Limitations on the use of the B. diminuta model

Given the complexity of the organism removal operation, it is doubtful whether a universal sterilizing filter can be devised. Certainly, there is no known absolute filter, one that will retain all organisms under all conditions, especially if viruses are included. Therefore, the successful attainment of a sterile filtration with regard to specified organisms of interest must in every individual filtration sterilization be corroborated by the documented experimental evidence that constitutes validation. The specifics of such operations are discussed in the Parenteral Drug Association's "Technical Report No. 26." Adherence to the teachings of this report is a second factor that is essential to attaining sterile effluent.

Obviously, the many organisms whose presence in pharmaceutical contexts deserves consideration may differ from one another so greatly as to compel individual techniques for their identification and cultivation. Conclusions cannot be made regarding the sterile filtration of microorganisms unless methods of quantifying them by culturing and counting are available. Organisms such as the L-forms, nanobacteria, and "viable but nonculturable" entities may not be amenable to such analyses. Concerns about their presence may be justified, but without the means to cultivate and count them, it is impossible to attest to their complete absence. It follows that a sterilizing filter can be judged only by its performance in the removal of identifiable and culturable organisms known to be present in the drug preparation (18).

It was not, nor is it as yet, sufficiently recognized within the pharmaceutical industry that organisms penetrating the 0.2/0.22-μm-rated membranes were likely to have undergone size reductions occasioned by exposures to particular drug preparations (Ralstonia pickettii, Burkholderia cepacia, and B. diminuta in particular). The dependability of the exclusion mechanism was predicated on the stability of the organism and pore sizes as measured in certain specific situations. It does require modification when organisms or pore sizes do not remain constant. Each may undergo change upon contact with particular drugs. At present, little is known about the kinetics of these size alterations.

Summary and conclusion

• There is no universal sterilizing filter. Sterile effluent results from a combination of many influences, not solely from the use of filters of any particular pore size. Each filtration is an independent event that arises from a proper combination of several stated requirements.
• Sterility must be defined in terms of specific organisms that are targeted in a given filtration. The ability to identify and cultivate the organism of interest is indispensable.
• The role of B. diminuta as the model organism likely to represent the bioburden of the drug preparations presented for aseptic processing by sterilizing filtration is limited to those microbes that do not undergo shrinkage upon contact with the drug preparations. Also, it is clear from considerable experience that risks arising from the use of the B. diminuta model are small enough in the vast majority of cases to be considered completely insignificant to public health.
• Choosing an actual bioburden organism to replace B. diminuta as a filtration model is unlikely to improve the situation. The substituting organism will replace the present problems with those peculiar to itself. It is questionable whether a universal model is at all possible.
• B. diminuta has served its model role adequately, except for those organisms like itself that undergo size diminution during their cultivation or that manifest size shrinkage through contact with drug preparations that are nutritionally poor for them. Thus far, these have been limited to relatively few, e.g., Ralstonia pickettii or Burkholderia cepacia. These Gram-negative rods can be kept to extremely low numbers in prefiltration bioburden with reasonable and effective bioburden control measures.