*2.1.5 Phage-colorimetric sensors*

Sensing based on changes in color allows the use of simple diagnostic systems like spectrophotometers, or even involving smartphones, and both of them are comparatively common and feasible. Designed colorimetric phage-based biosensors are mostly based and integrated on the utilization of reporter bacteriophages that carry genes coding for reporter enzymes. The foremost colorimetric sensor based on phage was to detect *Salmonella* ice nucleation sensor using reporter gene *inaW* [29]*.* Expression of ice nucleation protein was induced upon infection, interrupting the cell, and was consequently observed by the addition of an indicator dye (orange colored) [30]. Other serviceable reporter genes that have been successfully used with various colorimetric substrates are *celB* and *lacZ* segments encoding β-galactosidase and β-glycosidase [31]. More recently enhanced phage-based colorimetric technique has been reported to be integrating and coupling with novel technologies like surface plasmon [32], macroscope and smartphone [33], and lateral flow assay [11]. Other colorimetric phage-based biosensors established in recent years are briefed in **Table 1**.
