Metagenomic profiles and antibiotic resistance genes in gut microbiota of mice exposed to arsenic and iron
Introduction
Gut harbors diverse microbes that play a key role in well-being of their host. The gut microbiota acts in a concerted manner to achieve metabolic communication with the host, and many different bacterial genera and species are involved in metabolite production (Wikoff et al., 2009, Mestdagh et al., 2012, Martinez et al., 2013). Changes in gut microbiota are linked with inflammatory and metabolic disorders (Nicholson et al., 2012). Many researches have showed that metal exposure could change the gut microbiota (Dostal et al., 2012). On the other hand, the gut microbiota could change transportation and metabolism of metals (Wiele et al., 2010). Thus, it is necessary to identify impacts of metal on the gut microbiota under oral metal exposure. Arsenic (As) as ubiquitous metalloid has been paid much attention due to its high toxicity. Consumption of drinking water is the main source of As exposure. Iron (Fe) coagulation/flocculation has been widely applied in the actual treatment of As-contaminated water due to its low cost and high efficiency (Mohan and Pittman, 2007). Our previous study demonstrated that combined exposure of As and Fe in mouse could significantly reduce hepatic toxicity of As (Liu et al., 2013). In addition, some altered host-gut co-metabolites in serum and urine were identified, indicating the possible changes of gut microbiota. Thus, it is necessary to explore the impacts of As and/or Fe on gut microbiota to better understand their combined effects.
Gut microbiota is an important antibiotic resistance genes (ARGs) reservoir, playing important roles in host health. Recently, it has been suggested that environmental pollution could affect abundance of resistance traits. In fact, there are various known mechanisms by which the resistance traits may be retained or propagated in the presence of metals (Baker-Austin et al., 2006, Stepanauskas et al., 2006). Bacterial resistance mechanisms exist to mitigate toxicological effects of excessive bioavailable metals as part of their stress response strategy. Defense-associated metals are often closely associated with those responsible for antibiotic resistance on mobile genetic elements (MGEs) (Beaber et al., 2004). These genes can encode for generic detoxifying mechanisms (e.g. efflux pumps), which non-specifically reduce intracellular concentrations of both metals and antibiotics (cross resistance) (Berg et al., 2010, Knapp et al., 2011). Thus, constant exposure to metals can increase ARGs’ frequency in the gene pool in environmental or gut bacteria. There are many reports on correlation between tolerance to metals (including As and Fe) and antibiotic resistance in environment (Tuckfield and McArthur, 2008, Kaur et al., 2011, Ji et al., 2012). However, effects of As and/or Fe on the ARGs in gut microbiota are still unknown.
In the present study, we exposed pure water, As alone, Fe alone and As + Fe to male mice for 90 d, respectively. After exposure, gut microbiota were analyzed by high-throughput sequencing. Relationships between gut microbiota and mouse metabolic profiles were characterized by correlation analysis. The ARGs in gut microbiota were determined based on high throughput sequencing and verified by quantitative real-time PCR (qRT-PCR). This study firstly provides the effects of As and/or Fe exposure on gut microbiota and ARGs. Combined with results on metabolic profiles of mouse serum and urine, this study might be very useful for understanding of toxicological effects and mechanism of actions of As and/or Fe exposure.
Section snippets
Animal treatment
Five-week-old male mice (Mus musculus, ICR) were purchased from the experimental animal center of Academy of Military Medical Science of China. Forty mice (about 18 g) were randomly assigned to four groups (ten mice in one group). The mice in four groups were exposed to pure water, 3 mg L−1 As, 5 mg L−1 Fe and 3 mg −1 As + 5 mg L−1 Fe under ambient conditions (25 ± 3 °C, 50 ± 5% relative humidity, and a 12/12 h light/dark cycle) for 90 d, respectively. The concentrations selected for As and Fe were based on our
Histopathological maps
Representative images of H&E-stained histologic sections of mouse intestine are shown in Fig. 1. Compared with control group, As alone exposure induced small-bowel mucosal edema of intestine. Fe alone exposure caused the necrosis of intestine. For co-exposure of As and Fe, little damage was observed. We deduce that co-exposure of As and Fe has antagonistic effects on mouse intestine. The results are similar with our previous report, in which the biological effects of co-exposure of As and Fe
Conclusions
Exposure of As alone and Fe alone could alter the diversity and functions of gut microbiota and abundance of ARGs and MGEs. However, co-exposure of As and Fe had antagonistic effects on microbial community and ARGs to a certain extent. The alterations of gut microbiota might be an important reason of changes in metabolic profiles of mice serum and urine. The changes of gut microbiota should be considered during the risk assessment of As and/or Fe.
Acknowledgements
This research was supported by grants from the Natural Science Foundation of Jiangsu Province (SBK201320987), Foundation of State Key Laboratory of Pollution Control and Resource Reuse, Science Foundation of Nanjing University and National Natural Science Foundation of China (51208250). The authors declare no conflict of interest.
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