**4.1 Reporter phage-probes**

Reporter bacteriophages are genetically edited phages used to import and insert a specific gene into the genome of target bacteria. The foreign gene inserted to host bacteria is expressed, bacteria are marked based on available platforms as a colorimetric, optical, or as a fluorescent marker and thus bacterial screening is permitted [30]. Irrespective of, whether reporter bacteriophages are lysogenic or lytic, both can detect potentially the particular pathogenic bacteria. A number of genes, such as insertion of firefly *luc* or bacterial *lux* gene account for bioluminescence, β-galactosidase*-lacZ* gene, ice nucleation*-inaW* gene, and also green fluorescent protein (*GFP*) gene reported by researchers as reporter phages and detected many of Gram negative and Gram positive bacteria [31, 32].

#### **4.2 Stained phages**

Phages stained with different fluorescent dyes have been used for target bacterial detection involving various fluorescence sensing tools. Stained phage-probes can discriminate a target bacterium when they infect and attack host cells [33]. Like, phages were tagged with fluorescent quantum dots (QDs) and *E. coli* was detected at 20 colony forming units per mL in water samples within 1 h [34].

#### **4.3 Lytic phages**

Lytic phages infection results in cell burst and consequently intracellular organelles, descendant phages, and cell-lysis materials are released. Both the release of intracellular elements and released progeny phages provide a base to recognize the target bacterium [30]. For example, as a released cell component, adenosine-triphosphate can be detected through bioluminescence just after target bacterial cell lysis [35]. Also the amount of released progeny phages released after cell lysis by a particular phage is directly proportional to the amount of lysed cells and can be used for bacterial sensing [36]. The released progeny phages enumerated by various detection mechanisms such as plaque- or immuno assays, molecular methods like quantitative PCR (qPCR) and, or by isothermal nucleic acid amplification (ITNAA) [37].

**163**

**Figure 3.**

*Principle and Development of Phage-Based Biosensors DOI: http://dx.doi.org/10.5772/intechopen.86419*

as a sign for the existence of bacterial cells [41].

**4.5 Phage receptor-binding-proteins**

**5. Phages immobilization strategies**

Phages that are immobilized on solid matrix can be utilized for capturing specific bacterial cell from contained samples. Bacteriophages have a many functionally active groups like hydroxyl group (—OH), aldehyde group(—CHO), carboxyl group(—COOH), etc., on their exteriors, giving them inimitable characteristics permitting their interaction with other materials and to interact with bacterial surface receptor molecules [38]. Consequently phages have been successfully used to capture particular bacterial cells from different samples [39, 40]. Like streptavidin actuated gold nano-particles were used to immobilize GM T4 bacteriophage particles. Delay in impedance was observed due to bacterial cells binding that marked

Some phage components display natural magnetism to host cell for example receptor-binding proteins (RBPs), but they are highly subtle to variations in environmental conditions. Phage tail bears RBPs and helps in binding to host bacterium, proceeding to insert its genetic material within the cell and cell infection is established [42]. RPBs bind to cell surface with help of specific polypeptide or polysaccharide sequences that are present on the cell surface. Poshtiban and colleagues activated magnetic beads by immobilization of RBP protein Gp047 (from phage NCTC12673). These functionally active beads were then utilized for *Campylobacter*

It's obvious from the literature that different approaches have been developed for immobilization of phages on surface of electrodes **Figure 3**. The common phage immobilization strategies on solid surfaces include physical adsorption [44],

The quantity of randomly oriented phages on solid surfaces is the most straightforward way for enhancing signal in bio-sensing scheme [47]. Deposition

*Different ways to potentially orient phages on solid surfaces. Green highlighted-bacterial binding proteins, (a) tailed phages-side-ways, head-down, or tail-down, (b) asymmetric icosahedral phages, (c) filamentous phages-through either side-ways or, pole, (d) filamentous phages are likely to be bundled or aggregated (left).* 

*Oriented typically parallel on the substrate (right), adapted from [5].*

cells withdrawal from samples of milk and broth of chicken [43].

covalent bonding [45], entrapment of phages in solid matrix [46], etc.

**4.4 Capturing phages**
