We aimed to unravel BB action's protective mechanism and make a step forward to understand the selectivity in phage/bacteria systems. We designed a simple yet elegant experiment to prove that interactions between BB and virions are substantial for protection (Figure ). Two quartz cuvettes were bonded and covered so light could only pass through from one direction. Phages remained active only when BB was added directly to the suspension. No protection was observed when BB was placed in the first cuvette and phages in the second one. This confirmed that the mechanism was not simply the absorption of UV light by a solution of dye. If this was true, then all UV-absorbing molecules should protect virions, which was not the case (Figure ). The necessity of placing both components in a single cuvette was a strong suggestion that the UV protective properties required interactions between virions and protectants. We postulate that the protection originated in food dyes binding to the surface of the viral capsid, which resulted in the accumulation of molecules, absorption or scattering of a significant portion of photons, and finally, reduction of the exerted impact of UV irradiation on viral particles. We propose the name 'molecular sunscreen' mechanism. sulfonate (NSA), sodium isatin-5-sulfonate (ISA), and isatin along with the molecular structures of the compounds. The analysis allowed us to identify three domains required for the molecule to have phageprotective properties: i) sulfonic group, ii) UV-absorbing domain (here -aromatic rings), and iii) side group able to form hydrogen bonds.