Phage therapy is increasingly put forward as a "new" potential tool in the fight against antibiotic resistant infections. During the "Centennial Celebration of Bacteriophage Research" conference in Tbilisi, Georgia on 26-29 June 2017, an international group of phage researchers committed to elaborate an expert opinion on three contentious phage therapy related issues that are hampering clinical progress in the field of phage therapy. This paper explores and discusses bacterial phage resistance, phage training and the presence of prophages in bacterial production strains while reviewing relevant research findings and experiences. Our purpose is to inform phage therapy stakeholders such as policy makers, officials of the competent authorities for medicines, phage researchers and phage producers, and members of the pharmaceutical industry. This brief also points out potential avenues for future phage therapy research and development as it specifically addresses those overarching questions that currently call for attention whenever phages go into purification processes for application.

Autonomous Department of Microorganisms' Biology Faculty of Agriculture and Biology Warsaw University of Life Sciences SGGW 02 787 Warsaw Poland

Bacteriophage Laboratory Hirszfeld Institute of Immunology and Experimental Therapy Polish Academy of Sciences 53 114 Wroclaw Poland

Centre of Excellence in Biological Interactions Department of Biological and Environmental Science Nanoscience Center University of Jyväskylä Survontie 9C FI 40014 Jyväskylä Finland

Chemical Engineering Department Loughborough University Leicestershire LE11 3TU UK

Department of Clinical Immunology Transplantation Institute Medical University of Warsaw 02 006 Warsaw Poland

Department of Experimental Biology Faculty of Science Masaryk University 60000 Brno Czech Republic

Department of Fundamental Microbiology University of Lausanne 1015 Lausanne Switzerland

Department of Microbiology Institut Pasteur 75015 Paris France

Department of Microorganisms Leibniz Institute DSMZ German Collection of Microorganisms and Cell Cultures 38100 Braunschweig Germany

Department of Pharmaceutical and Pharmacological Sciences KU Leuven 3000 Leuven Belgium

Eliava Institute of Bacteriophage Microbiology and Virology Gotua Street 3 0160 Tbilisi Georgia

Faculty of Dental Medicine The Hebrew University of Jerusalem Jerusalem 9112001 Israel

Institute of Biochemistry and Biophysics Polish Academy of Sciences 00 901 Warsaw Poland

Laboratory for Bacteriology Research Faculty Medicine and Health Sciences Ghent University 9000 Ghent Belgium

Laboratory for Molecular and Cellular Technology Queen Astrid Military Hospital 1120 Brussels Belgium

Laboratory of Gene Technology Department of Biosystems 3000 Leuven Belgium

MB Pharma 120 00 Prague 2 Vinohrady Czech Republic

University of Turin 10124 Turin Italy

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Lytic and genomic properties of spontaneous host-range Kayvirus mutants prove their suitability for upgrading phage therapeutics against staphylococci