Factors controlling long-term survival and growth of naturalized Escherichia coli populations in temperate field soils

While Escherichia coli is widely used as an indicator of fecal contamination of waterways, recent studies suggest that this bacterium may become "naturalized" to soils. In this study, we investigated the survival and growth of naturalized E. coli in temperate soil in northern Minnesota. A spontaneously-occurring, antibiotic resistant E. coli strain, KS7-NR, was added to a field site at 103 cells (g soil)^-1. The survival and growth of E. coli KS7-NR were followed from June to October, by using colony counts on agar plates supplemented with antibiotics, and by using quantitative PCR (qPCR) with strain-specific primers developed based on suppressive subtractive hybridization. Both plate count and qPCR analyses indicated that E. coli KS7-NR survived for more than 2 months in the field soil. Laboratory experiments showed that soil temperature, moisture, nutrients, and the presence of other soil organisms influenced growth and survival of E. coli KS7-NR in soil. Moreover, our results indicated that summer drought conditions that occurred during the field study likely limited in situ growth of E. coli at the field site. Taken together, these results suggest that E. coli strains can survive long-term and grow in soils, provided that abiotic factors are within their tolerance limits.