Phages are useful where bacteria are unwanted. Already in 1917 when Felix d’Herelle discovered phages their potential application for medical therapy was imaginable and was soon dominating this early scientific approaches using phages; however, without being able to visualize the phages (a phage is only visible in an electron microscope). It was the lytic effect that made phages attractive. As phages specifically attack bacteria only while leaving all other organisms or cells intact and because phages are targeting a limited number of strains within a bacterial species, they are a logic, biological, natural, antibacterial agent with a high specificity of action.
Lytic phages for therapy purposes must either be isolated from natural habitats by “screening” them against the respective bacterial strain or, there might be an already available collection of existing phage preparations that might be able to target the unwanted bacteria, possibly by using a defined “phage cocktail” (mixture of different phages).
All expertise and knowledge of phage biology and the many decades of successful phage therapy allow the statement that phages do not cause side effects. Phages typically lyse target bacteria quickly and completely which means they contribute to a proper healing effect of wounds. Principally, the availability of potential therapy phages against a bacterial strain will never be limited as novel phages can be found in the environment again by screening. Therefore, the question on bacterial multiresistance against a phage is irrelevant. Furthermore, it is irrelevant for phage application if the bacterium is multiresistant against antibiotics: the phage only needs the specific bacterial cell surface to attack it.
Physicians applying phage therapy have profound expertise in phage biology and cooperate with phage biologists who have characterized the phages properly before application. Different methods are used by physicians to apply phages at the localization of the bacterial infection. However, this of course cannot be subject of this website.