Whole-Cell Biosensing and Remediation of Arsenic using Magnetically Separable E. Coli: Towards Sustainable Materials for Water Treatment

The World Health Organization (WHO) identifies arsenic as a carcinogenic toxin with exposure limits in drinking water set as low as 10 ppb. Arsenic is abundant in the Earth’s crust and its prevalence in groundwater originates from natural causes as well as anthropogenic activities. Millions of people around the world, especially those in rural areas, are exposed to dangerous levels of arsenic in their water supply each year. Preventing human exposure to arsenic with accessible, inexpensive, and sustainable technology is crucial. We describe a strategy for sensing and removing arsenic from drinking water that exploits the bioaccumulation of arsenic by engineered bacteria and its detection using a whole-cell biosensor (WCB). A gene coding for a chimeric protein with two different arsenic binding domains was transfected into wild type <i>E. coli</i>; the resulting microbes could reduce arsenic concentrations in water samples containing up to 500 ppb to our instrument detection limit (~ 10 ppt) within 60 minutes. These microbes remain alive while bioaccumulating arsenic and thus we refer to them as ‘living sorbents’. While the dry-weight sorption capacity of these biomaterials is less than commercial inorganic arsenic sorbents, their arsenic affinity and selectivity is much greater. As a result, it is possible to reduce even very low levels of arsenic to acceptable levels using reasonable sorbent loadings.<br/><br/>These organisms may be further engineered to detect arsenic through optical reporters, thus providing an important integrated solution to the problem of arsenic in drinking water. Environmental arsenite binds to the ArsR transcription regulator, a protein that is central to the arsenic bioaccumulation capability described earlier. This same protein may also be applied to upregulate a reporter protein with fluorescent or luminescent properties. Whole cell biosensors should indicate the presence of arsenic at the extremely low levels relevant to meeting global exposure standards, on the order of ppb. Such sensitivity can be achieved through signal amplification strategies such as the use of engineered positive feedback or the application of multiple optical reporters.<br/><br/>Because these living sorbents use proteins evolved for arsenic sequestration, the material performance is independent of local water chemistry. The technology can be applied without customization to a particular site, and is notably independent of silicate and phosphate concentrations. Unlike inorganic sorbents which must be transported to a given site, these WCBs can be stored as dehydrated powders and later reconstituted on-site to levels appropriate for water treatment. Reversible attachment of iron oxide particles to the <i>E. coli </i>allows for magnetic capture of the biomass and the production of drinking water with no measurable contamination. This biological approach, employing regenerative and biodegradable materials, provides a sustainable alternative to conventional, inorganic sorbents for arsenic removal.