Significant levels of infectious diseases caused by pathogenic bacteria are nowadays a worldwide concern implying considerable public healthcare challenges and huge economic detriment. Due to the fast spread of these bio-threat agents and the outbreak of diseases, a rapid detection of pathogens at early stages is crucial, particularly in low-resource settings. To this aim, we developed for the first time a new sensing approach carried out in a single-step for Escherichia coli O157:H7 detection. The detection principle is based on Förster Resonance Energy Transfer using gold nanoclusters as signal reporter and gold nanoparticles conjugated with antibodies as a quencher. The sensing platform includes and UV-LED to provide the proper excitation and consists in a microtube containing two pieces of fiber glass; one of them is embedded with label-free gold nanoclusters and the other one with gold nanoparticles conjugated with antibodies. Upon the addition of the sample containing bacteria the florescence of gold nanoclusters is recovered. The assay was evaluated by naked eye (on/off) and quantitatively using a smartphone camera. The biosensor proved to be highly specific and sensitive, achieving a limit of detection as low as 4.0 CFU ml-1. Additionally, recoveries of 110% and 95% were obtained when evaluating the platform in spiked river and tap water respectively.