Field of Science

Did the cells attach to the DNA-coated beads?

Yesterday I scored the results of Wednesday's test.  I had incubated competent H. influenzae cells with magnetic beads, using both DNA-coated beads and control beads that had gone through the same treatment without DNA.  Then I had washed the unattached cells away from the beads, taking samples for plating at every step so I could track the progress of the three washes.  I added DNase I to the beads after the last wash, so I'd be counting the total number of attached cells rather than the number of beads that had cells on them.  Then I plated all the samples, and later counted the colonies.

There were ten times more cells on the DNA-coated beads than on the control beads (about 10^5 vs 10^4).  This is good, but because I used about 10^8 beads, it means that, at best, only about one bead in 1000 had a cell attached.  I hadn't measured how much DNA was on these beads, lazily hoping that they had as much DNA as the similarly treated polystyrene beads.  Now I need to repeat the experiment with better-characterized beads.

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