Bacteriophage could be an alternative to antibiotics for the treatment of multidrug-resistant bacterial infections. Indeed, it has proven to be medically superior to antibiotic therapy in many ways 
. However, the emergence of phage-resistant variants was observed rapidly if only one phage strain was used against a particular bacterium 
. Fortunately, there are an abundance of other phage species which possess lytic ability against resistance-obtained variants 
In the present study, we report a SBS method. The method takes advantage of the occurrence of phage-resistant variants and ensures phages lytic for wild-type strain and its phage-resistant variants are selected. The phage cocktail established by this method can not only display broad lytic range, but also ensure that bacteria resistant to one phage remain susceptible to others. Hence, the cocktail established by SBS is significantly different from previous phage cocktails 
Three phages have been isolated using the SBS method in this study. If cells which resistant to phage “X” are sensitive to phage “Y,” it can be concluded that the adsorption receptors or the infection mechanism of phage “X” and “Y” are different 
. In this sense, GH-K1, GH-K2 and GH-K3 could identify different receptors or possess different infection mechanism.
The avirulence and reduced persistence of these variants in vivo were in agreement with previous studies 
. Perhaps, the occurrence of this phenomenon was because the variants could be cleared rapidly by innate immunity 
. Otherwise, in the case of K. pneumoniae
, the virulence is related to the surface-specific polysaccharide antigen (SPA), which acts as an initial surface binder that helps the organisms gather together or form biofilms 
. Maybe, compared with K7, SPA on the variant strains might have been changed or missed.
In most cases, the appearance of phage-resistant mutants was not considered important as earlier studies have reported that resistant variants tended to be avirulent and were easily taken care by phagocytes and immune system 
. However, the fast appearance of phage-resistant variants might limit the application of phage therapy. From the experiment in vitro, it was demonstrated that the phage cocktail significantly reduced the frequency of mutation compared with any monophage, and displayed higher lytic efficacy. Although the resistant variants also appeared when the cocktail was used, it needed a much longer time to accumulate enough mutations to develop resistance to three phages 
. In addition, the long-term inhibition function was sufficient for the immune system of mice to eliminate the small number of avirulent variants.
More importantly, compared with any single phage, the phage cocktail effectively rescued Klebsiella pneumoniae
bacteremia in the murine K7 strain challenge model. In addition, the dynamics of bacteria in the blood demonstrated that the cocktail needed a smaller dose, but displayed stronger elimination of bacteria. Maybe, it can be concluded that the phage cocktail proliferation threshold was higher than that of monophage in vivo 
. Since, a small amount of phage cocktail can proliferate quickly, resulted in the fast elimination of bacteria in infected mice and lower the development of phage resistance. Coupled with the data obtained from the experiment in vitro, it showed that these three phages might have a synergistic effect on killing K7, at least based on these limited studies.
In our opinion, it will be more reasonable to design corresponding phage cocktail according to the specified pathogen. A large number of phages need be isolated and identified and the phage library (warehouse) should be established. The phage cocktail could be formed using SBS method from the library or medical institution and prepared in advance.
Overall, the data presented in this study showed that phage cocktail established by SBS method has great therapeutic potential for multidrug-resistant bacteria infection.