This work reports the fabrication of an open-ended mesoporous porous silicon membrane (PSiM) for the fast and indirect detection of bacteria through their lysate. Porous Silicon (PSi) is a promising material for biosensing applications because of its tuneable optical properties and large surface area. The performance of PSi-biosensors is however hindered by the diffusion-limited infiltration of analyte, which lowers the sensitivity and increases the detection time. To overcome this limitation, Porous Silicon Membranes (PSiMs) have been fabricated using standard microfabrication techniques and previously studied for the detection of small biomolecules. In this work, PSiMs were developed for the optical detection of Bacillus cereus lysate. The protocol of detection starts with a bacterial suspension, which is incubated for 30 min at 30°C in the presence of PlyB221, an endolysin encoded by the bacteriophage Deep-Blue targeting B. cereus. In the meantime, a signal baseline is established by flowing PBS through the sensor. After the incubation, the lysate is pumped through the PSiM at a flow speed of ~15 µL/min. The PSiM is monitored optically and the detection is based on a shift in the effective optical thickness (EOT). The EOT equals 2nL, where n is the average refractive index of the PSi and L its thickness. The penetration of bacterial lysate induces an average EOT shift of ~600 nm after 1 hour, which is nearly 3 times higher than the noise level in PBS. The initial concentration of bacteria before the lysis was measured as 10⁶ CFU/ml. Not only does this biosensor enable the fast detection of bacteria, but the same technique can be adopted for other bacteria after their selective lysis.