## 3 Phage-Mediated Bacterial Detection Approaches
### Whole-Phage or progeny Virion Detection  
Lytic phages infect the host cell, and the number of progeny virions released from the infected cell is directly proportional to the number of bacteria infected. This approach was first reported by [PERSON] et al. (1998) [47], in which cells were infected with phagesfollowed by treatment with a virucidal agent to eliminate the added phage, thus allowing only progeny phage to be detected. The developed assay was sensitive and could obtain results in 4 h using plaque assays. Alternative assays, including molecular diagnostic tools such as quantitative PCR (qPCR), have been used to determine the number of progeny virions released from the infected cells [38] as well. For instance, _B. anthracis_ was detected by immuno-chromatography, which has been designed based on a lateral-flow assay and the amplification of the gamma phage (\(\gamma\)) in bacterial cells. The virions released from the infected cells have been detected via reporters made of polystyrene nanoparticles linked to anti-\(\gamma\) phage antibodies. The detection limit has been recorded as 2.5 \(\times\) 10\({}^{4}\) CFU/mL with a 2-4 h assay time [48].  
The plaque assay is one of the easiest/most straightforward methods for detecting foodborne pathogens to determine infection by increased titer [49]. If the titer of the phages rises, it relates to the effective binding or adsorption of phages to the host bacteria, resulting in lysis and the release of progeny virions, and thus indicating the existence of the viable target pathogens in the food matrices as initially described by [PERSON] et al. [47]. Recently, an assay was developed that employed phages coupled with qPCR for the detection of _S. enterica_ ser Enteritidis in spiked chicken meat samples [50]. Approximately 0.22 fg/mL of pure phage (vB_SenS_PVP-SE2) DNA and nearly10\({}^{3}\) pfu/mL of virions were detected using the combined technique with a detection limit of <10 CFU/25 g for 10 h of analysis, which included 3 h pre-enrichment, 6 h co-incubation, and 1 h DNA enrichment and qPCR.  
Despite its benefits, intact phages suffer from certain limitations that restrict their use in the development of whole-phage sensor systems. The fast adsorption of phages onto the host cell and their subsequent lytic activity may destroy the target bacterium before the completion of downstream detection steps. The size of phages is also another constraint that adversely affects the whole-phage detection system. Besides, some phages produce catalytic enzymes towards the receptors situated on the surface of the bacterial cell. For instance, the endorhaminosidase enzymes produced by the P22 phage can degrade the O-antigen of the outer membrane structure of Gram-negative bacteria, especially _Salmonella enterica_, which then affects the subsequent attachment process. The _S. flexneri_ phage, Sf6, shows similar endorhammosidase-mediated cleavage [51]. Such phage-encoded enzymes can interfere with biosensor performance, leading to poor signal output. Moreover, intact phages can dry up on the surface of the biosensor, which ultimately can collapse and prevent tail fibers from attaching to the target bacterium [52].