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.
Apr 30, 2007

A biological indicator (BI) is an "inoculated carrier contained within its primary pack ready for use and producing a defined resistance to the specified sterilization process"(1). The biological component of the BI typically is bacterial endospores.

Defined resistance is a quantitative measure generally expressed in units of time or dose. This value can be the point of BI inactivation (kill). More commonly, resistance is expressed as a D-value. D-value is defined as the time or radiation dose required to inactivate 90% of a population of the test microorganism under stated exposure conditions (1).


Figure 1
Many factors influence the measured resistance of a BI, including the test organism, the carrier material, the primary packaging, the culture medium used for the recovery of stressed spores, and the test system (2–6). The carrier material and packaging often are overlooked as influences on a BI's resistance. All of these items must be controlled to achieve predictable and consistent BI performance.


Figure 2
A major misunderstanding in the industry is that spores have an intrinsic D-value. This assertion is not true if for no other reason than it is impossible to test the resistance of individual spores suspended in space. For testing purposes, the spores are placed onto a surface or suspended in a liquid, and this system becomes the BI. It is the resistance of the system that is measured, not simply the spores. The bacterial spores integrate all known and unknown critical process variables. This article focuses on the effect of carrier materials on the measured resistance of spores on solid and in liquid carrier materials.


Figure 3
A myriad of solid and liquid carrier materials can be used. Solid carriers can be fibrous (e.g., paper, woven polymers, and fiberglass material) or nonfibrous (e.g., stainless steel, plastic, and rubber). Liquid carriers include water, oil, nutrient medium, and pharmaceutical products.


Figure 4
Commercially prepared spore suspensions are used as components in the BI-manufacturing process. Spore suspensions typically are labeled with a D-value. This D-value usually is collected on a carrier of convenience and should be stated on the certificate of analysis (C of A). The spore suspension by itself, however, is not the BI system that the user employs. These suspensions inoculate carriers that then become the BI system. The D-value of the resulting BI system is not specified on the C of A. The C of A, however, provides the user with a means of comparison with previous spore lots.