Dictyostelium discoideum as a model host for meningococcal pathogenesis

BACKGROUND: The aim of the present study was to evaluate the possibility of studying meningococcal virulence in a new model organism, Dictyostelium discoideum, a haploid social soil amoeba that is an established host model for several human pathogens, leading to the discovery of novel virulence mechanisms. MATERIAL/METHODS: A number of virulent and hyper-virulent N. meningitidis strains, including isogenic encapsulated, unencapsulated, and lipooligosaccharide (LOS) outer core-defective derivatives, were used to test the ability of D. discoideum to internalize and grow in the presence of bacteria. Intracellular survival of the internalized bacteria was also monitored. RESULTS: Meningococci were internalized and killed by D. discoideum cells. The presence of a capsule did not affect the internalization, but, as in human cells, it increased the resistance of the internalized bacteria. Although both encapsulated and unencapsulated meningococci supported the growth and development of D. discoideum on an agar surface, in liquid medium the encapsulated strains were toxic to the slime mould cells. Toxicity inversely correlated with meningococcal survival in the assay medium that was not favorable to bacterial replication, suggesting that it may be due to some toxic compound released after bacterial autolysis. Intriguingly, unencapsulated isogenic strains efficiently supported Dictyostelium growth in suspension, opening the possibility that the toxicity may be associated with the capsular polysaccharide. CONCLUSIONS: These results suggest that several meningococcal virulence determinants, such as the capsular polysaccharide, may be remarkably effective also in Dictyostelium cells, stimulating the use of this model host to search for novel meningococcal virulence determinants.