Interplays of mutations in , , and contribute to phage resistance in .

Plague caused by remains a public health threat worldwide. Because multidrug-resistant strains have been found in both humans and animals, phage therapy has attracted increasing attention as an alternative strategy against plague. However, phage resistance is a potential drawback of phage therapies, and the mechanism of phage resistance in is yet to be investigated. In this study, we obtained a bacteriophage-resistant strain of (S56) by continuously challenging 614F with the bacteriophage Yep-phi. Genome analysis identified three mutations in strain S56: * (9-bp in-frame deletion GTCATCGTG), (10-bp frameshift deletion CCGGTGATAA), and (1-bp frameshift deletion A). WaaA (3-deoxy-D-manno-octulosonic acid transferase) is a key enzyme in lipopolysaccharide biosynthesis. The mutation leads to decreased phage adsorption because of the failure to synthesize the lipopolysaccharide core. The mutation in (encoding cytidine monophosphate kinase) increased phage resistance, independent of phage adsorption, and caused growth defects in The mutation in inhibited phage adsorption while restoring the growth of the null mutant and accelerating the growth of the null mutant. Our results confirmed that mutations in the WaaA-Cmk-Ail cascade in contribute to resistance against bacteriophage. Our findings help in understanding the interactions between and its phages.