The Caudovirales 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 facilitates DNA delivery into the host bacterium.
Until recently, Caudovirales were categorized into three prominent families: Siphoviridae (long, non-contractile
tails), Myoviridae (long, contractile tails), and Podoviridae (short, non-contractile tails).
In 1936, a Siphoviridae phage named Chi (χ) was discovered to infect Salmonella enterica Typhi. Cryo-EM and
immuno-EM analyses of a Chi-like phage YSD1 revealed it to be a flagellotropic phage and provided insight
into its capsid, tail tube, and flagellum-binding tail fiber. More recently, a cryo-EM study revealed χ’s nearly
complete structure. Our current work investigates a Chi-like phage named PIN2, isolated from south-eastern
Australia. Structural analysis shows PIN2 shares Chi and YSD1’s capsid and tail components but lacks a
flagellum-binding tail fiber. Unlike traditional Chi-like phages, PIN2 infects Klebsiella — a non-flagellated host
— using an alternative receptor and infection mechanism.
Based on near-complete cryo-EM reconstructions at sub-nanometer resolution, this study defines the overall
architecture of the Chi-like phage PIN2, including its capsid, neck, tail, and tail tip modules. Our ongoing work
focuses on map interpretation and model refinement to further clarify the infection mechanism. These findings
deepen our understanding of Chi-like phage diversity and offer a structural foundation for future phage therapy
strategies.