f. Rinse down funnel walls with at least 35 ml of sterile buffered dilution water
for each rinse (see figure 3-7) after all the sample has passed through the membrane
filter. Repeat the rinse twice after all the first rinse to has passed through the filter.
Since the funnel is not routinely sterilized between successive filtration of a series of
samples, minute droplets must be removed from the funnel walls to prevent
Figure 3-7. Rinse down funnel walls.
Cut off suction on the filter assembly as soon as all the water has been filtered for any
sample. Continued suction with no sample may introduce unnecessary airborne
g. Remove the funnel from the filter holder.
h. Use sterile forceps to remove the membrane filter from the filter holder (see
figure 3-8). Carefully place it with the grid side up on the appropriate pad, which is
saturated with culture medium, but take care not to trap air bubbles between it and the
pad (see figure 3-9).
Then close the petri dish. Air bubbles interfere with the diffusion of the culture
medium through the membrane filter. Entrapment of air bubbles can be
minimized by having adequate culture medium on the absorbent pad and by
rolling the membrane filter into position. If necessary, reseat the filter on the
absorbent pad to eliminate bubbles.
Figure 3-8. Remove membrane filter from filter holder.