Acinetobacter phages use distinct strategies to breach the capsule barrier.

Acinetobacter baumannii is an opportunistic pathogen that is a growing threat in hospital settings due to its alarmingly high rates of antibiotic resistance. Alternative therapies are urgently needed to manage the growing burden of untreatable A. baumannii infections. Phage therapy is a promising avenue that has already seen some success in isolated compassionate-use cases, including the famous "Patterson case". Bacterial surface structures are the first determinants of susceptibility to phage infection and play a major role in phage host range. Most A. baumannii strains produce a protective capsule that is highly diverse both in structure and composition, and provides the first immunity barrier against phages. Here, we perform a detailed molecular characterization of three recently isolated, distinct A. baumannii phages, StAb1, StAb2, and StAb3, that breach the capsule via different mechanisms. Like many previously described A. baumannii phages, a specific capsule type is necessary for StAb1 infection. We found that StAb2 and its relatives adsorb to either a specific capsule type or the conserved outer membrane protein CarO, a porin normally occluded by the capsule. Thus, this phage has a narrow host range amongst capsulated strains, but can broadly infect A. baumannii strains lacking capsule. We also show that StAb3 requires a conserved and uncharacterized glycan that we have termed phage glycan receptor (PGR) that enables StAb3 to infect a broad range of A. baumannii strains irrespective of whether capsule is present. We demonstrate how rationally combining phages with distinct capsule interactions reduces the rapid emergence of phage escape mutants, with potential applications for more effective phage therapy.