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.
For example, water has greater surface tension than ethanol. The footprint of a water bead is smaller, but the height is greater.
When they are on the same surface, water has a greater contact angle than ethanol. This property affects how suspended spores
are deposited on the surface. Spore density increases when a defined number of spores are deposited over a smaller area. The
drying time of the diluent also affects the spore distribution within the footprint. In general, a longer dry time results
in a greater density of spores on the edge of the footprint. In the authors' experience, this "edge effect" (see Figure 10)
is always present but is more pronounced with aqueous liquids with high surface tension (resulting in a longer drying time).
The faster the liquid dries, the more homogenous the spore dispersion is.
The edge effect may consist of only a densely packed monolayer of spores (see Figure 11) or it may result in multiple layers
of densely packed spores (see Figure 12). The difference may influence BI resistance depending on the sterilization process.
The survivor kinetics of spores exposed to low-pressure plasma by Hury et al. demonstrated that D-values vary when one decreases the surface density of spores (7). Hury suggested that the diffusion of the sterilant was
limited by a "shield effect" of stacked spores.
Sterilant interaction with carriers
Table I: Summary of moist-heat D-values on rubber-stopper biological indicators.
Other factors that influence BI resistance include surface characteristics, surface topography, unit mass, heat capacity,
and hydrophobicity of the carrier. The influence of these characteristics depends on sterilization processes.
Table II: Summary of dry-heat D-values on rubber-stopper biological indicators.
Rubber stoppers respond differently to heat than glass or stainless steel. When stoppers are sterilized by moist heat, they
heat very slowly and therefore may exhibit higher measured D-values. Rubber stoppers also may have surface coatings, which are another variable. Resistance testing performed on rubber
stoppers demonstrated the range of D-values displayed in Tables I and II.