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).
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.
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.
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.
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.