Bacterial endotoxins such as lipopolysaccharides (LPS) are major contaminants of most pharmaceutical and consumer products. We report an antibiotic-mediated plasmonic biosensor for LPS detection based on a facile U-bent fiber optic probe (UFOP) technology. Biomimetic self-assembled layer of octadecyltrichlorosilanes (OTS) were functionalized on the surface of optical fiber probes to hydrophobically entrap LPS from aqueous solutions. The binding of LPS molecules was monitored in real-time by measuring the change in refractive index (RI) in the evanescent layer. To add specificity and signal amplification, the bound LPS molecules were further tagged with antimicrobial polymyxin-B conjugated gold nanoparticles (PMB-AuNPs) in a sandwich format. The assay was extensively optimized by investigating the role of experimental parameters like OTS concentration, incubation time and addition of a silver reduction step at the end of the assay. The lower limit of detection (LOD) for LPS was found to be 0.4 ng/mL with a 36-fold improved sensitivity upon silver enhancement. The total assay time was 1 h. The assay was also found to be highly specific in the presence of common biopharmaceutical components and could thus serve as an efficient endotoxin detection platform for quality control testing during therapeutic development.