Bacterial viruses (bacteriophages) shuttle their genomes into bacterial cells as a key initial step for the infection of the hosts. Many bacteriophages use a tail for adsorption and DNA translocation to the host’s cytosol. Tailed phages combine core tail architectures with diversified infection modules, reflecting that they encounter different envelope compositions in their wide range of bacterial hosts. Long, non-contractile tailed siphoviruses are a predominant tailed phage type. However, the nature of the infection signal these phages receive from the bacterial envelope is not well understood and needs more investigations. The goal of our French-German project is to elucidate, with cryo Electron Microscopy, the first prototype structure for a glycan-specific siphovirus tail and baseplate infecting a Gram-negative host, from Salmonella model phage 9NA. Structure in presence of the lipopolysaccharide receptor or outer membrane vesicles will allow to decipher how phage structural proteins are involved in infection initiation, opening of the tail and perforation of the cell wall upon membrane contact. We will thus gain understanding of the molecular rearrangements in the siphovirus tail leading to genome release. This will define structural differences to phages that use protein receptors for infection, for which we have determined the detailed structure, before and after interaction with the membrane receptor, of a representative: phage T5. With TIRF microscopy, and new Gram-negative model membrane set-ups, we will study siphovirus 9NA’s time-resolved genome release mechanism on a single particle level, and analyze the influence of Gram-negative membrane properties on successful infection initiation. We will moreover use these in vitro set-ups to study synergies of phage mixtures that compete for receptors on the same host and correlate this data with in vivo infection analyses on whole bacterial cells. Exploring the dynamics in bacteriophage-cell wall interactions will advance our view on phage tail structure rearrangements, leading to genome release as a key step in the phage life cycle. Elucidating the tail and baseplate structure of a model system representing a widespread type of Gram-negative, glycan-specific phages will improve structure-based genome annotation and phage receptor usage prediction from sequence data. Our work will contribute substantial molecular knowledge on how phage communities behave in a given infection ecosystem, important for the development of new antibiotic treatments that employ bacteriophages.