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.
It is well understood that not all surfaces are equal. Spores interact differently with a surface depending on many factors,
including surface topography, surface hydrophobicity, and spore-suspension diluent.
Spores are inoculated onto solid carriers by dispensing a specified volume of a calibrated spore suspension onto the carrier.
If the carrier is fibrous, the spores will be wicked throughout the depth of the carrier. Figures 1 and 2 display spore placement
on two different fibrous carriers.
If the carrier is a nonfibrous solid, the spores will be deposited on a single plane of the carrier. Spores placed onto surfaces
that are not perfectly smooth may settle in locations that prove difficult to sterilize, as demonstrated in Figures 3 and
4. Additionally, the surface quality of a carrier can affect BI resistance. Sigworth and Stark demonstrated that anodized
aluminum with good surface quality resulted in much lower D-values (H2O2 sterilization) than other aluminum carriers (2). Several nonaluminum carriers with rough surfaces resulted in "acceptable"
D-values, while other carriers with smooth surfaces showed increased resistance. These tests emphasized that a combination
of carrier properties influence BI resistance, not a single carrier property.
The spores can be distributed on the carrier in many forms, ranging from a well-dispersed monolayer to large aggregates, as
demonstrated in Figures 5 and 6, respectively. Spore aggregates can be problematic depending on the sterilization process.
The protection of spores in the center of these aggregates can lead to a perceived increased resistance and erratic results
(tailing), which are experimental artifacts. Aggregates can be minimized by understanding the surface characteristics of the
carrier and selecting an appropriate carrier liquid and spore population.
Spore concentration is the number of spores per volume of diluent. When a spore suspension is inoculated onto a solid surface, the inoculum forms
a small bead. Depending on the composition of the diluent and the carrier, the footprint (i.e., inoculated area) and spore uniformity (within the area) vary, as demonstrated in Figures 7 and 8. The diluents typically
are water or ethanol but may be other liquids.
The wettablity of the surface by the selected carriers is measured by the critical-contact angle of the carrier (see Figure
9). The lower the critical-contact angle, the more wettable the surface. Wettable surfaces result in more uniform spore distribution
on the carrier.