Ultrasensitive and accurate detection of microbial pathogens is of critical importance for healthcare,food safety,water quality control,and antibioterrorism.Bacteriophages(or phages for short)are viruses that exclusively infect bacterial host cells with high specificity.In 2011,we developed a new strategy for the sensitive and selective bacteria detection by utilizing a tetracysteine(TC)-tagged phage in conjunction with membrane permeable fluorescein arsenical helix binder(FlAsH)1.This methodology integrates the natural specificity of phages and rapid phage replication within their bacterial hosts with the highly sensitive fluorescence labeling of biarsenical dye to TC tagged proteins.Because phages propagate only in living host cells,the TC-phage-FlAsH strategy is further exploited to the discriminative detection of viable target bacteria from dead target cells and other viable but nontarget bacterial strains 2.Recently,with the demonstration of pathogenic bacteria(E.coli O157:H7)detection in food matrix,we mark an important step forward from the harmless engineered strains of E.coli in simple water matrix.This requires the construction of a recombinant PP01-TC phage by inserting the TC tag into a wild type PP01 phage genome rather than the widely used engineered phage of M13 or T7.Particularly,with 40 mL of sample volume,as low as 1 cfu/mL of E.coli O157:H7 in apple juice and with the presence of a large excess of non-target bacteria can be detected in less than 3 hours by fluorescence microscopy.We believe that the TC-phage-FlAsH approach opens a new avenue for the specific and sensitive detection of a wide variety of pathogens in different sample settings.