Title: In vivo structural characterization of bacteriophage 812K1‐420 virion lytic cycle intermediates
Abstract: Staphylococcus aureus is one of the most important bacterial pathogens causing a wide spectrum of diseases varying in severity. Treatment of the infections is often complicated by S. aureus resistant to antibiotics. Current development of antibiotics is not fast enough to counterpart the emergence of antibiotic‐resistance. Preclinical studies showed, that phage therapy is promising alternative for treatment of bacterial infections including those caused by resistant strains, but approval of bacteriophage‐based drugs requires a detailed functional understanding of the molecular mechanisms of phage infection which we are still lacking. Bacteriophage 812K1‐420 from the family Myoviridae is potential candidate to be used in phage therapy against S. aureus. Polyvalent and strictly virulent bacteriophage 812K1‐420 virion is composed of an icosahedral head containing linear 145,5 kb dsDNA, contractile tail and baseplate which specifically recognizes specific surface on the bacterial host. Replication and virion‐assembly mechanisms inside host cell are not fully explored yet, although few bacteriophage 812K1‐420 lytic cycle intermediates of virion assembly were reproduced in vitro. Here we present initial structural analysis of bacteriophage 812K1‐420 infection in vivo using cryo‐electron tomography. We used cryo FIB milling technique to thin plunge frozen S. aureus cells (600‐1000 nm in diameter) infected by phage 812K1‐420 into wedge geometry . The cryoET data acquired on FEI Titan Krios microscope and tomograms were reconstructed using program IMOD.