Sterilizing grade membrane filters

The earliest commercially available filters were manufactured in two pore sizes 0.45 and 0.8 pm. The 0.45 pm-rated membranes were considered to be sterilizing-grade filters and were successfully used in the sterile filtration of pharmaceuticals and parenterals. The membrane filters were qualified using Serratia marcescens, a standard bacterium, having dimensions of 0.6 x 1 pm. However, in the late 1960s it became apparent that the matrix of the 0.45 pm-rated filters could be penetrated by some pseudomonad-like organisms (1). For sterile filtration applications in the 1990s, 0.2 pm-rated membranes are the industry standard in the manufacture of sterile parenterals and pharmaceuticals. 

Although Brevundimonas (Pseudomonas) diminuta (ATCC 19146) is most commonly used for sterilizing-grade filter validation, in certain applications other bacteria are used. For example, when it is necessary to demonstrate removal of mycoplasma in applications involving sera and tissue culture media, membranes having a smaller pore size rating, eg, 0.1 Jim, are frequendy used. For these membranes, Picholeplasma laidlawii may be employed for validation purposes (9). 

Most membranes are made of porous cellulose acetate and are available in a range of porosities, from 0.22 pm to several micrometers. Sterilization and the complete removal of the smallest microorganisms (bacteria) requires the use of sterile membranes of porosities of 0.45 pm or less. Filter housings and clamps should be packaged in aluminum foil and then heavy-grade wrapping paper (butcher paper) prior to autoclaving and not opened until use. Alternatively, suppliers make disposable filter units that facilitate sterilization of small volumes of media. 

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