Presence and growth of naturalized Escherichia coli in temperate soils from Lake Superior watersheds

Abstract

The presence of Escherichia coli in water is used as an indicator of fecal contamination, but recent reports indicate that soil populations can also be detected in tropical, subtropical, and some temperate environments. In this study, we report that viable E. coli populations were repeatedly isolated from northern temperate soils in three Lake Superior watersheds from October 2003 to October 2004. Seasonal variation in the population density of soilborne E. coli was observed; the greatest cell densities, up to 3 x 10(3) CFU/g soil, were found in the summer to fall (June to October), and the lowest numbers, < or =1 CFU/g soil, occurred during the winter to spring months (February to May). Horizontal, fluorophore-enhanced repetitive extragenic palindromic PCR (HFERP) DNA fingerprint analyses indicated that identical soilborne E. coli genotypes, those with > or =92% similarity values, overwintered in frozen soil and were present over time. Soilborne E. coli strains had HFERP DNA fingerprints that were unique to specific soils and locations, suggesting that these E. coli strains became naturalized, autochthonous members of the soil microbial community. In laboratory studies, naturalized E. coli strains had the ability to grow and replicate to high cell densities, up to 4.2 x 10(5) CFU/g soil, in nonsterile soils when incubated at 30 or 37 degrees C and survived longer than 1 month when soil temperatures were < or =25 degrees C. To our knowledge, this is the first report of the growth of naturalized E. coli in nonsterile, nonamended soils. The presence of significant populations of naturalized populations of E. coli in temperate soils may confound the use of this bacterium as an indicator of fecal contamination.

FIG. 1.

Lake Superior watersheds used in this study and relation to Duluth, Minnesota. Sampling locations are marked (•). Soil and water samples were taken from KS, NW, and SC sites.

FIG. 2.

Topography of sampling sites used. (A) KS; (B) NW; (C) SC.

FIG. 3.

Seasonal shifts in the population density of E. coli at each sample site. Black bars, naturalized E. coli strains; gray bars, other E. coli strains. ND, not detected.

FIG. 4.

Partial dendrogram of naturalized E. coli strains from the KS site from October 2003 to October 2004. The dendrogram was generated from HFERP DNA fingerprints using Pearson's product-moment correlation coefficient and the UPGMA clustering method.

FIG. 5.

MANOVA of HFERP DNA fingerprints from E. coli strains. The first two discriminants are represented by the distances along the x and y axes. (A) MANOVA of HFERP DNA fingerprints from naturalized E. coli strains from the KS (▪), NW (•), and SC (▴) sites; (B) MANOVA of HFERP DNA fingerprints obtained from naturalized E. coli strains from the KS (▪), NW (•), and SC (▴) sites and E. coli isolated from feces of geese (□), terns and gulls (○), and deer (▵); (C) MANOVA of HFERP DNA fingerprints of naturalized E. coli obtained from soils at the KS (▪), NW (•), and SC (▴) sites and E. coli isolated from water at the same sites (×).

FIG. 6.

Influence of temperature on survival and growth of naturalized E. coli in Minnesota soils. Nalidixic acid- and rifampin-resistant naturalized E. coli strains KS-7NR, NW-13NR, and SC-20NR were inoculated into KS, NW, and SC soils, respectively. Values presented are means of CFU ± standard errors on A3 agar medium. (A) Soils were incubated at constant temperatures of 4°C (•), 15°C (○), 25°C (▾), 30°C (▿), or 37°C (▪); (B) E. coli strains KS-7NR (•), NW-13NR (○), and SC-20NR (▾) were inoculated into their respective soils of isolation and incubated at 15°C for 4 days and then shifted (↑) to 37°C for an additional 4 days.