**3.5 Bacteriophages**

Phages are virus particles, infecting and reproducing only within bacterial cells. Because of their associated evolution along with bacteria, phages have extremely specific machineries to identify and then infect their host bacteria for propagation. Phages generally have two distinctive chunks, the head comprising genetic material while the tail accountable to recognize and attach to bacterial cell [22]. Phages have several biomedical applications, and owing to their specificity they are extensively used for specific and sensitive detection of bacteria [23]. Most significant feature of phages is that they can only identify, and attack living bacterial cells. This exciting feature was well demonstrated by Fernandes et al., to detect viable, viable but not culture-able, or totally dead *Salmonella* cells on a biochip that was bio-functionalized with either phage or antibody as a bio-recognition element [24]. Interestingly dead cells were still capable to interact with the antibody, phage probes enabled a superior difference among viable and dead *Salmonella*. Additionally, some of phages are very specific and infecting only one bacterial specie [25]. This property will permit the recognition of target pathogenic bacterial species in a complex flora. Comprehensive explanation of bacteriophage based recognition elements employed as bio-probes in development of a biosensor to detect pathogenic bacteria, is outlined in the following section.
