Membrane Filter Technique

Membrane filters have a known uniform porosity of predetermined size sufficiently small to trap microorganisms. Membrane filters with a 0.45 µm pore size have long been recognized as the standard for growth of 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 facilitates the growth of organisms on the upper surface of the membrane. The discrete colonies that form on the surface of the membrane can be easily transferred to confirmation media. Special media and dyes can be used to make it easier to detect certain types of organisms than with the conventional plate count. During incubation the organisms grow into colonies which appear on the membrane surface.
The 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.
 Advantage of Membrane Filter Technique
  • Permits testing of large sample volumes.
  • Reduces preparation time as compared to many traditional methods.
  • Allows isolation and enumeration of discrete colonies of bacteria.
  • Provides presence or absence information within 24 hours.
  • Effective and acceptable technique to monitor drinking water.
  • Useful for bacterial monitoring in the pharmaceutical, cosmetics, electronics, and food and beverage industries.
  • Allows for removal of bacteriostatic or cidal agents that would not be removed in Pour Plate, Spread Plate, or MPN techniques.
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.

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