Phagocytosis and bacterial killing are major actors of innate immunity. When a phagocytic cell encounters a bacterium, it binds to it, ingests it by phagocytosis, and kills it inside a membrane enclosed phagosome. Phagocytes use various mechanisms to destroy bacteria: acidification of phagosomes, production of reactive oxygen and nitrogen species, permeabilization of bacterial membranes by antimicrobial peptides, and action of endosomal or lysosomal degradative enzymes. We do not fully understand the relative importance nor the degree of specificity of each of these antibacterial mechanisms against different species of bacteria. The investigation of antibacterial mechanisms can take two distinct approaches. Firstly, the genetic manipulation of cellular mechanisms involved in intracellular killing allows the assessment of their in vivo significance. This can be achieved through random mutagenesis, followed by the selection of mutants defective in intracellular bacterial killing, or through targeted mutagenesis of specific genes. Secondly, proteins exhibiting antibacterial activity can be discovered in vitro within cellular extracts, subsequently purified, and thoroughly characterized.
In the exploration of antibacterial cellular mechanisms, amoebae, specifically the unicellular soil amoeba Dictyostelium discoideum, emerge as an ideal model for genetic analysis of phagocytic cells. D. discoideum, co-evolving with bacteria, serves as a professional phagocyte, sharing conserved cellular mechanisms with innate immune cells. Its haploid genome is fully sequenced and thoroughly annotated, making D. discoideum a valuable host model organism to analyze intracellular killing of bacteria and study relevant host-pathogen interactions and to understand how phagocytic cells kill bacteria in phagosomes. Moreover, it represents a reservoir of unique antibacterial proteins not found in animals, with the potential to become valuable tools in the ongoing fight against infectious bacteria.
The publications included in this Privat Docent thesis reflect a conviction, and an overall aim, that the amoeba D. discoideum is a good model to address the following points: 1) exploring the participation of distinct mechanisms in the intracellular response of phagocytic cells against bacteria; 2) uncovering and delineating novel antibacterial effectors; 3) assessing the relative importance of various killing and antibacterial mechanisms; 4) and confirming that the repertoire of mechanisms employed by phagocytic cells actively contributes to the elimination of different bacterial species.
Our work, and that of others, has highlighted the utility of D. discoideum as a good host model organism to study relevant host-pathogen interactions. A detailed understanding of the determinants of the confrontation between phagocytic cells and bacteria paves the way to an informed study of bacterial infections.
