The membrane filter technique for estimating numbers of viable bacteria: some observed limitations with certain species.

The membrane filter, also known as molecular filter, is valuable for its ability to permit passage of large volumes of fluid and yet retain on its surface particles of bacterial size. The history of its development and some of its properties have been reviewed by Goetz and Tsuneishi (1951). Other properties and specifications are available from the manufacturers.3 At present details of manufacture are not available to the public. While use of the membrane filter (MF) has not been restricted to the field of bacteriology, this has been its chief area of application. Bacterial cells deposited from liquid or air suspension on the upper surface of the MF are able in many instances to grow to individual, discrete colonies when the filter is placed on a suitable nutrient surface such as an agar, or a blotting paper saturated with a liquid medium. Successful use of the MF has depended on the development of suitable nutrient media and this has progressed most rapidly in the field of sanitary bacteriology. Aside from the coliforms and enterococci, growth of several other organisms has been described. Rogers et al. (1955) and Morgante and Murray (1955) reported on cultivation of the tubercle organism; Orlando and Bolduan (1953) demonstrated growth of a variety of bacteria and fungi imperfecti on the MF. Gaspar and Leise (1955, 1956) studied growth of Brucella melitensis and Bacterium tularense