)

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