To perform the procedure, a total of 0.5 to 1.0 × 1010 bac- teria growing in 100–200 ml of minimal defined medium are fil- tered slowly (1–2 min) under vacuum onto the membrane filter at the appropriate temperature in a warm room or any convenient table-top incubator (Figure 4A). The entire filter holder is then inverted (Figure 4B), culture medium is poured into the top of the inverted holder (about 300 ml), and tubing from a reservoir of medium is connected to the stem of the funnel with a stop- per. To produce a sealed system, the tubing should be attached to a narrow glass or rigid plastic tube installed in a hole bored through a stopper of appropriate size, as shown. It is helpful to have most of the stem of the funnel cut off, as indicated in the figure, to ease pouring of the medium and connection of the stop- per. A peristaltic pump can be used to regulate the rate of medium flow into the apparatus after inversion. It should be set at about 15 ml/min for a few minutes to flush off weakly attached cells and then reduced to 2 ml/min thereafter. Alternatively, if you wish to perform tests before obtaining a pump, a reservoir bottle witha bottom outlet can be placed above the apparatus with a hose clamp acting as the flow regulator. In either case, once medium flow begins, the underside of the membrane will become con- vex with drops coming from the center as seen in Figure 4B. The smooth, convex shape of the membrane is VERY important because it enables the medium to flow uniformly over the surface of the attached cells, and to form drops that fall exclusively from a single point at the center of the membrane.