Survival of genetically marked Escherichia coli O157:H7 in soil as affected by soil microbial community shifts

Abstract

A loamy sand soil sampled from a species-rich permanent grassland at a long-term experimental site (Wildekamp, Bennekom, The Netherlands) was used to construct soil microcosms in which the microbial community compositions had been modified by fumigation at different intensities (depths). As expected, increasing depth of fumigation was shown to result in progressively increasing effects on the microbiological soil parameters, as determined by cultivation-based as well as cultivation-independent (PCR-DGGE, PLFA) methods. Both at 7 and at 60 days after fumigation, shifts in the bacterial, fungal and protozoan communities were noted, indicating that altered community compositions had emerged following a transition phase. At the level of bacteria culturable on plates, an increase of the prevalence of bacterial r-strategists was noted at 7 days followed by a decline at 60 days, which also hinted at the effectiveness of the fumigation treatments. The survival of a non-toxigenic Escherichia coli O157:H7 derivative, strain T, was then assessed over 60 days in these microcosms, using detection via colony forming units counts as well as via PCR-DGGE. Both data sets were consistent with each other. Thus, a clear effect of fumigation depth on the survival of the invading strain T was noted, as a progressive increase of depth coincided with a progressively enhanced inoculant survival rate. As fumigation depth was presumably inversely related to community complexity, this was consistent with the hypothesis that soil systems with reduced biological complexity offer enhanced opportunities for invading microbial species to establish and persist. The significance of this finding is discussed in the light of the ongoing discussion about the complexity-invasiveness relationship within microbial communities, in particular regarding the opportunities of pathogens to persist.

Factors Controlling Long-Term Survival and Growth of Naturalized Escherichia coli Populations in Temperate Field Soils

“…These results suggested that either some fraction of the inoculated E. coli might have entered the viable but not culturable (VBNC) state, or died off during the studies (29,31). While antibiotic resistant E. coli strains have been previously used in soil studies (9,11,15,17,22,26), the present study showed that genotype-or strain-specific qPCR may be a more sensitive tool for monitoring microbial population densities in the soil environment.…”

“…Results from several studies have shown that various stresses influence the survival and growth of E. coli in soils, including: high and low temperature (11,21,25), limited moisture (1-3, 9, 21), variation in soil texture (25), organic matter content (1,5,11), solar radiation (30), and competition and predation (2,26). Topp et al (25) observed that E. coli was capable of growing in soils amended with autoclaved swine manure slurry.…”

Microbial diversity determines the invasion of soil by a bacterial pathogen

“…In this context, the microbial communities that established after soil fumigation were shown to determine the fate of the invading species, whereby reduction in microbial diversity due to progressively enhanced fumigation depths resulted in higher pathogen persistence in soil (21). Similarly, in 25 different manures, the decline rate of E. coli O157:H7 was negatively correlated with Enterobacteriaceae richness (22).…”

“…After E. coli O157:H7 derivative strain T addition, we monitored the microcosms at different time points over a 60-d period, during which all cultivationdependent and -independent analyses were performed. The survival of strain T, expressed as the number of cfu g −1 dry soil, was then determined by selective dilution plating on tryptic soy agar (TSA) plus respective antibiotics, in accordance with van Elsas et al (21). E. coli counts were determined after 24 h incubation at 37°C.…”

“…Total bacterial biomass was determined using dichlorotriazinylaminofluorescein-assisted microscopy, and total protozoa were estimated using the most-probablenumber method of Darbyshire et al (33). We further determined the prevalence of different indicator PLFAs by using MIDI-FAME analysis (21).…”