The Effect of Carrier Material on the Measured Resistance of Spores
A biological indicator (BI) measures the effectiveness of the sterilization process to which it is subjected. Factors such as the test organism, the packaging, the culture material, and the test system all influence a BI's resistance. Carrier material is an often-overlooked factor that also influences BI resistance. The authors examine various solid and liquid carriers, describe their properties, and investigate how they influence BI resistance.
Liquid-carrier BIs generally consist of hermetically sealed glass ampuls filled with the inoculated liquid, which usually
is a culture medium. The physical and chemical properties of the liquid influence spore resistance. Properties such as pH,
ionic strength, the concentration of active ingredients, and the viscosity of these liquids can cause spores to clump. Spore
clumping may lead to erratic results and higher resistance. The effect of pH is demonstrated by phase-contrast microscopy
in Figures 13–15. Changes in the pH of some liqu-ids may cause spores to form aggregates. As the pH shifts from neutral to
acidic, the spores may form larger aggregates. The size of the aggregates can protect spores in the center of these clusters,
which results in a higher observed resistance. This resistance cannot be considered an artifact of experimental error because
the chemistry of the liquid has a similar effect on the bioburden that naturally occurs in the product.
A survivor-curve test performed on clumped spores in a pharmaceutical product resulted in a rapid drop in population during
the initial phase (i.e., the death of single spores or small clumps), followed by tailing because of the presence of large clumps of spores. (see
Figure 16). Data such as these initially may seem troublesome, but they are critical when developing a product-sterilization
Liquid pharmaceutical product inoculated with spores used in BIs
The liquid spore carrier of a BI can be the pharmaceutical product itself. The excipients and active ingredients both can
affect resistance. D-value studies performed on aqueous-based pharmaceutical products demonstrate that, in most cases, concentrations of active
ingredients have little influence on the D-value. These results indicate that the excipients have the greatest impact on BI resistance. Test units using the same spores
suspended in water for injection were prepared as a comparison (see Table III).
In the light of new and emerging materials and sterilization technologies, it is important to understand that spores are the
only tool that can measure the lethality of a sterilization process effectively. Spores account for the substrate effect and
the critical process parameters of the sterilization process itself.