Intracellular growth of Legionella pneumophila in Dictyostelium discoideum, a system for genetic analysis of host-pathogen interactions

Conditions were established in which Legionella pneumophila, an intracellul ar bacterial pathogen, could replicate within the unicellular organism Dict yostelium discoideum. By several criteria, L. pneumophila grew by the same mechanism within D. discoideum as it does in amoebae and macrophages. Bacte ria grew within membrane-bound vesicles associated with rough endoplasmic r eticulum, and L. pneumophila dot/icm mutants, blocked for growth in macroph ages and amoebae, also did not grow in D. discoideum. Internalized L. pneum ophila avoided degradation by D. discoideum and showed evidence of reduced fusion with endocytic compartments. The ability of L. pneumophila to grow w ithin D. discoideum depended on the growth state of the cells, D, discoideu m grown as adherent monolayers was susceptible to L. pneumophila infection and to contact-dependent cytotoxicity during high-multiplicity infections, whereas D. discoideum grown in suspension was relatively resistant to cytot oxicity and did not support intracellular growth. Some known D. discoideum mutants were examined for their effect on growth of L. pneumophila. The cor onin mutant and the myoA/B double myosin I mutant were more permissive than wild-type strains for intracellular growth. Growth oft. pneumophila in a G (beta) mutant was slightly reduced compared to the parent strain. This work demonstrates the usefulness of the L. pneumophila-D. Discoideum system for genetic analysis of host-pathogen interactions.