Blocking with BSA makes a big difference

The streptavidin-coated magnetic beads that hadn't been blocked with BSA aggregated into big clumps when mixed with cells, but the treated beads remained separate.  The clumping was independent of whether the beads had been incubated with biotinylated DNA, so I think the clumping was because most cells stuck to more than one bead and most beads stuck to more than one cell. 

So then I checked the streptavidin-coated polystyrene beads I'll use for the tweezers experiments.  Luckily adding cells didn't make them clump.

Now I'm making fresh batches of competent H. influenzae and B. subtilis to freeze, so I'll have consistent cells to work with.