The detection of clinically relevant bacteria as whole cells is subject of intense research. Besides established methods like ELISAs, even optical biosensors as surface plasmon resonance spectroscopy (SPR) have been proven to be applicable for such purposes. However, detection limit seems to be only at 105 cells mL−1, which is not sufficient for many applications. A possible reason for this limitation is that particles like bacteria could only immerse to a very restricted degree into the evanescent field. As an alternative to SPR, reflectometric interference spectroscopy (RIfS) can be used for whole cell detection. It is expected that this method can overcome the aforementioned limitation because the space for detection is only, theoretically, limited to the dimensions of the flow cell. Experiments were conducted using Legionella pneumophila as model organism. In this work, preliminary results are presented demonstrating that detection of whole cells using a RIfS-device combined with a flow-through system is generally possible. Legionella cells were either captured on the sensor surface by hydrophobic interaction or alternatively by specific antibodies. However, the results indicate that the detection limit is in the same range as observed for SPR.