Center for Biofilm Engineering
Abstract:
"Estimating the Antimicrobial Log Reduction: Part 2. Presence/Absence
Assays"
99-038 This is part 2 of a pair of papers on antimicrobial assays
conducted to estimate the log reduction (LR), in the density of viable microbes,
attributable to the germicide. Two alternative definitions of LR were defined in
part 1, one based on the mean of the log-transformed densities; the other is
based on the logarithm mean of densities. In this paper, we evaluate statistical
methods for estimating LR from an antimicrobial assay in which the responses are
presence/absence observations at each dilution in a series of dilutions. We
provide a model for the presence/absence data, and, for each definition of LR,
we derive the maximum likelihood estimator (mle). Using computer simulation
methods, we compare the mle to several alternative estimators, including an
estimator based on averaging the log-transformed most probable number (mpn)
values. Standard error formulas for the estimators are also derived and
evaluated using computer simulations. This investigation results in the
following recommendations. If the parameter of interest is based on the mean of
log-transformed densities, then the results favor use of the log-transformed mpn
method. If, however, the parameter of interest is based on the lagarithm of the
mean of densities, then the results show that the mle should be used.
DeVries, T. and M. Hamilton, "Estimating the
Antimicrobial Log Reduction: Part 2. Presence/Absence Assays," Quantitative
Microbiology, 1:47-62 (1999).
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