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Membrane Filter Applications
Date: 2016-10-18

The Membrane Filter Technique
The Membrane Filter Technique (MF Technique) offers the advantage of isolating discrete colonies of bacteria, whereas the MPN procedure only indicates the presence or absence of an approximate number or organisms.The concentration of larger samples on a membrane filter is a key benefit of the technique over the MPN procedure as well as over Pour Plate and Spread Plate techniques.

Many industry and U.S. EPA test procedures require samples of 100 mL or more to be analyzed for the presence of bacteria. The Pour Plate technique is limited to a sample volume of 2 mL and the Spread Plate technique has a limit of 0.5 mL.

Using the Membrane Filter Technique, a 100 mL sample is passed through a 47 mm membrane using a filter funnel and vacuum system. Any organisms in the sample are concentrated on the surface of the membrane. The filter is then placed in a petri dish with nutrient 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.

Municipal water treatment plants monitor drinking, waste, and surface water for the presence of coliform bacteria by the MF Technique. The key organism monitored in water treatment facilities is E. coli. The U.S. EPA considers this organism the leading indicator of fecal contamination.

In addition to its use by government labs for monitoring drinking water, the Membrane Filter Technique is also used for microbial monitoring in the pharmaceutical, cosmetics, electronics, and food and beverage industries. The MF Technique is used in these industrial labs to monitor the presence of microorganisms in process waters and final product.

The pharmaceutical and cosmetics industries typically focus on monitoring their process water for Pseudomonas species. The electronics industry monitors for any and all microorganisms because they must keep their process water free from even the smallest organisms. Microbial monitoring in the food and beverage industry typically employs several types of techniques because of the variety of samples that are encountered. Beverage samples can typically be monitored for microorganisms by the MF Technique, but when solid samples cannot be liquefied, alternative methods must be used.

 

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