The Caudalovirales order of bacteriophages (phages) is known for its structural complexity. These phages are sophisticated molecular machines, with a protein capsid that houses a double-stranded DNA genome, linked to a tail structure that interacts with bacterial receptors and provides for regulated delivery of phage DNA into the host bacterium. Until very recently, the Caudalovirales were categorized into three prominent families: Siphoviridae (long, non-contractile tails), Myoviridae (long, contractile tails), and Podoviridae (short, non-contractile tails). In 1936, a unique phage of the Siphoviridae family, named Chi (χ), was discovered to infect Salmonella enterica Typhi. A cryo-EM and immuno-EM were used to characterize a form of phage χ called YSD1, showing it to be a flagellotropic phage and providing an understanding of its capsid, tail tube, and flagellum-binding tail-fiber. Recently, a cryo-EM study revealed χ’s nearly complete structure, encompassing these same features as well as the neck and tail tip, illuminating the previously unexplored structural organization of phage χ/YSD1.
Our work now is directed at revealing the high-resolution cryo-EM structure of the Chi-like phage called PIN2, isolated from south-eastern Australia. Comparative genomic analysis shows that PIN2 shares conserved genomic regions with Chi and YSD1, but distinct differences in the baseplate proteins suggest specialized adaptations. Notably, unlike all other Chi-like phages that depend on binding bacterial flagella for entry into their host, PIN2 infects Klebsiella, a bacterium that has no flagellae, indicating it uses a unique alternative receptor and a distinct mechanism by which PIN2 infects its non-flagellated hosts. Our study aims to explore the step-by-step mechanism of PIN2 in infecting non-flagellated hosts, providing new insights into the diversity of Chi-like phages and their interactions with different bacterial hosts.