Membrane Filter Technique for Bacteriological Examination of Water

Membrane filters have a known uniform porosity of predetermined size (generally 0.45 µm ) sufficiently small to trap microorganisms.Using the membrane filter technique, sample is passed through the membrane using a filter funnel and vacuum system. Any organisms in the sample are concentrated on the surface of the membrane.  The membrane, with its trapped bacteria, is then placed in a special plate containing a pad saturated with the appropriate medium. The passage of nutrients through the filter during incubation facilitates the growth of organisms in the form of colonies, on the upper surface of the membrane. Discrete colonies thus formed  can be easily transferred to confirmation media.  
Membrane filter technique is an effective, accepted technique for testing fluid samples for microbiological contamination. It involves less preparation than many traditional methods, and is one of a few methods that will allow the isolation and enumeration of microorganisms.  Membrane filters are used extensively in the laboratory and in industry to sterilize fluid materials.
Step-by-step Procedures
  1. Collect the sample and make any necessary dilutions.
  2. Select the appropriate nutrient or culture medium. Dispense the broth into a sterile Petri dish, evenly saturating the absorbent pad.
  3. Flame the forceps, and remove the membrane from the sterile package.
  4. Place the membrane filter into the funnel assembly.
  5. Flame the pouring lip of the sample container and pour the sample into the funnel.
  6. Turn on the vacuum and allow the sample to draw completely through the filter.
  7. Rinse funnel with sterile buffered water. Turn on vacuum and allow the liquid to draw completely through the filter.
  8. Flame the forceps and remove the membrane filter from the funnel.
  9. Place the membrane filter into the prepared Petri dish.
  10. Incubate at the proper temperature and for the appropriate time period.
  11. Count and confirm the colonies and report the results.

Advantage of Membrane Filter Technique

  • Permits testing of large sample volumes. Theoretically almost any volumes of nontubrid water could be filtered through the disk, the organisms from any given volume being deposited in the disk.
  • The membrane can be transferred from one medium to another for purposes of selection or differentiation of organisms thus allowing isolation and enumeration of discrete colonies of bacteria.
  • Results can be obtained more rapidly than by the conventional MPN standard methods. It provides presence or absence information within 24 hours.

Uses of Memberane filters

  1. Membrane filters are used extensively in the laboratory and in industry to sterilize heat labile fluid materials.
  2. Effective and acceptable technique to monitor drinking water.
  3. Useful for bacterial monitoring in the pharmaceutical, cosmetics, electronics, and food and beverage industries.
  4. Allows for removal of bacteriostatic or cidal agents that would not be removed in pour plate, spread plate, or MPN techniques.
About tankeshwar 378 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion, I am working as a Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.


  1. I would be happy if you give information about various pore sizes for filtration of virus, fungi, bacteria.

  2. please i need to know the detailed microbial principle that is involved in the vacuum filtration for testing for Escherichia coli

  3. I was taught that after filtering, say E. Coli or K. Pneumoniae through a MF unit using a 0.45um membrane filter that it was safe to dispose of the filtered water down the drain. Is this true?

    • Tracy
      Filtration using Membrane Filter is not the recommended method for the waste water treatment. “I did not understand what do you mean by “safe to dispose”? There are other parameters (chemical pollution) that are equally important as microbial contamination.

  4. For samples with high coliforms counts eg raw and primary treated water from sewage treatment plant which method is preferable- membrane filtration of diluted sample or direct spreading/ pour plating of undiluted/ less diluted sample?

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