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Introduction
As the human population continues to grow, progress in medical and pharmaceutical sciences has led to a parallel increase in the global use of medications, including antibiotics. Along with other pharmaceuticals, increasing amounts of antibiotics find their way into the environment (Jones et al., 2001; Heberer, 2002; Kolpin et al., 2002) by diverse routes, usually after being excreted through urine and faeces (Daughton and Ternes, 1999; Hirsch et al., 1999). Through medical and agricultural applications, antibiotics spread in the environment at low concentrations (amoxicillin, for example, has been detected at approximately 30–80 ng ml−1; Kümmerer, 2004). Such concentrations are not necessarily bactericidal but may nonetheless contribute to the spread of bacterial antibiotic resistance (Ash et al., 2002; Baquero et al., 2008; Roberts, 2011), which may find its way into human food, gut flora or directly to pathogens (Silbergeld et al., 2008).
The traditional approach for detecting chemicals is based on chemical or physical analyses that allow highly accurate and sensitive determination of the exact composition of the tested sample. However, such methodologies fail to provide information regarding the bioavailability of pollutants, their effects on living systems, or their synergistic or antagonistic behaviour in mixtures. A complementary approach is based on the use of diverse living systems in a variety of bioassays. Unicellular microorganisms, in particular bacteria, are attractive for these purposes due to their large population size, rapid growth rate, low cost, easy maintenance and their amenability to genetic engineering (Belkin, 2003; van der Meer and Belkin, 2010).
Genetically engineered bacteria hold great promise as sensor organisms as their responses can be genetically ‘tailored’ to report either on specific biological effects or on the presence of pre‐determined classes of chemicals (Magrisso et al., 2008; van der Meer and Belkin, 2010). Reporter bacteria can be engineered to produce a dose‐dependent quantifiable signal (fluorescent, bioluminescent, electrochemical, etc.) in the presence of the target chemical or stress factor. These reporters are usually molecularly modified by fusing a promoter sequence, known to be responsive to the target compound, to a reporter system, such as the luxCDABE genes (Shapiro and Baneyx, 2007; Yagur‐Kroll et al., 2009;