O-antigen serves as a two-faced host factor for bacteriophage NJS1 infecting nonmucoid Klebsiella pneumoniae.

Klebsiella pneumoniae is an opportunistic pathogen commonly associated with nosocomial infections. In our previous study, we have demonstrated that colistin-resistant K. pneumoniae is more susceptible to killing by lytic tailed phages than the colistin-sensitive parent strain, including T1-like ФNJS1. This fitness cost associated with colistin resistance is due to the alteration of the surface charge that promotes phage adherence and infection. However, the receptor for phage adsorption has not been identified. In this study, we found that ФNJS1 specifically infected nonmucoid K. pneumoniae isolates, and the accelerated phage adsorption to colistin-resistant nonmucoid K. pneumoniae cells is reversible. Further research suggested that bacteria lipopolysaccharide may be involved in phage reversible adsorption, while capsule polysaccharide may block the receptors on cell surface from phage attachment. Transposon mutagenesis of colistin-resistant K. pneumoniae revealed that mutation in wecA and wecG, two genes involved in lipopolysaccharide O-antigen biosynthesis, significantly deceased phage adsorption capacity and infection efficiency. Inactivation of wzyE, which leaded to the shorten of O-antigen chain length, enhanced phage infectivity. Moreover, mutation of the outer membrane protein FepA slowed the phage lysis rate, suggesting that FepA may be an irreversible receptor for ФNJS1. In summary, our results show a delicate balance between ФNJS1 and its hosts, where the lipopolysaccharide O-antigen may serve as an essential reversible receptor for phage NJS1, while the long O-antigen chain hinders the bacteriophage infection.