Antibiotic microbial resistance is, according to the World Health Organization, one of the top 10 threats to global health. One way to prevent and control antibiotic resistance is the development of novel diagnostic tools. In this work, we present a biosensor for the rapid optical detection of bacteria after their selective lysis, based on open-ended Porous Silicon (PSi) membranes. PSi is widely used for biosensing applications, thanks to its large surface area and unique optical properties [1]. In this study, Bacillus cereus was selectively lysed by PlyB221, a recently characterized endolysin encoded by the Deep-Blue bacteriophage targeting B. cereus [2]. The bacterial lysate was then pushed through a PSi membrane, which generated changes in its optical properties. A method called Reflective Interferometric Fourier Transform Spectroscopy (RIFTS) was used to quantify these changes, by measuring the effective optical thickness (EOT) of the PSi membrane overtime. This EOT equals 2nL, with n the refractive index of the porous matrix, and L is thickness. With this detection protocol, we were able to selectively detect 105 colony forming units per mL (CFU/mL) of B. cereus lysate in 1h.