Antibiotic-resistant strains of Staphylococcus aureus cause human infections that are difficult to treat and lead to death [1]. Host-range mutants of bacteriophage (phage) phi812 infect 90 % of S. aureus isolates and, therefore, are promising phage therapy agents [2]. As the phage approaches its host cell, phage receptor-binding proteins attach to the cell wall. This interaction triggers a cascade of structural changes in the baseplate, resulting in phage tail contraction and genome ejection into the host cytoplasm [3]. Mechanistic description of the baseplate re-organization, however, remains unknown.
Using cryo-electron microscopy (cryo-EM), we reconstructed the phage baseplate in extended and contracted states. The quality of reconstructed maps enabled us to assign individual proteins to their densities. Selected proteins involved in the host cell wall binding and penetration were produced in recombinant form and their structures were solved using X-ray crystallography and cryo-EM single-particle reconstruction.
We present the first detailed structural characterization of a contractile phage infecting Gram-positive bacterium. Comparison of the two distinct baseplate states allows the description of the initial stage of phage infection on the molecular level. Finally, our results provide framework for engineering phage particles to combat S. aureus infections in humans.
We acknowledge the Cryo-electron microscopy and tomography core facility and the Biomolecular interaction and crystallization core facility of CIISB, Instruct-CZ Centre, supported by MEYS CR (LM2018127).