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

Abstract

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 (10(7) to 10(8) cells g(-1) dry pat), but cow pats disappeared within about 2 mo. In each pasture unit, constant levels of E. coli (10(3) to 10(4) 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 (10(3) to 10(4) cells g(-1) dry soil) than in well-drained soils (< 10(2) 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.

Environmental Patterns Are Imposed on the Population Structure of Escherichia coli after Fecal Deposition

“…Changes in E. coli population structure following fecal deposition have been reported previously (16,39,45), but the present work links those changes to patterns of fecal deposition coupled with environmental selection. The utility of E. coli as a fecal indicator bacterium (FIB) in pollution surveillance is dependent on the predictability of the population changes imposed by secondary habitats.…”

“…In the environment, ecological differences between E. coli pathogens and nonpathogens have been observed to be small (1,3,14), but concerns have been raised about the utility of nonpathogenic FIB strains in estimating fecal pollution risk, mainly resulting from reports of naturalized E. coli (39). In this context, the strong temporal trend in ECOR B2 genotypes is interesting, because this subpopulation contains the greatest proportion of virulence factors (38).…”

“…The contribution of fecal deposition to E. coli spatial patterns must be examined in addition to edaphic variables, because deposition is the process that controls E. coli introduction into soil (14,21). Therefore, landscape genetic analysis of E. coli distributions can help to clarify how deposition in soil changes extrahost populations (7,14,39). For example, if E. coli populations in soil were found to be structured along a pH gradient, then soil pH might be a useful predictor for mapping fecal pollution risk or potential environmental reservoirs of fecal bacteria.…”

Characterization of Environmentally Persistent Escherichia coli Isolates Leached from an Irish Soil

“…The integration of E. coli as a component of the indigenous microflora in soils of tropical and subtropical regions may be attributable to the nutrient-rich nature and warm temperatures of these habitats (21,39), combined with the metabolic versatility of the organism and its simple nutritional requirements (21). In addition to tropical and subtropical regions, the presence of autochthonous E. coli populations in the cooler soils of temperate and northern temperate regions has also been reported (6,20,22,37), with one report on an alpine soil (34) and, most recently, a report on a maritime temperate grassland soil (3). The growth of E. coli within soils can act as a reservoir for the further contamination of bodies of water (20,31,32), compromising the indicator status of E. coli within these regions.…”

“…The challenging nature of the soil environment and the disparity of conditions between the primary host and the secondary habitat raises the question of how these E. coli populations survive and compete for niche space among the highly competitive and diverse coexisting populations of the indigenous microflora (15,21). There is some evidence that naturalized E. coli may form genetically distinct populations in the environment (17,20,34,36). This suggests that autochthonous E. coli populations in soil may have increased environmental fitness, facilitating their residence in soil (20,34,38).…”