Effect of inoculation method on the determination of decontamination efficacy against Bacillus spores

Abstract

Decontamination studies investigating the effectiveness of products and processes for the inactivation of Bacillus species spores have traditionally utilized metering viable spores in a liquid suspension onto test materials (coupons). The current study addresses the representativeness of studies using this type of inoculation method compared to when coupons are dosed with a metered amount of aerosolized spores. The understanding of this comparability is important in order to assess the representativeness of such laboratory-based testing when deciding upon decontamination options for use against Bacillus anthracis spores. Temporal inactivation of B. anthracis surrogate (B. subtilis) spores on representative materials using fumigation with chlorine dioxide, spraying of a pH-adjusted bleach solution, or immersion in the solution was investigated as a function of inoculation method (liquid suspension or aerosol dosing). Results indicated that effectiveness, measured as log reduction, was statistically significantly lower when liquid inoculation was used for some material and decontaminant combinations. Differences were mostly noted for the materials observed to be more difficult to decontaminate (i.e., wood and carpet). Significant differences in measured effectiveness were also noted to be a function of the pH-adjusted bleach application method used in the testing (spray or immersion). Based upon this work and the cited literature, it is clear that inoculation method, decontaminant application method, and handling of non-detects (i.e., or detection limits) can have an impact on the sporicidal efficacy measurements.

Environmental Persistence of Bacillus anthracis and Bacillus subtilis Spores

“…Further, it is likely that the inoculation method could have a significant effect on inactivation rates. This was demonstrated in a study investigating chemical decontaminants, in which Ryan et al [ 37 ] showed that inactivation rates were indeed affected by the spore inoculation method.…”

A simple decontamination approach using hydrogen peroxide vapour forBacillus anthracisspore inactivation

“…Ryan et al . () found that the spore inoculation method did affect decontamination efficacy results, but the effect varied by material and decontaminant. Nevertheless, our B. anthracis Ames and B. atrophaeus decontamination efficacy results agree well with Rogers et al .…”

Evaluation of standardized sample collection, packaging, and decontamination procedures to assess cross-contamination potential duringBacillus anthracisincident response operations

“…Indeed, other studies have noted that differences in decontamination efficacy can been attributed to inoculation method. [42] Because liquid spores were deposited in ten discrete spots of 100 μL across a coupon in a predictable pattern rather than evenly distributed, it is possible that: (1) the location of contamination was known and therefore unintentionally targeted during the decontamination procedure; (2) physical removal of contaminants is more efficient when the liquid inoculum is dried into small, discrete locations rather than deposited as dry particles over the entire coupon surface; and (3) the actual amount of inoculated surface area was significantly smaller for liquid tests (the area under ten small droplets), and thus decontamination procedures had a much higher probability of treating 100% of the contaminated area. For example, if a small amount of coupon surface area was inadvertently neglected during the decontamination treatment, the impact on viable spore recovery could be much greater for the tests with dry inoculums.…”