How phagocytic cells kill bacteria: Lessons from a professional killer

Professional phagocytic cells such as neutrophils and macrophages, as well as free-living soil amoebae like Dictyostelium discoideum, employ evolutionarily conserved mechanisms to ingest and kill bacteria. Ingested bacteria are killed in phagosomes by four main antibacterial systems: production of reactive oxygen and nitrogen species, accumulation of bactericidal ions, deployment of lysosomal enzymes, and release of membrane-permeabilizing peptides. Despite extensive studies, important questions persist. How complete is the list of known antibacterial mechanisms? What is their relative contribution to bacterial destruction? How specific are they for different bacterial species? This review examines how D. discoideum amoebae and mammalian phagocytes ingest and eliminate non-pathogenic bacteria, focusing on three specific elements: the dual role of lysozyme, the specificity of antibacterial mechanisms, and the redundancy of the antibacterial arsenal. Overall, our current knowledge highlights the sophisticated strategies employed by phagocytes against diverse microorganisms. It also provides a framework to study the more complex interactions between phagocytic cells and pathogenic bacteria.