A biosensor system is provided for detecting biological species such as bacteria, proteins, viruses, spores and DNA or RNA. The biosensor system may also be able to distinguish between Gram positive and Gram negative bacteria. In some embodiments, the analyte solution flows past a detector surface to a filter, which traps the analyte. The analyte is then washed back past the detector surface, thus increasing the number of analyte species that become attached to the surface. The detector may include an optical microresonator that is optically coupled via waveguides to a light source and an optical detector. One of the waveguides may be provided with a wavelength selective reflector to increase the amount of probe light coupled into the microresonator or to increase the fraction of the signal light detected by the optical detector.