8.4.2 Membrane filtration
The membrane filtration (MF) technique is based on the entrapment of the bacterial cells by a membrane filter (pore size of 0.45 µm) (Figure 8.3). After the water is filtered, the membrane is placed on an appropriate medium and incubated (Tables 8.5, 8.6 and 8.7). Discrete colonies with typical appearance are counted after 24-48 hours, and the population density of the target bacteria, usually described as cfu per 100 ml in the original sample, can be calculated from the filtered volumes and dilutions used. This technique is more precise than the MPN technique, but the MF test can only be used for low-turbidity waters with low concentrations of background micro-organisms.
Figure 8.3 Preparation of dilution series and procedure for the membrane filtration method
The advantages of the MF technique include (Fujioka, 1997):
Savings in terms of time, labour, and cost compared with the MPN technique.
Direct determination of the concentrations of bacteria with high precision and accuracy.
The formation of the target bacteria as colonies which can be purified for further
identification and characterisation.
The ability to process large volumes of water samples to increase greatly the sensitivity
of this method.