S. aureus phage phi812/K1 is polyvalent T4-like bacteriophage with contractile tail. This phage is approved to use as an antimicrobial agent in Slovakia and the Czech Republic. Attachment of the phage to the host cell surface, tail contraction and penetration of cell wall are essential steps for successful infection. However, detailed mechanism of cell wall penetration is unknown. Our results show 3D reconstruction of phage tail tube penetrating the cell wall. The mechanism of genome delivery has been studied by Cryo-electron tomography. During attachment and contraction, many structural changes happen. Phage baseplate bind to receptors on cell surface, tail sheet rearrange and contract tail. Non-contracted tail tube is ejected and pierces outer cell membrane and cell wall. Then, virus genome is released and replication cycle starts. Determination of mechanism of cell wall penetration and genome delivery is necessary step to understanding of whole replication cycle of virus.