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Applied and Environmental Microbiology, August 2005, p. 4690-4695, Vol. 71, No. 8
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.8.4690-4695.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Statistical Analyses: Possible Reasons for Unreliability of Source Tracking Efforts

Clarivel Lasalde,* Roberto Rodríguez,{dagger} and Gary A. Toranzos

Environmental Microbiology Laboratory, University of Puerto Rico Department of Biology, San Juan, Puerto Rico

Received 16 July 2004/ Accepted 3 March 2005

Analyses for the presence of indicator organisms provide information on the microbiological quality of water. Indicator organisms recommended by the United States Environmental Protection Agency for monitoring the microbiological quality of water include Escherichia coli, a thermotolerant coliform found in the feces of warm-blooded animals. These bacteria can also be isolated from environmental sources such as the recreational and pristine waters of tropical rain forests in the absence of fecal contamination. In the present study, E. coli isolates were compared to E. coli K12 (ATCC 29425) by restriction fragment length polymorphism using pulsed-field gel electrophoresis. Theoretically, genomic DNA patterns generated by PFGE are highly specific for the different isolates of an organism and can be used to identify variability between environmental and fecal isolates. Our results indicate a different band pattern for almost every one of the E. coli isolates analyzed. Cluster analysis did not show any relations between isolates and their source of origin. Only the discriminant function analysis grouped the samples with the source of origin. The discrepancy observed between the cluster analysis and discriminant function analysis relies on their mathematical basis. Our validation analyses indicate the presence of an artifact (i.e., grouping of environmental versus fecal samples as a product of the statistical analyses used and not as a result of separation in terms of source of origin) in the classification results; therefore, the large genetic heterogeneity observed in these E. coli populations makes the grouping of isolates by source rather difficult, if not impossible.

* Corresponding author. Mailing address: Environmental Microbiology Laboratory, University of Puerto Rico Department of Biology, P.O. Box 23360, San Juan, PR 00931-3360. Phone: (787) 764-0000. Fax: (787) 764-3875. E-mail: clasalvel{at}hotmail.com.

{dagger} Present address: Department of Soil, Water and Environmental Sciences, University of Arizona, Shantz Building 38, Room 429, P.O. Box 210038, Tucson, AZ 85721-0038.

Applied and Environmental Microbiology, August 2005, p. 4690-4695, Vol. 71, No. 8
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.8.4690-4695.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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