Surrogate hosts: protozoa and invertebrates as models for studying pathogen-host interactions.

Animal models, primary cell culture systems and permanent cell lines have provided important information on virulence properties of pathogenic microorganisms. Recently, it has been shown that some inherent limitations of such models can be circumvented by using non-vertebrate hosts such as Caenorhabditis elegans, Drosophila melanogaster and Dictyostelium discoideum. These new models are helpful to follow infection processes at the molecular level. Persuasive support comes from the fact that processes such as phagocytosis, cell signaling or innate immunity can be studied in these surrogate hosts. This review describes the establishment and application of each of the three aforementioned and genetically tractable hosts. In addition, we will report on a number of approaches that led to the identification of host cell factors which influence the susceptibility of the hosts to infection.