Salmonella typhimurium harbours a Au-resistance system whose expression is controlled by GolS, a transcriptional regulator of the MerR family that selectively detects Au with high sensitivity. We developed both Salmonella and genetically engineered Escherichia coli strains as Au-selective whole-cell biosensors by coupling the strictly regulated GolS-dependent golB promoter to the gfp reporter gene. The bio-reporters were evaluated under different laboratory conditions and calibrated for their use as selective Au detectors. Due to the intrinsic characteristics of the regulatory protein, the transgenic E. coli sensor exhibits low background, high signal-to-noise ratio, and improved sensitivity for detection of Au ions in a wide range of concentrations (up to 470 nM) with a calculated detection limit of similar to 33 nM (6 mu g L-1 or parts per billion) Au(I). The fluorescent Au-sensing bacteria exhibit also minimal interference by chemically related metals such as Cu or Ag that are commonly found in Au deposits. These highly specific and sensitive Au detectors might allow the development of rapid and robust screening tools to improve discovery and extraction procedures. Biotechnol. Bioeng. 2011; 108: 2553-2560. (C) 2011 Wiley Periodicals, Inc.