Genetic Analysis of Legionella pneumophila Intracellular Multiplication in Human and Protozoan Hosts

Summary Several approaches have been taken to understand the genetic basis for intracellular multiplication by Legionella pneumophila. The first was based on the properties of avirulent variants that had lost the ability to replicate inside macrophages and were also defective in preventing phagosome-lysosome fusion. Researchers reasoned that introduction of a wild-type region of the L. pneumophila genome that restored the ability to replicate within and kill human macrophages would provide information about the genes that were defective in the variants. A genomic library of wild-type L. pneumophila DNA was introduced to one of the avirulent variants, and complemented bacteria that regained the abilities to replicate intracellularly and kill host cells were identified with the aid of a plaque assay. The striking homology between the icm/dot genes and the IncI tra and trb genes involved in conjugal DNA transfer prompted us to examine if L. pneumophila could conjugally transfer DNA. The current model for how the Icm/Dot complex influences the intracellular fate of L. pneumophila postulates that there are two classes of Icm/Dot gene products. Several possible mechanisms could be used to specifically interrupt the fusion of phagosomes with lysosomes.