Molecular Analyses of Changes Induced in the Microbial Populations of Murine Colon After As(III) Exposure

The gut microbiota is essential for mammalian health and metabolism. Thus identifying factors that influence the host microbiota is key to understanding the dynamic interplay between the host and its microbiota. Recent studies have shown the effect of chronic exposures of trivalent arsenic [As(III)] in environmentally relevant concentrations on host physiology, however, little is known of how it impacts the gut microbiota. This study examined the hypothesis that environmentally relevant concentrations of As(III) in drinking water will directly affect the murine colon microbial composition and physiology. The colon microbial communities from 10 and 250 ppb of As(III) exposed mice were compared to the control mice after 2, 5 and 10 weeks of As(III) exposure. Molecular analysis based on 16S rRNA gene and 16S- 23S intergenic region indicated a time and dose dependent shift in microbial community composition. Analysis of 16S rDNA clone libraries demonstrated an increase in Bacteroidetes and proportionally fewer Firmicutes in colon microbiota in response to As(III) exposure. Microbes have developed mechanisms to tolerate arsenic present in the environment. This study is the first to show that the gut microbes express arsenic resistant genes (arsA and arsB) in the colon. However, exposure to ppb concentrations of As(III) did not induce the expression of these genes in colon microbes. These data suggest that the selective effect of As(III) on colon microbiota was not due to direct exposure of colon microbes to As(III), but rather a response to changes in the host physiology. Since arsenic affects the NO levels in human cell lines, it was hypothesized that As(III) will affect the pathways that are linked to NO levels in human body. Many microbes present in human body have nitrogen metabolizing genes (nrfA) that contribute to NO levels, thus the expression levels of nrfA gene in colon microbes was examined. Real time RT-qPCR studies showed a time and dose dependent increase in nrfA expression in response to As(III) exposure. Together, the results presented in this study demonstrated an indirect effect of As(III) on the composition and physiology of murine colon microbiota that may further impact the host health