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Applied and Environmental Microbiology, July 2000, p. 2972-2980, Vol. 66, No. 7
Clancy Environmental Consultants, Inc., St. Albans,
Vermont1; University of Arizona, Tucson,
Arizona2; Thames Water Utilities,
Reading,3 and Scottish Parasite
Diagnostic Laboratory, Glasgow,4 United Kingdom;
and Technology Planning and Management Corporation,
Scituate, Massachusetts5
Received 28 December 1999/Accepted 8 March 2000
Several in vitro surrogates have been developed as convenient,
user-friendly alternatives to mouse infectivity assays for determining
the viability of Cryptosporidium parvum oocysts. Such viability assays have been used increasingly to determine oocyst inactivation following treatment with chemical, physical, or
environmental stresses. Defining the relationship between in vitro
viability assays and oocyst infectivity in susceptible hosts is
critical for determining the significance of existing oocyst
inactivation data for these in vitro assays and their suitability in
future studies. In this study, four viability assays were compared with mouse infectivity assays, using neonatal CD-1 mice. Studies were conducted in the United States and United Kingdom using fresh (<1
month) or environmentally aged (3 months at 4°C) oocysts, which were
partially inactivated by ozonation before viability and/or infectivity
analyses. High levels of variability were noted within and between the
viability and infectivity assays in the U.S. and United Kingdom studies
despite rigorous control over oocyst conditions and disinfection
experiments. Based on the viability analysis of oocyst subsamples from
each ozonation experiment, SYTO-59 assays demonstrated minimal change
in oocyst viability, whereas 4',6'-diamidino-2-phenylindole-propidium
iodide assays, in vitro excystation, and SYTO-9 assays showed a
marginal reduction in oocyst viability. In contrast, the neonatal mouse
infectivity assay demonstrated significantly higher levels of oocyst
inactivation in the U.S. and United Kingdom experiments. These
comparisons illustrate that four in vitro viability assays cannot be
used to reliably predict oocyst inactivation following treatment with low levels of ozone. Neonatal mouse infectivity assays should continue
to be regarded as a "gold standard" until suitable alternative viability surrogates are identified for disinfection studies.
0099-2240/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Comparison of Cryptosporidium parvum Viability and
Infectivity Assays following Ozone Treatment of Oocysts
*
Corresponding author. Mailing address: Clancy
Environmental Consultants, Inc., P.O. Box 314, St. Albans, VT 05478. Phone: (802) 527-2460. FAX: (802) 524-3909. E-mail:
zbukhari{at}together.net.
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