Abstract
In this study, we investigated gene expression changes in three bacterial strains (Escherichia coli C3000, Escherichia coli O157:H7 B6914, and Enterococcus faecalis ATCC 29212), commonly used as indicators of water quality and as control strains in clinical, food, and water microbiology laboratories. Bacterial transcriptome responses from pure cultures were monitored in microcosms containing water amended with manure-derived dissolved organic matter (DOM), previously exposed to simulated sunlight for 12 h. We used RNA sequencing (RNA-seq) and quantitative real-time reverse transcriptase (qRT-PCR) to compare differentially expressed temporal transcripts between bacteria incubated in microcosms containing sunlight irradiated and non-irradiated DOM, for up to 24 h. In addition, we used whole genome sequencing simultaneously with RNA-seq to identify single nucleotide variants (SNV) acquired in bacterial populations during incubation. These results indicate that E. coli and E. faecalis have different mechanisms for removal of reactive oxygen species (ROS) produced from irradiated DOM. They are also able to produce micromolar concentrations of H2O2 from non-irradiated DOM, that should be detrimental to other bacteria present in the environment. Notably, this study provides an assessment of the role of two conjugative plasmids carried by the E. faecalis and highlights the differences in the overall survival dynamics of environmentally-relevant bacteria in the presence of naturally-produced ROS.
Highlights
Survival of bacteria in the environment is dictated by their ability to grow or persist under diverse abiotic and biotic stressors
All three strains of bacteria differed in their growth dynamics following inoculation into Irradiated dissolved organic matter (DOM)-spiked Water (I-DOMW) and Non-irradiated DOM-spiked Water (NDOMW)
E. coli and E. coli O157:H7 grew by several orders of magnitude after 12 h of dark incubation in I-DOMW and N-DOMW (Figures 2A,C, Supplementary File 1)
Summary
Survival of bacteria in the environment is dictated by their ability to grow or persist under diverse abiotic and biotic stressors. Resource availability, sunlight, temperature, pH, and competition have been shown to be important drivers of bacterial population dynamics (Bradford et al, 2013, 2015; Pachepsky et al, 2014) Of these factors, substrate availability, in the form of dissolved organic matter (DOM), is necessary for bacteria to proliferate in surface waters. The colored fraction of DOM (CDOM) is primarily responsible for absorbance of UV light and production of labile nutrients from refractive fractions of DOM, which can be subsequently used for growth (Bushaw and Zepp, 1996; Moran and Zepp, 1997; Häder et al, 2011) These labile nutrients include low molecular weight nitrogen compounds, such as amino acids, that are of great importance to the biogeochemistry of natural waters and are involved in numerous biochemical processes. Phototransformation of labile organic compounds, such as thiols present in surface waters can result in a net increase in ROS steady state (Chu et al, 2016)
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