Water Analysis- Membrane Filter Method

2. Membrane Filter Method

The membrane filtration method is a direct method of determining the presence and numbers of bacteria.  It makes use of a filtration apparatus which separates bacteria from the water. The bacteria collected on the surface of a removable membrane filter are placed on an appropriate medium and incubated. Colonies have a distinctive appearance and are counted. This method is suitable for low-turbidity waters that do not clog the filter and have relatively few  bacteria that would interfere with the results. 

The membrane filter technique involves filtering a known volume (100 ml for drinking water samples) of water through a special sterile filter. These filters are made of nitrocellulose acetate or polycarbonate and have 0.45 µm diameter pores. A grid pattern is typically printed on these filter disks to facilitate colony counting. When the water sample is filtered, bacteria (larger than 0.45 µm) in the sample are trapped on the surface of the filter. The filter is then carefully removed, placed in a sterile petri plate (on a pad saturated with a liquid or agar-based medium,) containing appropriate medium and incubated for 24 hours at 35-37°C.

 Each bacterium trapped on the filter will grow into a separate colony. By counting the colonies one can directly determine the number of bacteria in the water sample that was filtered. 

Advantages

  §  Simplicity

  § shorter time needed to complete the test (1 day vs. 3 to 4 days in MPN)

  § direct test; not statistical - higher accuracy in counting

Disadvantages

  §  Particulate samples (containing silt or other organic matter) quickly clog the filter

  § Metals and phenols can stick to the filter inhibiting growth of microrganisms (filtration concentrates them on the filter)

Specific Coliform Count:

   Using specific media - EMB/Endo agar for coliforms

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