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This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

Date of Award

Summer 2013

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Department of Biology

Abstract

The Clean Water Act of 1972 raised awareness of the extent of the pollution in U.S. waters and the need for monitoring and identifying sources of contamination. Surface water quality is routinely evaluated using fecal indicators, such as Enterococcus spp. Recent studies in a south Texas watershed showed that fecal bacteria were being transported to water via agricultural land runoff following rainfall, suggesting that soil may be a source of these bacteria. In this study, soils from fields under different crop covers were sampled to determine seasonal levels of enterococci and species of enterococci were identified using the BIOLOG™ Microbial Identification System. A series of laboratory experiments were conducted to evaluate the ability of enterococci to survive in soil, using South Texas soil. Microcosms were set up with top soil placed in small column and inoculated with Enterococcus mundtii, a species which has been isolated from both soil and water in the watershed. Experiments were set up at different air temperatures (40°, 25°, and 10°C), soil moisture levels (dry or wet), and in non-autoclaved and autoclaved soil. Two strains of E. mundtii were used, an environmental strain from watershed soil, and ATCC 43186 strain. Enterococci were enumerated at several time intervals using standard membrane filtration on selective media for Enterococcus spp. (mEI), following extraction from soil. Field sampling showed higher numbers of enterococci in soil during cooler, wet months. Microcosm results demonstrated that survival of enterococci in non-autoclaved soil is affected by temperature and moisture at 25°C and 40°C, and that persistence is greater at 10°C. Survival of enterococci in autoclaved soil was affected only by temperature at 25°C and 40°C. Enterococci survived longer in the majority of the autoclaved soil experiments. This study has demonstrated the potential for soil to act as a reservoir for enterococci and therefore the need for Enterococcus standards for soil and the inclusion of soil assessments in watershed studies. These standards will allow policy makers to develop remediation strategies and best-management practices to minimize runoff and reduce the levels of bacteria entering water bodies.

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