A biosensor for detecting bacteria may use bacteriophages in a sandwich-assay system. The biosensor may include a capture element and a detection element. The capture element may include a substrate and a bacteriophage. The detection element may include a bacteriophage and a signal amplification element. The biosensor may be utilized such that the target bacterium is sandwiched between the capture element and the detection element, and a quantifiable signal may be generated to measure the amount of bacteria in a sample. The biosensor of the present invention utilizes direct sensing to detect the bacteria in the sample as opposed to indirect sensing methods.