Center for Biofilm Engineering
Abstract:
"Estimating the Antimicrobial Log Reduction: Part 1. Quantitative
Assays"
99-037 In quantitative
antimicrobial assays, the responses are counts of viable microbes in two
treatment groups. One group is treated with a chemical germicide and the other
group is control, treated with an inactive chemical. This is part 1 of a pair of
papers that pertain to assays that estimate the log reduction (LR), in the
density of viable microbes, attributable to the germicide treatment (part 2 is
concerned with presence/absence responses). Such assays are used by producers,
consumers, and regulatory agencies to assess the efficacy of liquid germicides.
We define and compare the two different mathematical formulations for LR that
are commonly used in practice when there are replicate density measurements. One
LR parameter is based on the mean of the log-transformed densities; the other is
based on the logarithm of the mean of densities. We build a statistical model
relating microbial count data to the LR parameters, derive maximum likelihood
and method of moments estimators for each LR parameter, and compare the
estimators according to both their asymptotic characteristics and the results of
a simulation study utilizing realistic sample sizes. Standard error formulas for
the estimators are derived, and they are evaluated via simulation studies. The
results of this investigation lead us to recommend the method of moments
estimator, regardless of which definition of LR is chosen.
DeVries, T., M. Hamilton."Estimating the
Antimicrobial Log Reduction: Part 1. Quantitative Assays," Quantitative
Microbiology, 1:29-45 (1999).
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