## 2 Phage-Based Biosensors  
According to the International Union of Pure and Applied Chemistry (IUPAC), the biosensor is defined as a self-controlled derivative material that contains a bio-recognition component (bio-receptor/bio-probe) linked to a transducer (sensor) to convert the biological signal into a digital signal in the computer system for interpretation [19]. Phage-based biosensor platforms generally consist of the network of the whole phage or partial phage particle, infection of the host bacterium, and finally production of colorimetric, electrical, fluorescent, or luminescent signals [20; 21; 22].  
Lytic bacteriophages are primarily classified under the order _Caudovirales_ (Figure 1) and are the principal biorecognition entities used as probes for phage-based biosensors. Apart from lytic phages, temperate phages also play a comparable role in the development of phage-based biosensors. Both lytic and temperate phages, such as HK620, P22, and \(\Phi\)V10, have been used to develop reporter (engineered) phages [14]. Reporter phages are genetically modified by incorporating a reporter gene sequence into the phage genome to generate a measurable signal inside the intact host cell without killing (lysing) the host cell for the detection of live pathogens [14]. Moreover, proteins such as phage receptor-binding proteins (RBPs) have been recognized to be efficient bio-probes for replacing antibodies or other biomolecules, and have been used in the design of various types of biosensors [23]. In comparison to the whole phages, RBPs provide better stability across a broad range of pH values, temperatures, and gastrointestinal proteases [24]. Remarkably, appropriate tags (amino acids, e.g., cysteine) can be added to the RBP sequence at a specific site without affecting the binding ability and can be employed for the oriented surface functionalization of the RBPs on the biosensor platforms [24].  
Bacteriophage-based biosensors offer several benefits for rapid bacterial detection [25]. They are highly specific towards their host organism, resist high temperatures (90-97 \({}^{\circ}\)C), and are stable across a wide range of pH values (3-14) and organic solvents. In comparison to antibodies, phages can be produced in large quantities easily and cheaply. They are eco-friendly and safe to use since they do not infect humans [26]. These characteristics make phages a novel bio-recognition tool for the development of biosensors for the detection of foodborne bacterial pathogens [27; 28].  
Figure 1: Schematic diagram showing the structure of major phage families: (**A**) _Myovidiidae_ (e.g., T4); (**B**) filamentous _Inoviridae_ (e.g., M13); (**C**) long and noncontractile _Siblovidiidae_ (e.g., \(\lambda\) phage); (**D**) _Leviviidae_; and (**E**) short _Podoviridae_ (e.g., T7).  
Today, phage-mediated biosensors have been developed as novel diagnostic tools in which specific phages are fixed to the device's surface and then enabled to detect the analyte found in the sample [29]. Bacteriophages can be immobilized on a solid material with the aid of chemical, physical, or other immobilization or tethering techniques. The capture of targeted bacterial cells by surface-immobilized virions is an event that ends up with specific detection. The detection of pathogens using phage-based sensors is not limited to clinical samples, but is also used in a wide range of nonclinical applications, including foodborne pathogens from water and various food matrices [30], such as milk [31] and other perishable and non-perishable foodstuffs [32].