Au-Ag alloy nanoparticles (NPs) were prepared by the reduction of metal ion mixtures in aqueous sodium citrate solution using sodium borohydride (NaBH4). The resulting Au-Ag alloy NPs were analysed by various techniques. Alloy-attached chips for the detection of microorganisms were fabricated simply by the attachment of Au-Ag alloy nanoparticles onto glass slides after silanization through self-assembled monnolayers (SAMs) for the formation of activated amine (-NH2) as a terminal function group. The alloy-attached chips were investigated for their ability to bind the target Escherichia coli (E. coli) in water. E. coli was detected in water as a function of time and concentration by UN-vis spectroscopic measurements based on the interaction between the alloy-attached chip and E. coli. Field-emission scanning electron microscopy (FE-SEM) was used to directly observe the E. coli captured on the alloy chips. These studies demonstrated that E. coli in drinking water can be directly detected with Au-Ag alloy microchips without requiring any interaction between an antibody and an antigen.