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TECHNICAL REPORTS

Vadose Zone Processes and Chemical Transport

Persistence of Culturable Escherichia coli Fecal Contaminants in Dairy Alpine Grassland Soils

^a UMR42 CARRTEL, INRA F74203, Thonon-les-Bains, France; Université de Savoie, F73376 Le Bourget du Lac, France
^b Université de Lyon, F69003, Lyon France; CNRS, UMR5557, Ecologie Microbienne, F69622, Villeurbanne, France

^* Corresponding author (trevisan{at}thonon.inra.fr).

Received for publication January 17, 2008. Our knowledge of Escherichia coli (E. coli) ecology in the field is very limited in the case of dairy alpine grassland soils. Here, our objective was to monitor field survival of E. coli in cow pats and underlying soils in four different alpine pasture units, and to determine whether the soil could constitute an environmental reservoir. E. coli was enumerated by MPN using a selective medium. E. coli survived well in cow pats (10⁷ to 10⁸ cells g^–1 dry pat), but cow pats disappeared within about 2 mo. In each pasture unit, constant levels of E. coli (10³ to 10⁴ cells g^–1 dry soil) were recovered from all topsoil (0–5 cm) samples regardless of the sampling date, that is, under the snow cover, immediately after snow melting, or during the pasture season (during and after the decomposition of pats). In deeper soil layers below the root zone (5–25 cm), E. coli persistence varied according to soil type, with higher numbers recovered in poorly-drained soils (10³ to 10⁴ cells g^–1 dry soil) than in well-drained soils (< 10² cells g^–1 dry soil). A preliminary analysis of 38 partial uidA sequences of E. coli from pat and soils highlighted a cluster containing sequences only found in this work. Overall, this study raises the possibility that fecal E. coli could have formed a naturalized (sub)population, which is now part of the indigenous soil community of alpine pasture grasslands, the soil thus representing an environmental reservoir of E. coli.

Abbreviations: ANOVA, analysis of variance • bp, base pair • DNA, deoxyribonucleic acid • K, saturated permeability constant • MANOVA, multiple analysis of variance • MPN, most probable number • MU/Ec, 4-methylumbelliferyl-β-D-glucuronide/Escherichia coli • PCR, polymerase chain reaction • UV, ultraviolet • 16S rRNA, small subunit ribosomal ribonucleic acid