Mannose-rhodamine (Rh) conjugate (80% yield) was synthesized in a one-pot reaction and immobilized onto magnetic polymeric nanoparticles (MPNP; 43% magnetic content) of poly(styrene/divinyl benzene/acrylic acid). The resulting nanoparticles contained MPNP as a substrate, mannose as an E. coli receptor and Rh as a fluorescent signaling unit. TEM imaging clearly demonstrated that multiple mannose-Rh MPNPs could be captured by E. coli strain ORN178. The fluorescent signal from captured nanoparticles was emitted at 580 nm. These results indicate that mannose-Rh MPNP offers a simple and rapid strategy for bacterial detection.