**2.1 Phage optical biosensors**

Optical phage-based sensors owing from their reasonably rapid screening, sensitivity, and flexibility to a broad-ranging assay situations have been extensively explored for bacterial detection. Optical methods are classified into two core subclasses on the basis of their working principles, label-free and labeled. The best frequently used optical methods for bacterial screening are fluorescence spectrometry [8], surface plasmon resonance (SPR) [10], and bio- or chemiluminescence [13]. In the subsequent subsection, our focus is on phage bio-probe-based optical biosensors for detection of pathogens with special emphasis on food safety and environmental monitoring. **Figure 1** represents a reporter phage-based optical sensing scheme.
