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In food quality, healthcare and biotechnology the presence of bacteria a nd viruses can have considerable consequences both willing and unwilling . Accurate identification and quantification of the bacteria and virusse s is key in this context. Currently, there are only a limited number of truly fast and reliable methods available. Sensitive diagnostic tests co uld have a large impact on several crucial healthcare problems such as a ntibiotic resistance or could further improve food quality control. Ther efore the aim of this disseration was to develop innovative, fast and hi ghly sensitive bioassays on a compact fiber optic SPR biosensor platform (FO-SPR), that can be used to identify and quantify bacteria and viruse s in a multiplexed way. First, the FO-SPR sensor was used to st udy the affinity and binding kinetics of small viral particles. These so -called bacteriophages, can display peptide libraries with affinity for almost any target molecule. The target molecule, in this case eGFP, was immobilized on the FO-SPR biosensor surface and afterwards exposed to th e bacteriophage library, allowing real-time monitoring of their interact ions. Although the bioassay based on the detection of the entire viruses showed limited sensitivity, it proved to be a valuable tool for compari ng binding kinetics of different viral particles, which expressed affini ty peptides in different densities on the surface. Following, k eeping in mind the need for both detecting and identifying pathogens wit hin the same test, a more universal bioassay was created using the genet ic material of micro-organisms. The thermal denaturation of DNA complexe s was measured using the FO-SPR technology. Here, the target DNA was use d to form complexes between two hybridization probes, each complementary to one half of the target molecule and immobilized both on the gold nan oparticles (Au NPs) and the sensor surface. Amplification with gold nano particle labels resulted in a clear signal, which superimposed the SPR s ignal caused by temperature changes during DNA melting analysis. This assay was then validated by measuring genetic variability in two genes of L. pneumophila, which is a common human pathoge n responsible for atypical pneumonia. Although the two selected genes ar e frequently used for the quantification of these bacteria, one of them is well documented as highly variable and therefore prone to introducing amplification bias in PCR based molecular diagnostic tests. The FO-SPR biosensor proved to be reliable for detecting mutations in those samples that previously displayed ambiguous qPCR quantification results. Moreov er, it showed advantages as a high resolution and fast genetic screening tool that can compete with the current standard techniques for single p oint mutation (SNP) detection.Next, the possibilities of the FO-SPR DNA biosensor were explored for target quantification. The FO-SPR biosen sor was able to monitor in real-time quantitative ligation. The gradualincrease in signal over multiple DNA ligation cycles was used to determi ne DNA concentrations in a broad dynamic range with a detection limit of 100 fM. Even more importantly, the obtained melting signal during each cycle of the ligation was sufficiently accurate to discriminate an SNP i n the amplified target molecule. Finally, the FO-SPR melting as say was applied to directly detect two bacterial species in one sample. To allow multiplex detection on FO-SPR, hybridization probes complementa ry to two regions of interest in different bacteria were immobilized on a single FO-SPR sensor and Au NPs. By choosing target regions with non-o verlapping melting curves in each bacteria, the presence of one or both bacteria could easily be identified. Even DNA targets with SNPs could be differentiated from the wild type targets. This achievement is a significant step towards a FO-SPR biosensor for pathogenic bacteria . Further development of this platform concerning both the FO-SPR biosen sor hardware and the implemented bioassays could make this technology an ideal candidate for the next generation point of care biosensor