The sterilizing filter concept
Until rather recently, it was believed that the sterilization of liquids could be achieved by their filtration through a sterilizing
membrane whose proper and pertinent identity was confirmed by its pore-size rating, which was itself determined by integrity
testing. The pore-size rating, plotted against organism size, indicated an inverse correlation. However arranged, organism
removal by a filter was seen as resulting from the mechanism of size exclusion, also known as sieve retention, wherein the
organism is retained because it is larger in size and shape than the filter's pores. The sizes of various organisms are available
from listings in the literature, and the size of a filter's largest pores derives from its bubble-point value. Therefore,
it was possible to accommodate the size of the restraining pore to the size of the organism to be retained. In the interest
of obtaining a maximum flow rate consonant with complete organism removal, the 0.2/0.22-μm-rated membranes were widely accepted
as being sterilizing filters.
The sterilizing filter was defined in 1987 by the US Food and Drug Administration on the basis of its retaining a minimum
of 1 × 107 colony-forming units (cfu) of Brevundimonas diminuta (at that time taxonomically identified as Pseudomonas diminuta) per square centimeter of effective filtration area (5).
The situation is complicated in its application by the absence of pore-size rating standards. The pore sizes may not be assumed
to have been rated in the same fashion by the various filter manufacturers. Thus, while filters may bear the same pore-size
designation, they may not be identical in this regard. Nevertheless, such filters may prove interchangeable in filtration
processing operations. Manipulative adaptations often make it possible to substitute one such filter for another.
Although the individual rating methods differ, bubble-point measurements performed in as similar a manner as possible enable
comparisons to be made among the various types of filters. Given the importance of gauging the sizes of the largest pores,
the integrity testing should be as accurate and reproducible as possible. Accordingly, using automated integrity test instruments
is recommended, because their use eliminates the subjectivity inherent in manual integrity testing (6). For the same reason,
the bubble point should be identified by the straight line plotted from multipoint diffusion measurements extended through
and beyond it to a robust flow of air. Single-point diffusive airflow measurements are not sufficiently reliable for this
Limitations of the B. diminuta model
Advantage of the model organism
There are numerous types and sizes of microorganisms. To explore the sterilizing capabilities of the available filters with
each of these different organisms would not be practical and might not even be meaningful from the standpoint of assessing
patient risk. Given the large number of organism types and their differences in size, it would be advantageous to select for
testing an organism that could serve as a model for all the others, or at least for those commonly encountered in pharmaceutical
operations. The smaller the test organism, the more its removal by a filter would ensure the sieve retention of larger organisms.
Properties other than size, however, also are important in the selection of the model microbe. Ease and safety in cultivation
and handling are real considerations. The likelihood of the selected organisms' being encountered in pharmaceutical operations
is another influence on its selection.