Structure and genome delivery mechanism of Staphylococcus aureus phage therapy agent phi812-K1 determined by cryo-electron microscopy

Jiří Nováček1, Marta Šiborová1, Martin Benešík2, Roman Pantůček2, Jiří Doškař2, Pavel Plevka1

1 Central European Institute of Technology, Masaryk University, Kamenice 5, 625 00 Brno, Czech Republic;

2 Department of Experimental Biology, Faculty of Science, Masaryk University, Kamenice 5, 62500 Brno, Czech Republic

Worldwide occurrence of multidrug-resistant pathogenic bacteria has increased interest in alternative treatments including bacteriophage-based therapy. Bacteriophage phi812 belongs to genus Twort-like virus, subfamily Spounavirinae and can infect at least 75% of Methicilin-resitant S. aureus strains (MRSA) and 95% of Methicillin-sensitive S. aureus strains. We have employed cryo-electron microscopy to determine structure and  genome delivery mechanism for polyvalent staphylococcal backteriophage phi812-K1. Phi812-K1 has a 90 nm diameter isometric head and 240 nm long contractile tail ended by a double layered baseplate. The tail and baseplate of the native phage are dynamic. Therefore, a divide-and-conquer strategy was employed to separately determine the cryo-EM reconstructions of the individual phage parts. The structure of the icosahedral head could be refined to 5.0 Å resolution and additional sub-averaging within the T=16 icosahedral asymmetric unit allowed determination of the major capsid protein to 3.8 Å resolution. The structures of the native tail and baseplate were solved to 8 Å and 12 Å resolution, respectively. In order to examine the mechanism of the infection process, we determined the structure of the phage in the contracted state. The phage head is not altered after the DNA ejection. However, both the baseplate and tail undergo large reorganizations documented in their 6 Å and 8 Å resolution structures. Comparison of the tail and baseplate structures in the native and contracted conformation allowed us to determine the changes  accompanying cell wall recognition and binding which is then followed by injection of the bacteriophage genome into the host bacteria.