A portable device for detecting the presence or amount of a pathogenic bacterium that is a source of a hospital acquired infection comprising: a portable enclosure containing a power source, an optical detector, a signal processor, and a signaling device for emitting a detectable signal upon detection of the pathogenic bacterium in an in vivo environment; a connecting device for attaching the enclosure to the clothing or body of a wearer; a fiber optic cable for inserting into the in vivo environment, the fiber optic cable being in optical communication with the optical detector, the fiber optic cable extending for a length exterior to the enclosure; and a delivery vehicle on at least a portion of the exterior surface of the fiber optic cable, characterised in that the delivery vehicle is adapted to deliver a recombinant bacteriophage to a targeted site, wherein the bacteriophage carries exogenous genetic material encoding a protein, the recombinant bacteriophage being configured to bind to the pathogenic bacterium and transfer the genetic material to the pathogenic bacterium, such that the pathogenic bacterium expresses the protein, and the expressed protein emits an optically detectable signal upon excitation by an excitation source or interacts with a cofactor to form a detectable marker that directly produces an optically detectable signal, wherein the delivery vehicle comprises a biodegradable microparticle having a biodegradable polymeric coating encapsulating the recombinant bacteriophage for release into the site following location of the delivery vehicle at the site; and wherein the fiber optic cable is configured to transmit the optically detectable signal to the optical detector, which comprises means to remove background light and to reduce the optically detectable signal to one or more diagnostically relevant emission peaks, the optical detector configured to produce an output signal proportional to the one or more emission peaks, and to transmit the output signal to the signal processor, which is configured to correlate the output signal from the optical detector to the concentration of the pathogenic bacterium at the site, to produce a detection signal and to transmit the detection signal to the signaling device which is configured to initiate the detectable signal if the detection signal reaches a predetermined threshold value corresponding to a known concentration of the pathogenic bacterium. a portable enclosure containing a power source, an optical detector, a signal processor, and a signaling device for emitting a detectable signal upon detection of the pathogenic bacterium in an in vivo environment; a connecting device for attaching the enclosure to the clothing or body of a wearer; a fiber optic cable for inserting into the in vivo environment, the fiber optic cable being in optical communication with the optical detector, the fiber optic cable extending for a length exterior to the enclosure; and a delivery vehicle on at least a portion of the exterior surface of the fiber optic cable, characterised in that the delivery vehicle is adapted to deliver a recombinant bacteriophage to a targeted site, wherein the bacteriophage carries exogenous genetic material encoding a protein, the recombinant bacteriophage being configured to bind to the pathogenic bacterium and transfer the genetic material to the pathogenic bacterium, such that the pathogenic bacterium expresses the protein, and the expressed protein emits an optically detectable signal upon excitation by an excitation source or interacts with a cofactor to form a detectable marker that directly produces an optically detectable signal, wherein the delivery vehicle comprises a biodegradable microparticle having a biodegradable polymeric coating encapsulating the recombinant bacteriophage for release into the site following location of the delivery vehicle at the site; and wherein the fiber optic cable is configured to transmit the optically detectable signal to the optical detector, which comprises means to remove background light and to reduce the optically detectable signal to one or more diagnostically relevant emission peaks, the optical detector configured to produce an output signal proportional to the one or more emission peaks, and to transmit the output signal to the signal processor, which is configured to correlate the output signal from the optical detector to the concentration of the pathogenic bacterium at the site, to produce a detection signal and to transmit the detection signal to the signaling device which is configured to initiate the detectable signal if the detection signal reaches a predetermined threshold value corresponding to a known concentration of the pathogenic bacterium.