K1 capsule-specific phages of
Escherichia coli
have been reported in recent years, but the molecular mechanism involved in host recognition of these phages remains unknown. In this study, the interactions between PNJ1809-36, a new K1-specific phage and its host bacteria
E. coli
DE058, were investigated. A transposon mutation library was used to screen for receptor-related genes. Gene deletion, lysis curve determination, plaque formation test, adsorption assay and inhibition assay of phage by lipopolysaccharide (LPS) showed that capsular polysaccharide (CPS) was the first receptor for the initial adsorption of PNJ1809-36 to
E. coli
DE058 and LPS was a secondary receptor for the irreversible binding of the phage. The penultimate galactose in the outer core was identified as the specific binding region on LPS. Through antibody blocking assay, fluorescence labeling and high-performance gel permeation chromatography (HPGPC), the tail protein ORF261 of phage PNJ1809-36 was identified as the receptor binding protein on CPS. Given these findings, we propose a model for the recognition process of phage PNJ1809-36 on
E. coli
DE058: The phage PNJ1809-36 tail protein ORF261 recognizes and adsorbs to the K1 capsule; then the K1 capsule is partially degraded, exposing the active site of LPS which is recognized by phage PNJ1809-36. This model provides insight into the molecular mechanisms between K1-specific phages and their host bacteria.
IMPORTANCE
It has been speculated that CPS is the main receptor of K1-specific phages belonging to
Siphoviridae
. In recent years, a new type of K1-specific phage belonging to
Myoviridae
has been reported, but its host recognition mechanisms remain unknown. Here, we studied the interactions between PNJ1809-36, a new type of K1 phage, and its host bacteria
E. coli
DE058. Our research showed that the phage initially adsorbed to the K1 capsule mediated by ORF261 and then bound to the penultimate galactose of LPS to begin the infection process.