Contact-Dependent Assay. Predator and prey were mixed and treated as described in the killing assay at anMOIof 10 with the addition of a sterile 0.22-μm filter (Millipore). The filter was placed either directly on the LB-agar plate or on top of the predator cells. Prey were spotted on top of the filter. After 4 h incubation, cells were harvested by suspending the filter in 1 mL LB and vortexing for 10 s. The cfu per milliliter of surviving prey were determined by serial dilution and growth on selective media as described above.