CHARACTERIZATION OF A LEGIONELLA-MICDADEI MIP MUTANT

The pathogenesis of Legionella micdadei is dependent upon its ability to infect alveolar phagocytes. To better understand the basis of intra cellular infection by this organism, we examined the importance of its Mip surface protein. In Legionella pneumophila, Mip promotes infectio n of both human macrophages and freshwater protozoa Southern hybridiza tion and immunoblot analyses demonstrated that mip sequences were pres ent and expressed within a panel of virulent L. micdadei strains. Usin g allelic exchange mutagenesis, we then constructed an L. micdadei str ain that completely and specifically lacked Mip. Although unimpaired i n its ability to grow in bacteriologic media, this Mip mutant was defe ctive in its capacity to infect U937 cells, a human macrophage-like ce ll line. Most significantly, the Mip(-) organism displayed a 24-fold r eduction in survivability immediately after its entry into the phagocy te. Similarly, the mutant was less able to parasitize Hartmannella amo ebae. Taken together, these data argue that Mip specifically potentiat es intracellular growth by L. micdadei.