Understanding the role of the general stress response regulator, RpoS, in the environmental persistence of Escherichia coli
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The use of E. coli as an indicator organism for faecal contamination in the environment, particularly in water sources, is based partly on the assumption that E. coli survives transiently outside of the mammalian gastrointestinal tract. However, studies have shown that E. coli can persist and grow in various external environments, and raises questions about its continued use as an indicator organism. The general stress response regulator, RpoS, helps E. coli overcome various stresses, however, mutations are known to accumulate within rpoS in low nutrient environments causing loss of RpoS function. Since soil is less abundant in nutrients compared to the host gut, it was necessary to understand if loss of function mutations were present in the rpoS of soil-persistent E. coli. Hence, this study investigated a collection of long-term soil-persistent E. coli strains to understand if RpoS is retained in them and the contribution of the general stress response to soil survival. All the soil-persistent strains tested had an intact rpoS gene and retained a fully functional RpoS-regulated general stress response. RpoS was demonstrated to be important for soil survival and specifically, crucial for overcoming protozoan predation, low moisture and pH stresses in soil. Production of curli, an important component of E. coli biofilm, which is RpoS-dependent, increased the ability of E. coli to resist predation by Acanthamoeba polyphaga and Tetrahymena pyriformis, and promoted the attachment of E. coli to sand. Ten percent (17 out of 170) of the soil isolates were curli-deficient. Some of the curli-negative E. coli, which were defective in biofilm production and sand attachment, had mutations in two genes associated with c-di-GMP metabolism, dgcE and pdeR, although this did not significantly impair their survival in soil compared to curli-positive strains. Loss of curli production in soil-persistent E. coli may promote their dissemination into other environments.