Signal transduction in the protozoan host Hartmannella vermiformis upon attachment and invasion by Legionella micdadei.

The intracellular pathogens Legionella micdadei and Legionella pneumophila are the two most common Legionella species that cause Legionnaires' disease. Intracellular replication within pulmonary cells is the hallmark of Legionnaires' disease. In the environment, legionellae are parasites of protozoans, and intracellular bacterial replication within protozoans plays a major role in the transmission of Legionnaires' disease. In this study, we characterized the initial host signal transduction mechanisms involved during attachment to and invasion of the protozoan host Hartmannella vermiformis by L. micdadei. Bacterial attachment prior to invasion of H. vermiformis by L. micdadei is associated with tyrosine dephosphorylation of multiple host cell proteins, including a 170-kDa protein. We have previously shown that this 170-kDa protein is the galactose N-acetylgalactosamine (Gal/GalNAc)-inhibitable lectin receptor that mediates attachment to and invasion of H. vermiformis by L. pneumophila. Subsequent bacterial entry targets L. micdadei into a phagosome that is not surrounded by the rough endoplasmic reticulum (RER). In contrast, uptake of L. pneumophila mediated by attachment to the Gal/GalNAc lectin is followed by targeting of the bacterium into an RER-surrounded phagosome. These results indicate that despite similarities in the L. micdadei and L. pneumophila attachment-mediated signal transduction mechanisms in H. vermiformis, the two bacterial species are targeted into morphologically distinct phagosomes in their natural protozoan host.