Bioreporters and biosensors for arsenic detection. Biotechnological solutions for a world-wide pollution problem Davide Merulla 1 , Nina Buffi 2 , Siham Beggah 1 , Fre ´ de ´ ric Truffer 3 , Martial Geiser 3 , Philippe Renaud 2 and Jan Roelof van der Meer 1 A wide variety of whole cell bioreporter and biosensor assays for arsenic detection has been developed over the past decade. The assays permit flexible detection instrumentation while maintaining excellent method of detection limits in the environmentally relevant range of 10–50 mg arsenite per L and below. New emerging trends focus on genetic rewiring of reporter cells and/or integration into microdevices for more optimal detection. A number of case studies have shown realistic field applicability of bioreporter assays. Addresses 1 Department of Fundamental Microbiology, University of Lausanne, 1015 Lausanne, Switzerland 2 Laboratory of Microsystems, LMIS4-STI, Ecole Polytechnique Fe ´ de ´ rale de Lausanne, 1015 Lausanne, Switzerland 3 Institut Syste ` me Industriel, Haute e ´ cole spe ´ cialise ´e de Suisse occidentale, 1950 Sion, Switzerland Corresponding author: van der Meer, Jan Roelof (janroelof.vandermeer@unil.ch) Current Opinion in Biotechnology 2013, 24:534–541 This review comes from a themed issue on Environmental biotechnology Edited by Robert J Steffan and Juan Luis Ramos For a complete overview see the Issue and the Editorial Available online 19th September 2012 0958-1669/$ – see front matter, # 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.copbio.2012.09.002 Introduction Arsenic is an abundant but tightly bound element in the Earth’s crust, which, however, under particular hydro- geological conditions and/or as a result of human activities can lead to serious and widespread contamination [1,2,3  ]. Arsenic has a complex chemistry and can occur in different inorganic forms, such as trivalent AsIII in As 2 O 3 , AsO 2  , AsO 3 3 (arsenite) or AsH 3 (arsine), or as five-valent AsV in AsO 4 3 (arsenate). In addition arsenic can be bound in organic form, such as in arsenobetaine, trimethylarsine or arsenosugars, often as a result of enzy- matic processes [4]. Arsenicals are generally considered as extremely toxic [5], and the provisionally tolerable weekly intake for inorganic arsenic is set at 15 mg/kg of body weight [6] whereas the current WHO recommended drinking water limit is 10 mg/L [1]. The drinking water limit for inorganic arsenic is by far surpassed in potable water sources in large geographical areas such as the deltas of the Ganges, Brahmaputra, and Meghna (India, Bangladesh), Mekong (Loas, Cambodja) or Red River (Vietnam) [3  ]. Unfortunately, many of the most affected zones with arsenic groundwater contamination are also those where people have the least access to centralized drinking water systems, and where the logistics of measuring arsenic levels in potable water sources is cumbersome [7]. In addition, accurate quantification of arsenicals requires sophisticated chemical analytics and chemical field-tests are not satisfactory for a variety of reasons [7–9]. Consequently, many research groups have proposed alternative methods for assaying arsenic based on biological systems (e.g. bioreporters and biosensors). The purpose of this current opinion article is thus to review the most recent developments in bioreporters and biosensors for arsenic detection. We will briefly recapi- tulate the principal concepts of the biological detection of arsenic, then will focus on recent genetic engineering efforts, synthetic biology designs and micro-engineering strategies to incorporate and improve bioreporter assays, and finally will discuss the merits of biological assays under realistic field conditions. Biological detection of arsenic Arsenic is not only toxic to higher organisms but also to prokaryotes, and most prokaryotes have evolved exquisite resistance systems against arsenic [10]. Probably the best characterized resistance system to arsenic (and simul- taneously to antimonite) was discovered on the plasmid R773 in Escherichia coli [11], and is encoded by the arsRDABC operon (Figure 1). Expression of the operon is controlled by the ArsR protein, which in absence of arsenic binds as a dimer to an operator DNA called ArsR- binding- site (ABS, Figure 1A) slightly upstream of the 10/35 promoter [12]. Binding of ArsR to the ABS prevents transcription, but in the presence of arsenic the affinity of the dimer for the ABS decreases, allowing transcription to occur [12]. ArsD is a chaperone protein, which may help to bind and present arsenite to the ArsA ATP-ase part of a specific arsenite efflux pump (ArsAB) [13–15]. Cells can further reduce arsenate to arsenite by means of a reductase (ArsC), which can then again be removed from the cell through the ArsAB efflux pump [15]. Different varieties of the arsRDABC operon are found, some of which lack an arsD and arsA homologue [16], or which carry an additional gene arsH, an NADPH- dependent FMN reductase that might prevent re- oxidation of arsenite to arsenate [17]. Arsenic-resistance Available online at www.sciencedirect.com Current Opinion in Biotechnology 2013, 24:534–541 www.sciencedirect.com