THE PHAGOSOME CONTAINING LEGIONELLA-PNEUMOPHILA WITHIN THE PROTOZOAN HARTMANNELLA-VERMIFORMIS IS SURROUNDED BY THE ROUGH ENDOPLASMIC-RETICULUM

Legionella pneumophila is an intracellular parasite of protozoa and hu man phagocytes. To examine adaptation of this bacterium to parasitize protozoa, the sequence of events of the intracellular infection of the amoeba Hartmannella vermiformis was examined, The previously describe d uptake phenomenon of coiling phagocytosis by human monocytes was not detected, At 1 h postinfection with wild-type strain AA100, mitochond ria were observed within the vicinity of the phagosome, At 2.5 h posti nfection, numerous vesicles surrounded the phagosomes and mitochondria were in close proximity to the phagosome, At 5 h postinfection, the b acterium was surrounded by a ribosome-studded multilayer membrane, Bac terial multiplication was evident by 8 h postinfection, and the phagos ome was surrounded by a ribosome-studded multilayer membrane until 15 h postinfection, The recruitment of organelles and formation of the ri bosome-studded phagosome was defective in an isogenic attenuated mutan t of L. pneumophila (strain AA101A) that failed to replicate within am oebae, At 20 h postinfection with wild-type strain AA100, numerous bac teria were present in the phagosome and ribosomes were not detected ar ound the phagosome, These data showed that, at the ultrastructural lev el, the intracellular infection of protozoa by L. pneumophila is highl y similar to that of infection of macrophages. Immunocytochemical stud ies provided evidence that at 5 h postinfection the phagosome containi ng L. pneumophila acquired an abundant amount of the endoplasmic retic ulum-specific protein (BiP), Similar to phagosomes containing heat-kil led wild-type L. pneumophila, the BiP protein was not detectable in ph agosomes containing the mutant strain AA101A, In addition to the absen ce of ribosomes and mitochondria, the BiP protein was not detected in the phagosomes at 20 h postinfection with wild-type L. pneumophila. Th e data indicated that the ability oft. pneumophila to establish the in tracellular infection of amoebae is dependent on its capacity to resid e and multiply within a phagosome surrounded by the rough endoplasmic reticulum, This compartment may constitute a rich source of nutrients for the bacteria and is probably recognized as a cellular compartment. The remarkable similarity of the intracellular infections of macropha ges and protozoa by L. pneumophila strongly supports the hypothesis th at adaptation of the bacterium to the intracellular environment of pro tozoa may be the mechanism for its ability to adapt to the intracellul ar environment of human alveolar macrophages and causes pneumonia.