Adhesion, penetration and intracellular replication of Legionella pneumophila: an in vitro model of pathogenesis.

Legionella pneumophila attached to, penetrated and replicated within the three eukaryotic cell lines, MRC-5, HEp-2 and Vero. Multiplication occurred rapidly in these cells for an initial 48 h after inoculation and declined thereafter. Infected MRC-5 cell monolayers developed lytic-type cytopathic changes, with organisms being readily released. HEp-2 cells showed a more chronic infection, with slowly developing granular changes in the monolayers, and slow release of intracellular bacteria. In Vero cells, organisms were released rapidly along with a more progressively developing granular cytopathic effect in the monolayers. L. pneumophila was unable to grow in cell-free culture fluids. Uptake and intracellular development was similar for each cell type, and was initiated by 'bacteriopexis', a process in which the organisms bound via receptors and were surrounded by cellular microvilli which eventually fused, leading to bacterial engulfment. Replication of organisms in vacuoles within the cytoplasm of infected cells was confirmed by thorium labelling. These vacuoles were lined with ribosomes and, at the early stages of intracellular development, were found in close proximity to mitochondria, cytoplasmic filaments and banded enclosures. Ruthenium red staining showed that acid mucopolysaccharide capsular material was not present on these organisms during the attachment process or intracellular phase. Organism release was by lysis of the infected cells.